Influence of Co and B(12) 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.