N-2 fixation by non-heterocystous cyanobacteria

Many, though not all, non-heterocystous cyanobacteria can fix N-2. However, very few strains can fix N-2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N-2 fixation by laboratory cultures and natural populations of non-heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N-2 fixation to environmental factors such as fixed nitrogen, O-2 and the pattern of illumination. The integration of N-2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of nonheterocystous cyanobacteria are highlighted. [KEYWORDS: non-heterocystous cyanobacteria; N-2 fixation; nitrogenase; immunolocalization; ATP and reductant; diurnal rhythm; natural environment Aerobic nitrogen-fixation; filamentous nonheterocystous cyanobacterium; synechococcus sp rf-1; marine microbial mat; anabaena-variabilis atcc-29413; plectonema-boryanum pcc-73110; circadian gene-expression; sp strain pcc-6803; sp pcc 6909; unicellular cyanobacterium]

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

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 μmol m2 s1 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 μmol m2 s1 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.

Nitrogen fixation by Baltic cyanobacteria is adapted to the prevailing photon flux density

N-2 fixation, measured as acetylene reduction, was studied in laboratory cultures and in natural assemblages (both as a mixed population and as individually picked colonies) of the heterocystous cyanobacteria Aphanizomenon sp. and Nodularia spp. from the Baltic Sea. During a diurnal cycle of alternating light and darkness, these organisms reduced acetylene predominantly during the period of illumination, although considerable activity was also observed during the dark period. In both laboratory cultures and natural populations N-2 fixation was saturated below photon flux density of 600 mu m(- 2) s(-1). In cyanobacterial blooms in the Baltic Sea, nitrogenase activity was mostly confined to the surface layers. Samples collected from greater depths did not possess the same capacity for acetylene reduction as samples from the surface itself, even when incubated at the photon flux density prevailing in surface waters. This suggests that, with respect to N-2 fixation, Baltic cyanobacteria are adapted to the intensity of illumination that they are currently experiencing. [KEYWORDS: Baltic cyanobacteria; N-2 fixation; Aphanizomenon; Nodularia Fe-protein; planktonic cyanobacteria; anabaena-variabilis; nodularia-spumigena; gloeothece nageli; alternating light; n-2 fixation; gas vesicles; sea; blooms]

New record of Trichodesmium thiebautii Gomont ex Gomont (Oscillatoriales - Cyanophyta) for the continental shelf of northeastern Brazil

This study reports the first occurrence of Trichodesmiumthiebautii Gomont ex Gomont on the coast of northeastern Brazil. Samples were gathered during the rainy season (July 2005) and the dry season (November 2006) at six stations distributed along two perpendicular profiles of the coast of Pernambuco (8º18'S - 34º56'W and 8º32'S - 35º00'W). T. thiebautii was found during both periods of the year and at the six collection stations, forming colonies in the form of clusters or more rarely, bundles, with straight cylindrical trichomes, quadratic or slightly longer cells, with widths between 5-10 µm and heights of 3-15 µm. The hydrological variables presented small variations, contributing to wide distribution of the species