Nutrient control of cyanobacterial blooms in the Baltic Sea

Cyanobacterial blooms in the Baltic Sea were investigated with respect to growth Limitation and nitrogen fixation. The community was composed predominantly of Synechococcus spp., and large, heterocystous, nitrogen-fixing cyanobacteria (Aphanizomenon spp, and Nodularia spp.), that usually formed buoyant macroscopic aggregates. Although conspicuous, these aggregates often represented less than 20 to 30% of the total chlorophyll a. Nitrogenase activity was not Limited by molybdate availability, but, instead, by high concentrations of sulfate. This may explain inhibition of nitrogenase activity at high salinities. Inhibition of nitrogenase activity at high salinity did not occur when sulfate concentration was kept low. Nitrogen fixation and growth of the diazotrophic cyanobacteria were limited by iron. Synechococcus spp. was primarily nitrogen Limited but iron appeared to be the secondary limiting substrate, particularly when these organisms depended on nitrate as the source of nitrogen. Nutrient limitation of the picoplanktonic community was particularly apparent when a wind- induced mixing event occurred. These organisms responded by a subsequent doubling of their biomass within 24 h. Mixing of the water column apparently transported nutrients from greater depth into the euphotic zone, causing a temporary relieve of nitrogen limitation. [KEYWORDS: Baltic Sea; cyanobacteria; bloom; picoplankton; nutrients; iron; nitrogen; nitrogen fixation; molybdate; sulfate Nitrogen-fixation; planktonic cyanobacteria;marine-phytoplankton; iron limitation; gas vesicles; cell-size; ocean; eutrophication; synechococcus; communities]

Diurnal behaviour of nitrogenase activity in a benthic cyanobacterial mat community in the Wadden Sea (North Sea)

Versicoloured microbial mats (Farbstreifen-Sandwatt) in the intertidal zone of the North Sea island of Mellum were investigated with respect to oxygenic photosynthesis and nitrogen fixation. In laboratory cultures of the cyanobacterium Oscillatoria limosa a temporary separation of both incompatible processes occurs. Also in situ a negative correlation of sedimentary oxygen concentration and nitrogenase activity was found. If the sediment turned anaerobic during the night, no acetylene reduction was measurable. In systems, however, which stayed oxygenated during the night, nitrogenase activity was found at night. Peaks of nitrogenase activity were often detected at sunrise. Sometimes acetylene reduction was measured during the day when oxygenic photosynthesis was interrupted or depressed, due to light conditions (clouding effects). The diurnal pattern of nitrogenase activity as well as the acetylene reduction rate was influenced by the variation of all environmental conditions. The highest rate of nitrogenase activity (25 µmol C2H4/mg Chl a/h) detected in situ agreed well with maximal nitrogenase activity found in pure cultures of Oscillatoria limosa

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]

Oekosystemforschung Niedersaechsisches Wattenmeer. T. B: ELAWAT -Elastizitaet des Oekosystems Wattenmeer. Projektsynthese Abschlussbericht

The aim of the basic research part of the Ecosystem Research Lower Saxonian Wadden Sea was to detect fundamental ecological properties of this ecosystem. Therefore, the concept of ELAWAT (Resilience of the Wadden Sea Ecosystem) realized, for the first time in the history of ecosystem research, an approach based on stability properties. Processes and mechanisms relevant for a long-term preservation of the Wadden Sea were investigated. 12 projects were involved in this interdisciplinary approach. Field investigations and experiments were carried out in the back barrier tidal flat behind the island of Spiekeroog to detect temporal and spatial patterns and effects of disturbances. Most parameters revealed high variability. Therefore no reference situation can be named for the Wadden Sea. Instead, the 'state' of the Wadden Sea is its dynamic. Several abiotic components showed a resilience on intermediate scales, e.g. a return of basic morphological features of the tidal flats 6 months after a severe ice winter. Stability properties could also be named for biotic components of the Wadden Sea, being specific for the studied variables and scales. Mechanisms to react to disturbance events include a high functional morphology, high reproductive output and high mobility of the Wadden Sea biota. These stability mechanisms can only function in a natural system with an undisturbed tidal regime. To preserve this ecosystem, entire tidal basins shoule be protected to ensure a variety of habitats, source populations for recolonization and undisturbed dispersion. (orig.)SIGLEAvailable from TIB Hannover: F98B1440 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

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

Lay-up optimization of composite plates to delay mode-jump instabilities

All cyanobacterial mats that have been investigated have been proven to be diazotrophic, i.e., use atmospheric dinitrogen (N2) as the source of nitrogen. Many cyanobacteria possess the capacity to fix N2 and different species have evolved various ways to cope with the sensitivity of nitrogenase toward oxygen which is produced by these oxygenic phototrophs. These different strategies give rise to complex patterns of nitrogenase activity in microbial mats. Nitrogenase activity may exhibit complex variations over a day–night cycle but different types of microbial mats may also have their own characteristic patterns. Besides the cyanobacteria, numerous other members of the Bacteria as well as some Archaea are known to be diazotrophic. The complexity of the microbial community and of the observed patterns of nitrogenase activity makes it difficult to understand how the different groups of organisms contribute to N2 fixation in microbial mats. Cyanobacteria have ample access to energy (sunlight) and reducing equivalents (water) and therefore easily satisfy the demands of nitrogenase. As well, since they also fix CO2, they are able to synthesize the acceptor molecules for the fixed nitrogen. However, it is also feasible that other diazotrophs in a joint venture with cyanobacteria are responsible for the bulk of the fixed nitrogen. In this review we discuss the importance of cyanobacteria as diazotrophs in microbial mats, their interactions with other potential N2-fixing microorganisms, and the factors that control their activities.