MEPS 626:29-42 (2019)  -  DOI: https://doi.org/10.3354/meps13077

ABSTRACT: In order to evaluate the role of vitamin B₁₂ availability in microbial plankton communities, 10 microcosm experiments were performed in which inorganic nutrients (nitrate, ammonium and phosphate) and vitamin B₁₂ were added, separately or in combination, to surface seawater samples drawn from an on-shelf station over an annual cycle. The responses of both autotrophic and heterotrophic microbial plankton were highly variable. Vitamin B₁₂ addition enhanced phytoplankton biomass in early spring (March and April) and autumn (October) and bacterial biomass in winter (January) and summer (June and September). Our data reinforce the idea that inorganic nutrient limitation experienced by autotrophic and heterotrophic microbial communities is mostly dependent on the initial background nutrient conditions. The single (addition of vitamin B₁₂ alone) or secondary vitamin B₁₂ limitation (combined with inorganic nutrients) of phytoplankton growth implies that the availability of this molecule might modulate the increase in primary production associated with enhanced nitrogen loads to the ocean derived from human activities_. These findings may have significant implications for our understanding of carbon and nutrient cycling through the planktonic microbial compartments of coastal ecosystems.

KEY WORDS: Vitamin B₁₂ · Nutrient addition · Limitation · Competition · Phytoplankton · Bacterioplankton