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

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.