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

Abstract

Cobalamin (vitamin B 12 ) is an important cofactor of different enzymes used by marine bacterioplankton (Helliwell et al., 2016), and has been shown to alter phytoplankton growth and physiological state (Koch et al., 2012). Vitamin B 12 is synthesized by specific prokaryotes, yet required by all auxotrophic phytoplankton (Tang et al., 2010). The availability of B 12 in seawater affects the cell size of autotrophs in different functional groups, where replete levels "favor" the growth of larger phytoplankton such as diatoms and dinoflagellates (Sañudo-Wilhelmy et al., 2006), while under depleted conditions, cyanobacteria usually prevail (Bertrand & Allen, 2012). The ambient concentration of B 12 in the photic layer is typically close to or even below its limit of detection (∼0.2-∼40 pmol L −1 ) (Okbamichael & Sañudo-wilhelmy, 2004), which may lead to a severe limitation of this micronutrient to phytoplankton/bacteria (Sañudo-Wilhelmy et al., 2012). Previous studies have shown that in addition to background nutrient conditions, phytoplankton productivity is limited by B 12 (Barber-Lluch et al., 2019), while in the High Nutrient Low Chlorophyll regions, B 12 has been shown to co-limit with iron and cobalt as well (Bertrand et al., 2007).The SEMS coast is a "Low Nutrient Low Chlorophyll" (LNLC) marine environment (Kress et al., 2019). In this region phytoplankton communities are dominated by small-size cyanobacteria and picoeukaryotes (Raveh et al., 2015), while nano-phytoplankton species such as small diatoms/dinoflagellates are usually found in low