Assessment of phytoplankton community composition and cobalamin nutritional status using protein abundance measurements on the Scotian Shelf and Slope

Phytoplankton growth fuels marine food webs and has a profound influence on global biogeochemical cycles. While we now have an unprecedented ability to determine the diversity and genomic potential contained within phytoplankton communities, tools to quantitatively assess their contributions to biogeochemical cycling and the controls on their growth, in a taxon-specific manner, are lacking. Here we describe novel protein-based assessments of diatom contributions to phytoplankton communities on the Scotian Shelf and Slope of the Northwest Atlantic. We compared taxon-specific peptide metrics to pigment-based assessments and community composition derived through chloroplast 16S sequencing. We observed the expected seasonal trends in diatom contributions to the phytoplankton community using all three methods. Fucoxanthin concentrations and diatom specific Actin (dACT1) showed a significant positive relationship. Additionally, we showed that the abundance of a general Eukaryotic phytoplankton Rubisco peptide (RBCL1) correlates well with Chlorophyll A. We demonstrate the utility of these peptide-based measurements for assessing taxon-specific expression patterns of biomarker proteins within complex planktonic communities. The proteins assessed here describe the use of and starvation for cobalamin (vitamin B12) in diatoms. These measurements suggest that cobalamin was consistently utilized by diatoms and that cobalamin limitation of diatom growth may be rare in this region. Therefore, quantitative protein-based metrics hold promise for high-throughput determination of taxon-specific abundances and metabolic assessments in plankton communities. Further development of protein probes for cobalamin starvation will clarify the role of this organometallic micronutrient in the Northwest Atlantic.