Influence of Co and B12 on the growth and nitrogen fixation of Trichodesmium

We investigated the influence of varying cobalt (Co) and B-12 concentrations to growth and nitrogen fixation of Trichodesmium, a major diazotroph in the tropical and subtropical oligotrophic ocean. Here we show that sufficient inorganic Co, 20 pmol L-1, sustains the growth of Trichodesmium either with or without an additional B-12 supply. We also found that in these culture conditions, nitrogen levels fixed by Trichodesmium were higher in treatments with insufficient B-12 than in treatments with higher B-12 availability. Under limited inorganic Co availability, ranging from 0.2 to 2 pmol L-1, Trichodesmium growth was significantly compromised in cultures without B-12. In these low Co concentrations, addition of 400 pmol L-1 of B-12 supported phytoplankton growth indicating that B-12 supply augmented for the low Co concentrations. Our study demonstrates that Trichodesmium has an absolute Co requirement, which is not replaceable with Zn, and that B-12 supply alleviates stress in cases where Co is limiting. These results show that the interlocking availabilities of Co and B-12 may influence the growth and nitrogen fixation of Trichodesmium in the ocean.