On the role of oxygen for nitrogen fixation in the marine cyanobacterium Trichodesmium sp.

Abstract

The marine, non-heterocystous, filamentous cyanobacterium Trichodesmium shows a distinct diurnal pattern of nitrogenase activity. In an attempt to reveal the factors that control this pattern, a series of measurements were carried out using online acetylene reduction assay. Light response curves of nitrogenase were recorded applying various concentrations of oxygen. The effect of oxygen depended on the irradiance applied. Above a photon irradiance of 16 mumol m(-2) s(-1) nitrogenase activity was highest under anoxic conditions. Below this irradiance the presence of oxygen was required to achieve highest nitrogenase activity and in the dark 5% oxygen was optimal. At any oxygen concentration a photon irradiance of 100 mumol m(-2) s(-1) was saturating. When Trichodesmium was incubated in the dark, nitrogenase activity gradually decreased and this decline was higher at higher levels of oxygen. The activity recovered when the cells were subsequently incubated in the light. This recovery depended on oxygenic photosynthesis because it did not occur in the presence of DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Recovery of nitrogenase activity in the light was faster at low oxygen concentrations. The results showed that under aerobic conditions nitrogenase activity was limited by the availability of reducing equivalents suggesting a competition for electrons between nitrogenase and respiration.

Modeled contributions of three types of diazotrophs to nitrogen fixation at Station ALOHA

“…The potential contributions of these three diazotrophs to N 2 fixation were modeled as a function of field-measured cell abundances as well as measured and derived growth and biomass characteristics (Goebel et al, in revision). Consistent with laboratory experiments (Staal et al, 2007) and other published models of the cyanobacteria Trichodesmium (for example, Hood et al, 2001Hood et al, , 2002Hood et al, , 2004, we set N 2 fixation to be a function of lightdependent growth for all diazotrophs modeled. In addition, N 2 fixation was assumed to follow elemental stoichiometries (C:N) of cyanobacterial biomass.…”

“…The model also includes irradiance-dependent growth both vertically and in time. N 2 fixation for Trichodesmium has been shown to vary with irradiance in laboratory experiments (Staal et al, 2007) and this relationship has been parameterized in numerical models (Fennel et al, 2002;Hood et al, 2001Hood et al, , 2002Hood et al, , 2004. Fundamentally, it assumes that photosynthesis supplies the energy necessary to fix N 2 on a daily basis.…”

A novel cohabitation between two diazotrophic cyanobacteria in the oligotrophic ocean

“…(Hawser et al, 1992;Roman, 1992, 1994); therefore, these large filamentous colonies offer protective habitat in the open ocean, and they are known to harbor a community of heterotrophic bacteria and other filamentous cyanobacteria (Paerl and Bebout, 1989;Siddiqui et al, 1992;Sheridan et al, 2002;Hewson et al, 2009). Measurements of diurnal nitrogen fixation have shown that Trichodesmium nitrogen-fixation rates drop to below detection level at the start of the dark cycle (e.g., Capone et al, 2005;Staal et al, 2007). However, heterocystous diazotrophs are known to express nifH transcripts into the middle of the dark cycle , a time when Trichodesmium are reaching N-storage depletion.…”

Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean

“…Diel patterns of N 2 fixation activity in the marine cyanobacteria Trichodesmium, UCYN-A, and Crocosphaera watsonii have been reported, and while probably partially regulated by circadian rhythms (Chen et al, 1996), the cycles of nitrogenase activity are directly linked with access to energy supporting the enzyme and strategies for protection against inhibition by O 2 (Postgate, 1990;Rabouille et al, 2006). The diel cycle in the marine filamentous cyanobacterium Trichodesmium is well characterized, with the majority of the N 2 fixation occurring during the day (Staal et al, 2007b).…”