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Environmental vulnerability of the global ocean plankton community interactome

Marine plankton form complex communities of interacting organisms at the base of the food web, which sustain oceanic biogeochemical cycles, and help regulate climate. Though global surveys are starting to reveal ecological drivers underlying planktonic community structure, and predicted climate change responses, it is unclear how community-scale species interactions will be affected by climate change. Here we leveraged Tara Oceans sampling to infer a global ocean cross-domain plankton co-occurrence network – the community interactome – and used niche modeling to assess its vulnerabilities to environmental change. Globally, this revealed a plankton interactome self-organized latitudinally into marine biomes (Trades, Westerlies, Polar), and more connected poleward. Integrated niche modeling revealed biome-specific community interactome responses to environmental change, and forecasted most affected lineages for each community. These results provide baseline approaches to assess community structure and organismal interactions under climate scenarios, while identifying plausible plankton bioindicators for ocean monitoring of climate change.


MAIN TEXT Introduction
Marine plankton and associated processes are at the core of global biogeochemical cycles, shaping ecosystem structure and influencing climate regulation (1). While global biodiversity maps for viruses, prokaryotes and microbial eukaryotes are beginning to emerge (2)(3)(4), identifying and understanding the complex network of interactions between these organisms and their environment is in its infancy (5). These interactions are critical to establish the ecosystem trophic links that underpin biogeochemical cycles and feedbacks that drive climate regulation and response (6,7). While abiotic factors, such as temperature, can explain a large fraction of microbial community composition in the global ocean (8), biotic interactions can differentially shape ecosystem diversity (9), and can even influence the adaptation to new environments (10). Thus, determining how plankton ecological interactions are structured and affected by environmental change remains a significant challenge.
Large-scale holistic marine ecosystem sampling facilitates conceptualization of plankton community interactomes as co-occurrence networks that are useful to model the complex community structure of ecological associations (11,12). Such networks have enabled the detection of communities assembled through niche overlap across biomes (13), and also the prediction of putative interactions such as parasitism or symbioses (14). Likewise, plankton co-occurrence networks have been instrumental in detecting interrelated changes in community structure from surface to depth (15), as well as to identify specific communities of key lineages (e.g. Synechococcus, its phages, and Collodaria) associated with global open ocean processes such as carbon export (16). Community interactomes are also useful to identify central, highly connected lineages that may play significant ecological roles and confer stability to the community (17). These central lineages can correspond to keystone taxa that are good indicators of community shifts (18). Understanding the mechanisms affecting these central taxa may help us to predict responses of microbiome structure and functioning to perturbations (19).
While community interactomes inferred from global-scale samplings summarize well the complexity and potential interactions within microbial assemblages (12), they usually do not reflect dynamic processes shaping the observed system, as measured by longitudinal high-frequency sampling (20). Thus, alternative strategies need to be developed to capture ecosystem dynamics and responses from spatial samplings. Indeed, plankton species display various ecological and evolutionary responses to global environmental change (21,22). Within marine ecosystems, the interplay between species ecological niche and climate change can induce abrupt community shifts, which may lead to long-term reconfiguration of marine metazoan communities or biodiversity rearrangements (23,24). Recently, environmental drivers of ocean plankton diversity were inferred from Tara Oceans data and used to predict the effects of severe warming on surface ocean biodiversity (3). While species niche distribution models combined with climate models are useful to project fine-scale future distributions of species (25,26), species interactions are generally not included in these models (27), certainly due to our lack of knowledge about organismal interactions. Nevertheless, plankton network metrics can capture emergent properties (e.g. connectivity) relating to ecological characteristics of the community (28), which can serve as proxies of ecosystem and community-level resilience (29). Given that biotic interactions can influence species distributions at macroecological scales (30), and that climate change may cause trophic cascading effects on plankton community structure by directly impacting the top and bottom of marine food webs (31), ecological interactions need to be considered for assessing plankton community stability under climate change scenarios.
Here, we bridge the gap between ecological and climate modeling by combining network analyses (32) with species niche models (33) into a novel computational framework for predicting ecosystem-scale vulnerabilities to environmental change. By leveraging Tara Oceans data from all major oceanic provinces, including the Arctic Ocean, we inferred a comprehensive global-ocean cross-domain plankton cooccurrence network from sequencing data. We built statistical niche models to predict realized niche widths of planktonic taxa, across kingdoms, and from pole-to-pole. These were then mapped onto the network and used to evaluate both local and global robustness of plankton community structures to simulated environmental changes. Noticeable efforts have used the ecological niche concept to identify open ocean physical conditions governing phytoplankton biogeography (34), and also to better formalize central biogeochemical processes through the definition of key plankton functional types (35). The niche representation of planktonic diversity affords a more effective integration of abiotic and biotic constraints to better predict perturbations of primary productivity under climate change scenarios (26,36). A cross-kingdom plankton interactome from pole to pole  To reconstruct a global marine plankton co-occurrence network across kingdoms of  life, we analyzed data from 115 stations from the Tara Oceans expeditions (2009-2013) covering several organismal size fractions and all major oceanic provinces (37) across an extensive latitudinal temperature gradient from pole to pole (Fig.  1A). Via a dedicated probabilistic learning algorithm (38) (see M&M), we predicted ecological interactions between plankton taxa from compositional abundances inferred from sequencing data. The resulting integrated species association network (referred to as the Global Ocean Plankton Interactome -GPI) counts a total of 20,810 nodes corresponding to Operational Taxonomic Units (OTUs) and 86,026 edges corresponding to potential biotic interactions (Fig. 1A). In comparison to a previous plankton interactome generated from Tara Oceans data (14), GPI doubled the number of recovered known interactions from the literature (see supplementary text). A vast majority of positive associations (98.5%) were predicted, probably underlying a prevalent role for biotic interactions in shaping marine plankton communities (14). Very few direct associations between OTUs and environmental parameters were detected (n = 325, see supplementary text). However, by estimating robust ecological optima (or niche value) and tolerance ranges (or niche widths) (39) for each OTU and environmental parameter (see M&M and Table S3), we observed a strong influence of temperature in structuring predicted interactions (Fig. 1A). The GPI displayed a very high temperature optima assortativity coefficient (ACt=0.87), which quantifies the tendency of nodes being connected to similar nodes (here with similar temperature niche optima) in a network. Thus, it confirms that the latitudinal temperature gradient indirectly shaped the global ocean plankton interactome (3), and demonstrates the substantial effect of both environmental forcing and habitat filtering in structuring marine plankton communities at global scale.

Abiotic factors differentially shape the plankton interactome structure
To further investigate the influence of abiotic factors in shaping the GPI structure, we extracted local subnetworks corresponding to potential interactomes at each sampling site and computed graph topological metrics. These local metrics (see supplementary text for a detailed description) were then correlated to environmental parameters (see M&M). Globally, the GPI network connectivity assessed by these metrics was negatively associated with temperature and salinity ( Fig. 1B and supplementary text), pointing towards their potential impact in altering the structure of predicted interactions (40). We also observed a differential association between temperature and interactome connectivity in polar (Fig. S1A, negative association with strength) versus non-polar regions (positive association with strength). This difference may be linked to the observation that community turnover, which dominates in polar versus non-polar prokaryotic communities (4), is accompanied by stronger biotic dependencies between species. It also suggests a potential role for temperature in reducing polar community size structure in response to ocean warming, which we modeled and discuss below.
Given the observed differential association between temperature and community structure along the latitudinal axis, we compared local interactome topological metrics across biomes (Fig. 1C). The network stability (mean weight) and connectivity (transitivity) were significantly higher for the polar biome compared to other marine biomes (Dunn's test, FDR < 0.05 for all tests), and were associated with a lower mean (cross-domains) species diversity. This higher connectivity of the polar interactome further supports the more prevalent role of biotic interactions in structuring less diverse plankton communities in the extreme polar environment. It may also reflect the lower complexity and simpler structure of pelagic ecosystems in polar oceans, which are usually characterized by shorter pathways of energy flow in food webs (41). The lower diversity observed in polar ecosystems has also been linked to higher prokaryotic species turnover (42), which may translate into the higher connectivity observed between distinct species in the polar interactome. As recently proposed for a fluvial river system, environmental heterogeneity may determine the ecological processes assembling bacterial metacommunities (43).
Here, the environmental heterogeneity of polar systems may cause the higher network connectivity through higher heterogenous selection and community turnover.

Biome-specific communities emerge from the plankton interactome
To further our understanding of the role of temperature in shaping the interactome structure along the latitudinal axis, we used an unsupervised approach to delineate network communities and test their association with specific biomes. Using a deterministic community detection algorithm (see M&M), five communities emerged from the GPI, which were enriched in OTUs assigned to specific biomes, and displayed distinct predicted biotic associations (Fig. 2). Through comparison of community abundance profiles, these five communities were indeed preferentially observed in specific biomes ( Fig. 2A and Table S4). GPI Communities 0 and 3 (TC0 and TC3) occurred preferentially in Trades stations, Community 2 (WC2) prevailed in Westerlies stations, while Community 1 (PC1) clearly emerged in Polar stations. Community 4 (UC4) was more abundant in Polar stations but displayed a clear ubiquitous distribution. This unsupervised approach to community detection demonstrates that the GPI is self-organized across marine biomes, and that it captures the biogeography of cross-domain plankton associations. It also indicates that Longhurst's primary biomes partitioning (44), which is based on chlorophyll and phenology, is also biologically meaningful for planktonic associations across plankton domains and size spectra.
All GPI communities (with the exception of the ubiquitous UC4) displayed mostly exclusive associations, even at the high taxonomic level of the main planktonic lineages considered (Fig. 2B). All GPI communities clearly differed in their associations (Fig. S3), which were enriched between distinctive taxa (Table S6). Most prevalent associations in communities TC0, TC3 and WC2 (see supplementary text) included radiolarians (e.g. Spumellaria, Acantharea and Collodaria) and Dinophyceae, detected in associations with parasitic organisms (e.g. MALV, see supplementary text for details). Both PC1 (Polar-like) and UC4 (ubiquitous) communities clearly formed two distinct systems as compared to TC0, TC3 and WC2 with respect to co-occurring lineages (Fig. S3). The PC1 community displayed a significantly lower contribution of MALV and was particularly enriched in Bacillariophyta (diatoms) associations with several eukaryotic lineages, including Ciliophora, Cryomonadida, Choanoflagellatea and Mamiellophyceae, but also with bacterial lineages such as Bacteroidetes and Gammaproteobacteria, suggesting widespread diatom-bacteria interactions (45) in polar ecosystems. Notably, Bacillariophyta -Cryomonadida associations may correspond to ecologically important interactions in sea-ice influenced waters. Several Cryomonadida in cold waters can feed on diatoms and some Cryothecomonas spp. are diatom parasitoids (46). The UC4 community was significantly enriched in associations involving heterotrophic bacterial lineages (Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes) between themselves, and with major phytoplankton taxa such as Dinophyceae, Haptophyta, and Bacillariophyta, among the most abundant photosynthetic eukaryotes (47). Notable UC4 over-represented associations (Table  S6) included Haptophyta with MAST and MALV lineages, emphasizing the promiscuous nature of MALV parasitic interactions, not only in tropical and temperate ecosystems (14), but also in polar regions. Several cross-domain associations were enriched in UC4, such as Bacillariophyta and Dinophyceae with Bacteroidetes, and Copepoda with Alphaproteobacteria, revealing the pervasive role of phytoplankton-and zooplankton-bacteria ecological interactions (48) shaping the plankton microbiome from pole to pole. Core associations detected across all GPI communities were also identified (n=56, Fig. 2B) and reflected strong dependencies between clades that have co-adapted to specific environmental conditions encountered in each biome. These core associations were dominated by MAST and MALV lineages, underlying their broad biogeography (49,50), and very successful adaptation from pole to pole, through grazing and parasitism, respectively.
Communities emerging from the GPI underline niche differentiation by biome and implies that community-specific ecologically central species may be identified. To identify species whose impacts appear to be particularly important compared to their abundances, we computed the integrative general keystone index for each GPI community (51). Focusing on the ubiquitous UC4 community, the top 10 OTUs delineated by the index (Table S7) included Eukarya (n=6), among which several Copepoda (Cyclopoida, Corycaeus sp.) and Dinophyceae (Phalacroma, HM581743 sp.) taxa. It also included bacterial OTUs (n=4) belonging to AEGEAN-169, NS5 marine group, Polaribacter and SAR116 lineages. The AEGEAN-169 group was previously shown to be abundant and ecologically important at the SPOT station (52). Polaribacter environmental genomes were recently shown to be prevalent and active in the euphotic zone at both poles (Royo-Llonch et al., submitted).
Although each GPI community was more abundant in a given biome, their occurrence goes beyond these partitions, which probably reflects the importance of physical processes (e.g. advection by ocean currents) influencing their distribution through dispersal (53,54). This is also reflected by the biogeography of the WC2 community (Westerlies-like), and especially UC4 that is ubiquitous and appears to interface with other communities. The broader biogeography of these associations reflects the interconnected evolutionary history of phytoplankton-and zooplanktonbacteria ecological interactions and their pervasive role in influencing fundamental processes such as primary production, nutrient regeneration and biogeochemical cycling (48), not only in low-nutrient regions of the ocean but from pole to pole.

Community-specific vulnerabilities to environmental change
Given that the GPI captured the global biogeography of cross-domain plankton associations, we sought to investigate the potential influence of environmental change on community stability across biomes. Unlike previous studies that mapped global biodiversity and investigated ecological drivers, we used the GPI as a basis to develop a novel computational framework integrating OTU niche inference and community network analyses, to assess how plankton communities and lineages may be affected under environmental change. First, for each OTU, we calculated the ecological optimum and tolerance range for a selection of environmental parameters including salinity, nutrient concentrations (NO2+NO3, PO4), pH and temperature. These abiotic factors are projected to change significantly under ongoing climate change scenarios (36). For the temperature niche, we observed smaller OTU tolerance ranges towards the poles and the equator (Fig. S4), which supports the general assumption of higher environmental stability and narrower temperature ecological niches in both Polar and Trades biomes compared to the Westerlies (44). The environmental optima and tolerance ranges of OTUs inform us about the realized ecological niches of the taxa they represent and their potential sensitivity to environmental variations. OTUs from taxa with narrower tolerance ranges (i.e. specialists) are more likely to be affected by environmental changes, while OTUs from taxa with larger tolerance ranges (i.e. generalists) are more likely to be less sensitive to environmental changes. Based on this general assumption, we then simulated the effect of environmental changes on plankton interactome stability. Specifically, we perturbed GPI by progressively removing nodes ranked by their environmental tolerance ranges, from the narrower to the wider, for each parameter. We also attacked GPI's nodes by their degree (i.e. from the most connected to the least connected nodes) to simulate the potentially most damaging perturbation of the network, and repeated random attacks to obtain a random expectation reference. The GPI perturbations were systematically performed at global scale, to study both global and community-specific impacts of these attacks on the stability of the network (see M&M).
In response to in silico environmental perturbations, we observed an overall global robustness of the network (Fig. S5). However, at local scale, we found evidence for differential effects of specific abiotic factors on GPI communities (Fig. 3A). While the UC4 (ubiquitous) community was found to be the least sensitive to simulated environmental changes (Table S8, p > 1 x 10 -2 ), community TC0 (Trades-like) displayed significant vulnerabilities (Fig. S6) to temperature (Wilcoxon-rank test, p = 3.8 x 10 -10 ), salinity (p = 3.8 x 10 -10 ) and PO4 (p = 4.5 x 10 -6 ), as compared to random attacks. The TC3 community (Trades-like) also displayed a significant vulnerability to temperature (p = 1.7 x 10 -3 ). The WC2 community (Westerlies-like) was predicted as being the most vulnerable (Fig. S6) to nutrient concentration changes (NO2+NO3, p = 6.7 x 10 -9 ; PO4, p = 5.2 x 10 -9 ), while the PC1 community (Polar-like) displayed a clear vulnerability (Fig. 3B) to temperature (p = 3.8 x 10 -10 ). The PC1 community was also found sensitive to pH (data not shown), but with lower confidence given the higher amount of missing data for pH. These distinct predicted sensitivities of GPI communities imply that taxa represented by central, most connected OTUs display lower environmental tolerance ranges for distinct abiotic factors in each community. Thus, these findings suggest that the plankton interactome will be impacted differently by environmental change in specific ecological marine regions, which are themselves predicted to be impacted differently by warming and nutrient distributions (55). Both Trades communities (TC0 and TC3) appeared to be more sensitive to temperature, and to a lesser extent to salinity, which are both currently increasing in tropical ocean regions (56). On the other hand, the Westerlies community (WC2) appeared to be more vulnerable to nutrient concentration variations, which is a coherent scenario with climate change projections (36). These predictions also confirm the vulnerability of the Polar community (PC1) to temperature changes that are currently occurring with the rapid warming of the Arctic over recent decades and that is projected to be amplified (57,58).

Plankton lineages potentially most impacted by environmental change
By combining environmental tolerance range inference with network stability analyses, plankton communities most affected by environmental perturbations were predicted, as well as vulnerabilities of the respective plankton taxa and marine plankton groups (MPGs, see M&M). For temperature vulnerability predictions, we considered relatively abundant OTUs displaying a temperature niche width smaller than 2.1 ºC, which corresponds to the global mean sea surface temperature anomaly projected for the end of the century by the CMIP6 model scenario SSP2-4.5 (59).
Marine plankton vulnerabilities to temperature, salinity and nutrient concentration changes were predicted for communities TC0 and WC2 (see supplementary text). Focusing on the PC1 polar community, which appeared the most sensitive to temperature change, we identified specific plankton lineages from all domains of life predicted to be impacted (Fig. 4A). The bacterial phyla Verrucomicrobia and Marinimicrobia were found most sensitive with a vulnerable fraction above 50%. Verrucomicrobia lineages are poorly characterized but are ubiquitous in the ocean, and may be essential for the biogeochemical cycling of carbon (60). Conversely, several Marinimicrobia clades have been shown to participate in the biogeochemical cycling of sulfur and nitrogen (61). Abundant Eukaryotic lineages for which the vulnerable fraction was above 50% included Dinophyceae, Bacillariophyta, and Ciliophora, which are all key planktonic groups in the ocean, considerably impacting global biogeochemical cycles. Notably, all MAST (Marine Stramenopiles) groups, some of which are heterotrophic and bacterivorous flagellates that interact with key photosynthetic picoplankton (62), are also predicted to be significantly impacted. When resuming PC1 community lineages into MPGs (Fig. 4B), we predicted a large impact from temperature changes on Archaea, phototrophs and phagotrophs, and in particular on gelatinous filter feeders. The critical role of gelatinous zooplankton within ocean trophic webs is increasingly being recognized as they may channel energy from picoplankton to higher trophic levels (63). The temperature sensitivity we predict for gelatinous filter feeders questions the paradigm that gelatinous zooplankton have been increasing in the past decades (64), and points toward the overall vulnerability of corresponding lineages to ocean warming in polar regions.
PC1 polar lineages predicted to be most sensitive to temperature were also identified at a lower taxonomic level (Fig. 4C) to infer potential species indicators of polar ecosystem change in response to ocean warming (65). Predicted bacterial genera as being most vulnerable to temperature change in polar regions were Lentimonas and Methylotenera, along with several uncharacterized OTUs (n=30). Lentimonas spp. are specialized degraders of fucoidans and other complex polysaccharides (66). Their observed sensitivity to temperature variations may increase the recalcitrance of algal biomass to microbial degradation, which would affect the turnover of carbon sequestered in glycans that is vital for global carbon cycling (67). Methylotrophs of the family Methylophilaceae play a crucial role in the carbon cycle of aquatic habitats (68), and several Methylotenera spp. isolates are methylotrophic bacteria that can use a range of one carbon compounds in coastal ocean ecosystems (69). Thus, these two genera appear to encompass rather specialist microbes with regard to their metabolism, and are predicted to be impacted by ocean warming in the polar ocean. Eukaryotic lineages predicted to be most sensitive to temperature included several abundant diatom genera: Chaetoceros, Porosira and Proboscia, and other genera belonging to Rhizosolenids and Mediophyceae. A single abundant genus of dinoflagellate was predicted to be impacted by temperature change: Protoperidinium. For copepods, the genus Pseudocalanus and genera from the family Paracalanidae were found to be the most vulnerable. Picomonadida was the only heterotrophic protist family predicted to be vulnerable to temperature change.
Monitoring pelagic ecosystems under environmental stress due to ongoing climate change is challenging, but plankton species indicators may provide an accurate diagnosis of ecosystem health (70). Previous evidence suggests that genera we predict as being most sensitive to temperature in polar ecosystems may be good candidates for plankton indicators of ocean warming. Chaetoceros constitutes a very large genus of marine planktonic diatoms, and is a dominant component of phytoplankton communities contributing an estimated 20% of total oceanic primary production (71). Chaetoceros is abundant in polar oceans and is affected by temperature in lab experiments (72). A species distribution model previously showed that the annual median probability of occurrence of another diatom species Rhizosolenia stolterfothii was predicted to shift in the North Atlantic Ocean (21), suggesting that it too will be affected by anthropogenic climate change. Considering copepods, the abundance of the genus Pseudocalanus has continuously decreased within a decade (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) in East Greenland waters (73). Another line of evidence for the temperature sensitivity of the predicted genera comes from mesocosm experiments, in which the relative biomass of a diatom from the genus Proboscia (Proboscia alata) was negatively impacted by temperature (74). As for dinoflagellates, a species of the genus Protoperidinium was shown to be less tolerant to prolonged temperature shifts in laboratory experiments (75).
Overall, these results underlie the differential responses of biome-specific plankton communities and associated functions to specific environmental changes. These findings provide new insights into community-specific environmental vulnerabilities of plankton lineages and associated functions. Plankton MPGs play central roles in the ecology and biogeochemistry of the polar (and global) oceans.
Here, we predicted that specific plankton lineages and MPGs will be affected, which has substantial implications and may even worsen under currently projected scenarios of climate change (59) in the Arctic and in nutrient-rich oceanic regions.

Conclusions
This study provides a comprehensive cross-kingdom plankton interactome covering all major oceanic provinces, including the Arctic Ocean, a region that has lacked systematic standardized sampling. Our integrated network constitutes a unique resource providing putative key ecological associations among mostly uncultivated plankton taxa identified at the molecular level. Still, this resource is limited because predicted ecological associations do not demonstrate ecological interactions (76), and because it does not capture the dynamics of plankton interactions that are usually assessed using temporal or longitudinal samplings (77). Our knowledge of plankton microbiomes, symbioses, and host-parasite relationships remains limited (78). While planktonic interactions remain challenging to validate, our predictions are useful to further our understanding of ecosystem functioning. Today, highthroughput co-culture experiments using microfluidics (79) and fabricated synthetic microbial ecosystems may help fill this gap (80). Our resource constitutes useful guidance for co-culture experiments, as well as a database for hypothesis testing.
Climate scenarios predict global changes in temperature, pH, and nutrient concentrations, which all greatly influence plankton physiology. Temperature can directly impact bacterial growth (81), grazing rates (82), and phytoplankton metabolism (83). Nitrogen availability is a primary limiting factor for marine phytoplankton (84). Ocean acidification caused by rising atmospheric CO2 can impact phytoplankton growth rates, and is predicted to have a greater impact than warming or reduced nutrient supply on plankton ecological functions (85). Herein, we identified and predicted distinct community vulnerabilities of the plankton interactome by studying its robustness to environmental perturbations. Overall, our findings imply differential effects of environmental change on biome-specific plankton communities resulting from biotic interactions and environmental stresses. While the influence of temperature is central, at the biome-specific community scale, salinity and nutrient concentrations were found to significantly influence plankton community structures as well. These associations support previous lines of evidence linking temperature and nutrient concentrations as the principal drivers of microbial plankton community variability (54). These findings further advocate for the development of novel modeling paradigms targeting multiple biological scales (86), from genes to species and community levels (87). Our computational framework combining network analyses with niche modeling is generalizable and can be applied to various microbial ecosystems for assessing and predicting robustness to environmental perturbations. Here, specific lineage vulnerabilities were identified, but it remains an open question whether taxonomy, rather than function, is essential or not for predictive models given the potential functional redundancy in open microbial systems (88). Similar studies should be performed at the genomic level given that the molecular functions rather than the microbes themselves sustain marine biogeochemical processes (89).

Data description
From 2009 to 2013, the Tara Oceans expedition collected samples at more than 200 stations across all significant oceanic provinces from oligotrophic to polar regions. Sampling stations were selected to represent distinct marine ecosystems at global scale (90) describes the sampling strategy and the methodology applied, and sample provenance is described in Table S1. Environmental data measured or inferred at the depth of sampling are available in Table S2 and  . When both size fractions were available for the same sampling site, the 0.8-5 µm size fraction was preferred. For the 3-20/5-20 µm size fractions, only one station (TARA_124_SRF) was found to be in conflict, and we discarded the 3-20 µm sample. By doing so, we analyzed 115 sampling sites at which all considered size fractions were available.

Data processing and taxonomic annotations
For the prokaryotes-enriched size fraction (0.2-1.6µm and 0.2-3µm), taxonomic profiling was performed using 16S ribosomal gene fragments directly identified in Illumina-sequenced metagenomes (4). Extracted 16S reads, named miTags, were mapped to cluster centroids of taxonomically annotated 16S rRNA gene reference sequences from the SILVA database (59) (4), and we used the OTU-level abundance matrix as provided by the authors. For the Eukaryotic taxonomic profiling, we used the same methodology as in (50) to define OTUs using 18S rRNA gene V9 amplicons with the Swarm2 algorithm (91), with an updated version of the PR2 reference database (3). 18S rRNA gene V9 region PCR primers also amplify some 16S rRNA gene V9, thus we decided to filter out prokaryotic OTUs in the Eukaryote data that had been assigned to the following taxonomic groups: 'Bacteria' (35,448), 'nan' (31,406), 'Archaea' (1,806), 'root' (58) and 'Organelle' (583). For the prokaryotic abundance matrix, we filtered out miTags assigned to 'Eukaryota' (5,283), Chloroplast (468) and Mitochondria (74). After this filtering, we worked with six distinct matrices corresponding to each size fraction considered (see supplementary files online). Based on these taxonomic affiliations, we classified all taxa into marine plankton groups (MPGs) as in (3). For prokaryotes, photosynthetic bacteria (i.e. cyanobacteria) were distinguished from heterotrophic/chemotrophic bacteria and archaea. For protists, an extended version of the functional database of (50) was used. It encompasses a wide variety of protist taxa that are assigned to major functional groups: photosynthetic/mixotrophic protists, endophotosymbionts, hosts with endophotosymbionts (photohosts), parasitic protists, and free-living heterotrophs or phagotrophs (heterotrophic protists). For the mesozooplankton, the categories used corresponded to the most abundant taxonomic groups (such as copepods and chaetognaths) or feeding strategies. Shannon diversity indices were calculated for each sample and provided by (3).

Data transformation and filtering
All OTU abundance matrices were transformed using the centered log-ratio (CLR) transformation (92). The CLR transformation is widely used in microbiome data analysis, especially in association network reconstruction (93), as it copes with the compositional nature of microbiome data. As log transformation cannot be applied to zero values, we added beforehand a pseudo count of one to all elements of the matrix. Finally, to reduce the high dimensionality of our data, which may be the source of false-positive predicted associations, we filtered each abundance matrix using a top-quartile filtering approach. For each sample, the upper quartile (Q3) of its non-zero abundance values was computed. An OTU was retained when its observed abundance was higher than Q3 in at least 5 samples.

Network inference and stability procedure
The network inference was performed using FlashWeave (FW) v0.13.1 with default parameters (38). FW relies on the Local-to-Global learning framework and infers direct associations by searching for conditional dependencies between OTUs. Several heuristics are then applied to connect these "local" dependencies and infer a network. FW is significantly faster than other methods while achieving better or similar results, and gives the possibility to include meta-variables (such as the temperature). Although the latter feature seemed appealing, very few OTUenvironmental factor associations were detected which advocates for developing a complementary approach to study the environmental influence (see Network attack below). While FW includes a heterogeneous mode (FlashWeaveHE) and the Tara dataset is heterogenous itself, the low number of samples prevented its use. Thus, we used FW in 'sensitive' mode without its embedded normalization since it was performed upstream to comply to our network inference strategy designed to deal with the multiple size fractions context described below. We reconstructed graphs for each size fraction separately, running FW on the corresponding CLR-transformed abundance matrix. This first step only allows to discover intra-fraction edges. To connect together the 5 resulting graphs, and thus infer inter-fraction edges, we considered all 10 combinations of two size fractions and ran FW on the according concatenated matrices. This results in a meta-graph with OTUs from different size fractions being connected together.
To assess the robustness of intra-and inter-fraction edges, and reduce the number of putative false-positive associations, we implemented a stability procedure inspired by the STARS model selection approach (94). As we did for two size fractions matrices, we built every combination of three size fractions matrices and obtained 10 three-fractions graphs. We then evaluated the stability of every meta-graph edge by computing its frequency in the three-fractions graphs. This procedure computes a relative stability metric reflecting a given edge robustness to variation in both the number of samples and the number of OTUs. Edges with relative stability below 50% were removed from the meta-graph.

Estimation of false discovery rate
Three null models were generated using two R packages (picante v1.7 and HMP v1.6). The HMP library provides the Dirichlet.multinomial function which allows data matrix generation of OTUs following a Dirichlet distribution. picante comes with a randomizeMatrix function and several methods to randomize the matrix. We used the frequency (that maintains OTUs occurrence frequency), and trialswap (maintaining OTUs occurrence frequency and sample OTUs richness) approaches. Then, networks were inferred from these matrices using FW and the same procedure as for the observed matrices. We then estimated a False Discovery Rate by comparing common edges between the observed and simulated networks. The highest FDR we obtained was 3.6% (with a number of iterations set to 10 e 8) using the trialswap method.

Literature-based validation of predicted interactions
To compare the performance and sensitivity of FW to similar co-occurrence network inference methods such as SPIEC-EASI (93), we estimated the graph accuracy by comparing edges with known (marine) biotic interactions. We limited our comparisons to Polar networks and compared edges with known interactions from the PIDA (78) (https://github.com/ramalok/PIDA) and GLOBI (95) databases (https://www.globalbioticinteractions.org/). We used the NCBI taxonomy for prokaryotes and PR2 taxonomy for eukaryotes to identify Superkingdom, Family, Genus, and Species levels. Then, we searched for known interactions from these databases in the networks by detecting all combinations of OTUs at the four taxonomic levels considered. Results are presented in Fig. S11. Conserved associations across taxonomy ranks were estimated as follows. First, taxonomic ranks were extracted from NCBI Taxonomy database for prokaryotes and from PR2 database for eukaryotes. Next, for each pair of ranks, we counted the number of edges between nodes of each rank. Next, we repeated the procedure but now applied to the subnetwork induced by considering only nodes from a particular biome. Finally, we calculated the proportion of edges for each rank pair in each biome with respect to the total network.

Station-specific network extraction
To further explore the association between plankton community structures and abiotic factors, we extracted sampling station-specific subnetworks corresponding to local GPI interactomes containing only nodes of OTUs detected at a given sampling station. This procedure enabled the computation of graph topological metrics (mean degree, edge density, mean weight, mean strength and transitivity) for each sampling station and enabled us to directly associate environmental parameters to local community structures.

Marine biome assignations to OTUs
In the Tara Oceans data set, each sample is associated with one specific marine biome (Coastal, Trades, Westerlies or Polar). Using this information, we assigned each OTU to a biome or a combination of biomes according to its abundance profile. We did this by identifying biome(s) in which a given OTU is overrepresented, based on relative abundance, compared to other biomes using a Kruskal-Wallis (KW) test implemented in the Python package SciPy (version 1.2.1). Adjustments for multiple testing were performed using the Benjamini-Hochberg (BH) procedure implemented in statsmodels (version 0.9.0). For significant tests (FDR < 0.05), a post hoc Dunn's test implemented in scikitposthocs (version 0.6.1) was performed to determine in which biome(s) a given OTU was significantly over-represented (FDR < 0.05). To determine the direction of the over-representation, we compared the mean values to identify and discard the "lower mean biome(s)" from the list of the OTU-associated biomes. In the GPI we were able to assign biome(s) to a significant fraction of OTUs (41.1%). Numerical and categorical assortativities were determined with the corresponding functions from networkx 2.3.

Network community detection and biome assignation to communities
We detected five communities in the GPI using an eigenvector-based network community detection algorithm (96) implemented in the networkx 2.3 python package. To assign biomes to these communities, OTUs abundance tables were CLR transformed and aggregated by community for each size fraction. CLR values for each community were grouped by biome, and a KW test was run to verify mean differences of communities among biomes (KW test column in Table S4). As all pvalues were significant while controlling the FDR using the BH procedure, post hoc Dunn tests were performed to detect community pairwise differences between biomes (Dunn test p-value columns in Table S4). Biomes that were found significantly lower via the Dunn test were discarded from the biome assignation (Dunn test z-score column in Table S4). The five GPI communities were found prevalent in the Polar (n=2), Westerlies (n=1) or Trades (n=2) biomes.

Environmental optimum and tolerance range inference
Environmental optimum and tolerance range were calculated with the robust optimum (RO) method described in (39). For each OTU and a selection of environmental parameters, we determined the ecological optimum reflecting the optimal OTU living conditions relative to a given environmental parameter, and a tolerance range around this optimum defined by lower and upper bounds. Here, Total Sum Scaling (i.e. read count divided by the total number of reads in each sample) normalization was applied to raw matrices. For each OTU, the proportion of observed counts in a given sample is computed relatively to all samples. We use these proportions to fill a weighted vector of a fixed size (n = 10000) with environmental values accordingly (i.e. if the proportion of observed counts for OTU1 in sample 1 represents 5% of the OTU1 abundance across all samples, then the weighted vector will be filled at 5% with the environmental value measured for sample 1). The ecological optimum is then defined as the median value (Q2) of this vector, the lower and upper limits as the first (Q1) and third quartile (Q3) respectively, and the tolerance (niche) range is given by the interquartile range (Q3-Q1). Some environmental parameter values are missing (non-available, NA) for some samples. To avoid inferring spurious ecological optima and tolerance ranges for OTUs for which many values are missing, we set a minimum threshold of 10 OTU observations with non-NAs and overall with a minimum of 30% non-NAs values for it to be computed.

General keystone index
The generalized keystone index (51) combines several centrality metrics in a single measure, which can then be used to rank nodes, revealing their topological importance in the network. Degree, betweenness, closeness and subgraph centralities have been calculated using the Python library networkx (version 2.3), capturing the relevance of each node at different topological scales. Factor analysis was performed with the Python library sklearn (version 0.20.3) on those centralities to get the generalized keystone index associated with each node.

Network-based robustness analyses
In order to simulate the effects of environmental change and predict their impact on the stability of plankton community structures, we designed a network attack procedure mimicking the potential effect of each environmental parameter's variations onto the GPI. We progressively removed network nodes by bins (n = 200 nodes until the 10000 th node, then n = 1000 nodes) corresponding to environmental ranges, ordered from the smallest to the largest tolerance ranges for each parameter (within a given range, the nodes are randomly sorted). At each step we computed the graph natural connectivity (97), a graph robustness metric, for the global interactome and for subgraphs corresponding to communities extracted from the GPI (see M&M section 'Biome assignment and network community detection'). By doing so we could evaluate the vulnerability (or loss of robustness / stability) of the GPI at the global and community levels, and detect OTUs and lineages that were actually targeted/impacted first in the process. Importantly, temperature and nutrient concentration changes are generally not independent; temperature increases metabolic rates, which may in turn increase nutrient uptake and cycling through the food web. Thus, both parameters may show a synergistic effect on plankton community structure (98). The potential for this abiotic synergy points toward a limitation of our in silico perturbation experiments since we did not integrate per se the whole set of environmental parameters that are necessary to properly define the ecological niche of a given OTU -nor the synergistic interactions between them. While an equal combination of different environmental tolerances can be assumed to define a niche (i.e. the cardinal product of limiting abiotic factors), we argue this would bias our predictions since plankton species are differentially adapted and respond to environmental conditions. Lineagespecific adaptation may explain the differential sensitivity in the Polar biome where temperature is significantly lower compared to non-polar regions, and in the Westerlies biome where nutrients are usually not limiting factors as compared to the Trades biome.

Predicting most vulnerable community lineages and MPGs
In order to predict community-specific groups (phyla, PMGs and MPGs) most vulnerable to environmental change, we focused on most "abundant" OTUs, for which the total mean abundance was above 0.001. This cutoff corresponds to the mean relative abundance of all groups. The proportion of affected groups was computed as the factor between the total mean abundance and the affected mean abundance of a given group. For computing these affected proportions we limited ourselves to environmental ranges corresponding to global mean anomalies projected for the end of the century by the CMIP6 model scenario SSP2-4.5 (59). Thus, environmental ranges considered here were 2.1ºC for temperature, 0.5 PSS-78 for salinity, 0.7 µmol/L for NO2 and 1.0 µmol/L for PO4.

Statistical analyses
Spearman correlations, followed by BH procedure (FDR < 0.01), were performed to test associations between network topology metrics and environmental parameters ( Fig. 2A). KW tests followed by Post-hoc Dunn's tests were performed using R (version 3.2.2) to determine significant differences across biome-specific interactome topological metrics (Fig. 2C). A Pearson's Chi-squared test was performed to detect taxa associations enriched in each interactome community (Fig.  S3). Here, only pairs of taxa which co-occur in at least 3 communities and occur at minimum 50 times in total were tested. For these pairs, we performed a post hoc analysis for Pearson's Chi-squared test on the residuals (99) using the chisq.posthoc.test R package (https://chisq-posthoc-test.ebbert.nrw/) to identify within each community taxa pairs with a number of associations significantly diverging from random expectation. Wilcoxon's rank sum tests were performed to compare distributions of natural connectivity for network environmental perturbations versus random perturbations (Fig. 3).    In all panels, the fraction of lineages, MPGs, and genera (from 1 for most impacted, to 0 for not impacted) predicted to be impacted by temperature variations are depicted within each size fraction. Plankton lineages (Prokaryotes and Eukaryotes), MPGs and genera are ordered according to the cumulative mean relative abundance of the corresponding OTUs across size fractions (note that these relative abundances are not directly comparable between size fractions).