B-vitamins and trace metals in the Pacific Ocean: Ambient distribution and biological impacts

Abstract

B-vitamins and trace metals have been implicated as important controllers of phytoplankton abundance and composition in the marine environment. In order to further establish the distribution and biological importance of dissolved B-vitamin in the Pacific Ocean, I determined the environmental concentrations of B-vitamins, the vitamin B₁₂-dependent amino acid methionine, and the B₁₂-precursor cobalt in the Eastern Tropical Pacific (ETSP) and the subtropical North Pacific. The environmental relevance of some toxic trace metals was also established in the coastal ocean off Los Angeles, CA. The field data was complemented with targeted laboratory and field manipulation experiments to assist in the interpretation of environmental distributions. In the ETSP, I collected and analyzed depth-profile measurements of B ₁₂, methionine, B₇, B₆, and cobalt at 6 stations with environmental conditions ranging from nutrient rich coastal stations to oligotrophic open ocean stations. Vitamin B₁₂ and methionine showed similar geographical distributions suggesting a potential control of vitamin B₁₂ on the synthesis of the amino acid likely due to B ₁₂-dependent methionine synthase. Despite low cobalt levels in the ETSP (<20pM), vitamin B₁₂ distribution was only related to the trace metal's distribution in a nutrient rich coastal station suggesting that cobalt may only regulate B₁₂ in locations where other key macronutrients are plentiful. Vitamin B₆ showed a strong correlation with chlorophyll indicating that this vitamin may be related to photosynthetic activity. Vitamin B₇ showed a coastal input and incubation experiments showed that some phytoplankton may be limited by vitamin B7 in this region. Large areas of the ETSP were depleted of B-vitamins and vitamin concentrations were not clearly correlated with microbial abundance.

In the subtropical North Pacific, the availability of dissolved thiamin (vitamin B₁) was related to nitrogen fixation rates due to the genomically identified thiamin auxotrophy of abundant group A cyanobacteria. Field B₁ amendment incubation experiments showed a 46% increase in nitrogen fixation and laboratory culture studies with an identified B ₁ auxotroph showed a 127% increase in nitrogen fixation.

Finally in the Los Angeles coastal ocean, the distribution of dissolved and particulate trace metals was examined in order to establish current levels of trace metal contamination in this region of the Pacific Ocean. Particulate levels were shown to be reduced dramatically compared to levels reported in the 1970s with decreases of ∼100-fold for Pb and ∼400-fold for Cu and Cd. Dissolved levels were found to be low with concentrations within the same range as an uncontaminated site in Punta Banda, Mexico. A trace metal uptake experiment with Synechococcus sp. CC9311 showed rapid internalization of multiple metals (within 3 hours) highlighting the importance of monitoring environmental concentrations of toxic and nutrient metals in the marine environment.