Abundances of Iron-Binding Photosynthetic and Nitrogen-Fixing Proteins of Trichodesmium Both in Culture and In Situ from the North Atlantic

Marine cyanobacteria of the genus Trichodesmium occur throughout the oligotrophic tropical and subtropical oceans, where they can dominate the diazotrophic community in regions with high inputs of the trace metal iron (Fe). Iron is necessary for the functionality of enzymes involved in the processes of both photosynthesis and nitrogen fixation. We combined laboratory and field-based quantifications of the absolute concentrations of key enzymes involved in both photosynthesis and nitrogen fixation to determine how Trichodesmium allocates resources to these processes. We determined that protein level responses of Trichodesmium to iron-starvation involve down-regulation of the nitrogen fixation apparatus. In contrast, the photosynthetic apparatus is largely maintained, although re-arrangements do occur, including accumulation of the iron-stress-induced chlorophyll-binding protein IsiA. Data from natural populations of Trichodesmium spp. collected in the North Atlantic demonstrated a protein profile similar to iron-starved Trichodesmium in culture, suggestive of acclimation towards a minimal iron requirement even within an oceanic region receiving a high iron-flux. Estimates of cellular metabolic iron requirements are consistent with the availability of this trace metal playing a major role in restricting the biomass and activity of Trichodesmium throughout much of the subtropical ocean

Light dependency of nitrogen fixation in a coastal cyanobacterial mat

The fixation of nitrogen in cyanobacterial mats situated along the littoral gradient on a Dutch barrier island was investigated by using a high-resolution online, near-real-time acetylene reduction assay. Light-response curves of nitrogenase activity yielded a variety of physiological parameters that changed during a day–night cycle. The fitted parameters were used to calculate nitrogen fixation from the incident natural irradiance over several days in two different mat types. Mats occurring in the higher regions of the littoral were composed of a diverse community of cyanobacteria, consisting of both heterocystous and non-heterocystous filamentous species, whereas closer to the low water mark the mats contained mainly non-heterocystous filamentous cyanobacteria. Although the daily cycles of nitrogenase activity differed considerably between the two types of mats, the daily integrated rates of nitrogen fixation were the same. Moreover, the daily integrated nitrogen fixation seemed to be independent from the daily incident photon flux. The measurements further suggest that different types of diazotrophic cyanobacteria become active at different times of the day and that the composition of the mat community affects maximal and daily patterns of nitrogenase activity. Notwithstanding the apparent light independence of nitrogen fixation, the light-response curves as well as light action spectra unequivocally showed that cyanobacteria were the predominant nitrogen-fixing organisms in these mats. It is concluded that the diversity of nitrogen-fixing cyanobacteria leads to an optimization of this process