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Characterization of the metalloproteome of Pseudoalteromonas (BB2-AT2): biogeochemical underpinnings for zinc, manganese, cobalt and nickel cycling in a ubiquitous marine heterotroph.

Pseudoalteromonas (BB2-AT2) is a ubiquitous marine heterotroph, often associated with labile organic carbon sources in the ocean (e.g. phytoplankton blooms and sinking particles). Heterotrophs hydrolyze exported photosynthetic material, a component of the biological carbon pump, with the use of diverse metalloenzymes containing zinc (Zn), manganese (Mn), cobalt (Co) and nickel (Ni). Studies on the metal requirements and cytosolic utilization of metals for marine heterotrophs are scarce, despite their relevance to global carbon cycling. Here, we characterized the Zn, Mn, Co and Ni metallome of BB2-AT2. We found that the Zn metallome is complex and cytosolic Zn is associated with numerous proteins for transcription (47.2% of the metallome, obtained from singular value decomposition of the metalloproteomic data), translation (33.5%), proteolysis (12.8%) and alkaline phosphatase activity (6.4%). Numerous proteolytic enzymes also appear to be putatively associated with Mn, and to a lesser extent, Co. Putative identification of the Ni-associated proteins, phosphoglucomutase and a protein in the cupin superfamily, provides new insights for Ni utilization in marine heterotrophs. BB2-AT2 relies on numerous transition metals for proteolytic and phosphatase activities, inferring an adaptative potential to metal limitation. Our field observations of increased alkaline phosphatase activity upon addition of Zn in field incubations suggests such metal limitation operates in sinking particulate material collected from sediment traps. Taken together, this study improves our understanding of the Zn, Mn, Co and Ni metallome of marine heterotrophic bacteria and provides novel and mechanistic frameworks for understanding the influence of nutrient limitation on biogeochemical cycling.