H2 accumulation and N2 fixation variation by Ni limitation in Cyanothece

Ni is an essential cofactor in NiFe-uptake hydrogenase, an enzyme regulating H-2 metabolism in diazotrophic cyanobacteria, the major H-2 producers in the surface ocean globally. Here, we investigated the effect of Ni supply on H-2 production and N-2 fixation by using a model marine cyanobacterial diazotroph, Cyanothece. By mediating total dissolved Ni concentrations from 100 to 0.03 nmol L-1 in a trace metal-defined culture medium, we demonstrated that Ni deficiency results in H-2 accumulation, coupled with decreasing Ni quotas, growth rates, and occasionally relatively low N-2 fixation rates. These results indicate that Ni deficiency limits the growth of the Cyanothece to some extent, considerably decreases H-2 uptake by hydrogenase and leads to H-2 accumulation and N-2 fixation variation in the diazotroph. The findings show that Ni availability is a critical factor on controlling H-2 production and N-2 fixation in marine diazotrophic cyanobacteria. The information of Ni bioavailability for diazotrophic cyanobacteria is thus essential to evaluate the importance of Ni for H-2 cycling and N-2 fixation in oceanic surface waters.