Award: OCE-1435666

The relationship between the availability of nutrients and phytoplankton dynamics in the ocean has been studied for several decades. Yet, our fundamental ability to predict the occurrence of blooms, and which particular species will dominate any given assemblage, is still undeveloped at best. This poor predictive ability is undoubtedly related to the fact that we have not yet fully understood the role of other substances in controlling the magnitude, duration or composition of algal blooms. B-vitamins and other essential organic compounds required at very low concentrations by certain members of the microbial plankton represent one set of these uncharacterized substances. This project tried to establish the role that B-vitamins may play in controlling biological dynamics in marine ecosystems today, a prerequisite to understanding potential future changes in different marine regimes. To address the ecological relevance of B-vitamins in the marine environment, we developed a new analytical protocol that allows to simultaneously measure all chemical forms of dissolved B-vitamins at pico-and-femto molar levels in seawater, as well as in particulate samples such as plankton. The observational part of the project showed that B-vitamins are, indeed, present in multiple chemical forms in seawater although large B-vitamin depleted marine regions were also observed. Our measurements also showed that their distributions and concentrations were both spatially and temporally variable, especially in coastal areas influenced by upwelling events and sediment sources. However, modeling results showed that despite their low concentrations, the availability of different chemical forms of the B-vitamins influenced the microbial community composition differently in several areas of the world ocean such as the Mediterranean Sea, Atlantic and Pacific Oceans. The variable community response to the different chemical forms of the B-vitamins is consistent with the fact that different organisms have different vitamin requirements. The ecological impact of the availability of B-vitamins in the marine environment was further tested by conducting field incubation experiments with natural microbial plankton assemblages. These experimental additions allowed to study the effect of vitamin enrichments on phytoplankton and bacterial community structure in a coastal upwelling system off Southern California. Those experiments showed that the native phytoplankton species present in seawater were co-limited by inorganic nutrients (e.g. nitrogen) and vitamin B12. Furthermore, our findings indicate that increasing B-vitamin availability causes shifts in phytoplankton community structure. For example, the numerically dominant chain-forming centric diatom Chaetoceros transitioned to the pennate diatom Pseudo-nitzschia, known to produce harmful algal blooms, in response to vitamin B12 availability. Therefore, the addition of the vitamins seems to provide a competitive advantage to certain species at the phytoplankton genus level. The scientific and societal impacts of this project include elucidating the impact of B-vitamins on the functioning of the marine environment. This research therefore advances our understanding of one group of the essential compounds needed to sustain major biological processes in the ocean. For example, phytoplankton supports the base of the oceanic food web, and mediates carbon flux from the atmosphere to the deep ocean via the so-called biological carbon pump. Ultimately understanding the mechanisms behind these fundamental processes are required for the well being of human society. Last Modified: 12/21/2018 Submitted by: Sergio A Sanudo-Wilhelmy