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Algal-driven priming of cellulose decomposition along a phosphorus gradient in stream mesocosms

Primary producer-based and detrital-based energy pathways in streams are intertwined. Interactions between algae and heterotrophic microorganisms may accelerate or inhibit recalcitrant organic detritus decomposition depending on the environmental conditions. The magnitude and direction of this priming effect have been attributed to nutrient availability, but the influence of nutrients can be difficult to disentangle. To date, no studies have examined priming across a single-nutrient gradient where every other variable is tightly controlled. The purpose of this study was to investigate the role of algal priming on decomposition of standardized cellulose substrates across an extensive phosphorus (P) gradient in laboratory mesocosms. We placed unbleached cotton strips into 11 recirculating mesocosms, each divided into a light section and a dark section. Each mesocosm was subjected to a different P concentration ranging from 1 to 1024 µg P/L, with all other nutrients maintained at saturating concentrations. P stimulated algal biomass in the light, but fungal biomass responded more to P in the dark. Decomposition, measured as loss of tensile strength (an indicator of cellulose catabolism), and extracellular cellulase enzyme activities were lower in the light treatment than the dark treatment above ∼16 μg P/L, indicating a suppression of decomposition by algae. This negative priming was likely due to preferential use of algal exudates by fungi in the light treatments under P-replete conditions, which may be caused by maximum algal accrual and possibly bulk labile organic matter exudation above these concentrations. A lack of positive priming in this study may have been due to negligible nutrient concentrations in the cotton and saturating nitrogen (N) concentrations in the water, suggesting that positive priming is primarily associated with N mining and, to a lesser extent, P mining. This study highlights how complex interactions between algae and microbial heterotrophs coupled with altered light and nutrient regimes may have consequences for stream food webs.