H₂ accumulation and N₂ fixation variation by Ni limitation in Cyanothece

Abstract

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.

The effects of Ni availability on H2 production and N2 fixation in a model unicellular diazotroph: The expression of hydrogenase and nitrogenase

“…As Tuo et al (2020) conducted a similar experiment with shorter time duration for the measurement of N 2 fixation and H 2 production, we have compared our data with the data of Tuo et al (2020). Most observations of the parameters, including Ni quota, specific growth rates, and H 2 /N 2 ratios, were comparable between the two studies (Table 5).…”

“…Comparison of the ratios of daily H 2 net production to fixed N 2 obtained from the 1 st and 2 nd experiments and Tuo et al (2020), which are presented by the symbols of triangle, square, and circle, respectively. The red highlighted zone indicates a speculative range of the Ni threshold concentration which results in two distinctive ratios.…”

“…There is no statistical difference within the same Ni concentration between 1 st and 2 nd experiments except for Ni 2 treatment (ANOVA, F 1,4 = 77.0, p < 0.001). The time frames used to calculate the ratios are 22 and 30 h for our 1 st and 2 nd experiments, respectively and only 12 h in dark period for the study of Tuo et al (2020) (Table 5).…”

“…H 2 net production and N 2 fixation rates were estimated by subtracting the value of a prior time point from the value obtained at any given time point and dividing by the time difference between the two time points. More detailed information about these measurements is provided by Tuo et al (2020).…”

“…Besides Ni superoxide dismutase and urease, the effects of Ni availability on NiFe uptake hydrogenase in marine diazotrophs are poorly understood. For the first time, Tuo et al (2020) applied trace metal‐defined culture techniques at low nanomolar Ni concentrations to examine the effects of Ni availability on H 2 production and N 2 fixation in Cyanothece . They found that total dissolved Ni concentrations below 10 nM (with 20 μM ethylenediaminetetraacetic acid, EDTA) caused high H 2 net production.…”