Cobalamin Co-Limits Phytoplankton and Bacterial Biomass and Activity in Eastern Mediterranean Coastal Waters

Abstract

In this study, we examined the response of phytoplankton and heterotrophic bacteria to B12 amendments in microcosm experiments in the coastal southeastern Mediterranean Sea (SEMS) during the summer, where ambient levels of dissolved B12 ranged from as low as ∼2 pmol L−1 to 60 pmol L−1 (median 5.3 pmol L−1). Additions of B12 (20 pmol L−1) to surface seawater triggered a 4-fold increase in NO3+NO2 uptake compared to unamended seawater, resulting in proliferation of mainly pico/nano-eukaryotic phytoplankton (∼30%) and increase in primary and bacterial productivity (40%–50%). Complimentary experiments that tested the combined effects of nutrient (NO3 and PO4) and B12 additions suggest that phytoplankton were primarily NO3 and B12 co-limited, whereas heterotrophic bacteria were PO4 and B12 co-limited. These results provide valuable information about the marine distribution of nutrient limitation in low nutrients low chlorophyll (LNLC) environments such as the SEMS, and how bacterioplankton might respond to environmental perturbations.