It is well known that the B-vitamins are important growth factors involved in a large number of metabolic pathways. However, to date, very little work has been done regarding the importance of those growth factors in regulating biological activity in marine systems. Genomic data indicates that the supply of B-vitamins in marine systems depends mainly on biosynthesis by prokaryotes. In contrast, half of the currently tested eukaryotic phytoplankton have been determined to be auxotrophic for at least one vitamin. Therefore, the lack of an exogenous source of these growth factors from prokaryotic cells may strongly impact the efficiency of the so-called biological pump, which is dominated by large eukaryotic cells.
In this project, researchers at the University of Southern California will carry out the first in-depth evaluation of the sources and cycling of arguably one of the most important growth factors in the ocean, the B-vitamins (B1, B7, and B12). This will be accomplished by combining field studies in three different oceanographic regimes (San Pedro Ocean Time Series site (SPOTS) off Southern California, The Hawaii Ocean Time-Series (HOT) station ALOHA, and a cruise of opportunity to the East Tropical South Pacific (ETSP) with lab manipulation experiments of cultures of different marine prokaryotic species. The field study sites are selected to give the maximum amount of information about the cycling of vitamins in a broad range of marine environments.
Based on preliminary results, the research team hypothesizes: (1) that there are several potential producers of B-vitamins in the ocean: non-diazotrophic cyanobacteria and heterotrophic bacteria in the upper euphotic zone of the world ocean, and that the availability of B-vitamins is further augmented by diazotrophic cyanobacteria in subtropical-tropical LNLC regions of the world ocean; (2) that the synthesis of some B-vitamins is controlled by the availability of specific dissolved trace metals such as Co (for B12) and Fe (for B2 and B7) and organic and inorganic nutrients; and (3) that the balance between B vitamin production (found in the particulate phase) and release (found in the dissolved phase) dictates vitamin availability and environmental cycling, and that this balance is set by three factors: the activity of a subset of the pico and microplankton, physical transport, and uptake by auxotrophic plankton. In order to test these hypotheses, they developed a new method, using a triple quadrupole mass spectrometer coupled to a liquid chromatographer, which allows them to measure directly all B-vitamins simultaneously in less than a liter of seawater as well as in the intracellular and particulate pools.
Broader impacts: The scientific and societal impacts of this project include elucidating the cycling of B-vitamins, without which we will never have a complete understanding of all of the essential substances needed to sustain major biological processes such as primary production and nitrogen fixation in the ocean. Both graduate and undergraduate students will participate in this project. The principal investigators are active participants in the COSEE-West program, which involves a network of marine scientists, K-12 educators, and the general public.
Enzymes that require coenzymes (i.e., organic cofactors) catalyze many important chemical reactions occurring in every cell, and a large number of coenzymes are B-vitamins (thiamin (B1), biotin (B7) and cobalamin (B12)). Despite the biochemical relevance of B-vitamins and their absolute requirements for many pivotal biochemical pathways (i.e., the Calvin Cycle), the "omics" revolution of cultured and uncultured marine bacterioplankton have shown that many species do not have the genes to synthesize some of those vitamins, suggesting that they require an exogenous source to survive. In order to evaluate those exogenous sources of B-vitamins, we developed a new method to measure dissolved B-vitamins in seawater. The application of this new technique showed that in large areas of the world ocean B-vitamins are undetectable, while in others the concentrations seem sufficient to sustain life. These data suggest that vitamin limitation could be important for the efficiency of carbon and nitrogen fixation in those depleted regions. Using recently available genomic data combined with culture-based surveys of vitamin auxotrophy (i.e., vitamin requirements), we show that auxotrophy is widespread in the marine environment and occurs in both autotrophs and heterotrophs residing in oligotrophic and eutrophic environments. Our analysis shows that soluble vitamins originate from the activities of some bacteria and algae, and that taxonomic changes observed in marine phytoplankton communities could be the result of their specific vitamin requirements and/or vitamin availability.
