ALBERT

Description: We experimentally evaluated the temporal (interday and interseason) and spatial variability in microbial plankton responses to vitamin B12 and/or B1 supply (solely or in combination with inorganic nutrients) in coastal and oceanic waters of the northeast Atlantic Ocean. Phytoplankton and, to a lesser extent, prokaryotes were strongly limited by inorganic nutrients. Interday variability in microbial plankton responses to B vitamins was limited compared to interseason variability, suggesting that B-vitamin availability might be partially controlled by factors operating at seasonal scale. Chlorophyll a (Chl a) concentration and prokaryote biomass (PB) significantly increased after B-vitamin amendments in 13 % and 21 %, respectively, of the 216 cases (36 experiments × 6 treatments). Most of these positive responses were produced by treatments containing either B12 solely or B12 combined with B1 in oceanic waters, which was consistent with the significantly lower average vitamin-B12 ambient concentrations compared to that in the coastal station. Negative responses, implying a decrease in Chl a or PB, represented 21 % for phytoplankton and 26 % for prokaryotes. Growth stimulation by B1 addition was more frequent on prokaryotes than in phytoplankton, suggesting that B1 auxotrophy in the sampling area could be more widespread in prokaryotes than in phytoplankton. Negative responses to B vitamins were generalized in coastal surface waters in summer and were associated with a high contribution of Flavobacteriales to the prokaryote community. This observation suggests that the external supply of B12 and/or B1 may promote negative interactions between microbial components when B-vitamin auxotrophs are abundant. The microbial response patterns to B12 and/or B1 amendments were significantly correlated with changes in the prokaryotic community composition, highlighting the pivotal role of prokaryotes in B-vitamin cycling in marine ecosystems.

Description: We evaluated the temporal (inter-day and inter-season) and spatial variability in microbial plankton responses to vitamins B12 and B1 supply in coastal and oceanic waters. Inter-day variability in microbial plankton responses to B-vitamins was not of great concern, suggesting that B-vitamins availability was controlled by factors operating at larger temporal scales, such as those driving microbial community seasonal succession. Most positive responses were produced by treatments containing either B12 alone or B12 combined with B1 in oceanic waters, which was consistent with the significantly lower average vitamin B12 ambient concentrations compared to that in the coastal station. Growth stimulation by B1 addition was more frequent on bacteria, which is coherent with their widespread dependence on exogenous sources for this growth factor. Negative responses to B-vitamins were generalized in coastal waters in summer, and were associated to a high contribution of Flavobacteriales to the prokaryote community. This observation suggests that the external supply of B12 and/or B1 may promote negative interactions between microbial components when B-vitamins auxotrophs are abundant. The microbial response patterns to B12 and/or B1 amendments were significantly correlated with changes in the prokaryotic community composition, highlighting the pivotal role of prokaryotes in B-vitamins cycling in marine ecosystems.