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Baltic sea cyanobacteria


The cruises of BASIC have yielded an extensive dataset of physicochemical and biological parameters. It was demonstrated that primary productivity in the Baltic Sea was limited by nitrogen, although the concentrations of phosphate were also low. Extensive microscopy and a sophisticated HPLC method were applied to provide information about the phytoplankton composition. The dominant nitrogen-fixing species were Aphanizomenon and Nodularia.
The species composition varied, but it was noted that in the more southern (and more saline) stations Nodularia was more common. Quantitatively more important were the picocyanobacteria (Synechococcus spp.) which were present in large numbers (up to 2 106 ml-1). The BASIC consortium has obtained a large number of cyanobacteria in culture. The red and green picocyanobacteria form a coherent group, within which a considerable degree of diversity is present. Genetic analysis of the picocyanobacteria of the Baltic Sea revealed a large variety of hitherto uncultivated species.

During the 1998 cruise a drift station was made. The variations of all relevant parameters of a cyanobacterial bloom were recorded over a full day and night cycle and over the depth. These experiments were repeated twice, so that 3 full day-night cycles were obtained. Throughout the experiment the phytoplankton bloom was nitrogen limited. This was concluded from the nutrient data, but also from the data on cellular physiology. It appeared that nitrogen fixation was high during the first experiment, while photosynthesis was relatively low. During the following days the opposite was the case. During the first experiment nitrogen fixation exceeded the demands, while this was later used by the elevated primary productivity. Comparison of nitrogen fixation measured by the acetylene reduction assay with the fixation of the stabile isotope 15N2 revealed that during the first diel the ratio amounted approximately 20, while it was close to the theoretical number of 4 during the second and third experiments. The high ratio was caused by the excretion of fixed N as NH3. Numerical analysis of data of nitrogen fixation, photosynthesis, biomass distribution, light and temperature, revealed that the nitrogen-fixing cyanobacteria fixed N in excess of 12% of their own demands. Fully independently, by using 15N2, it was demonstrated that about 10% of the fixed N was transferred to the picocyanobacteria. The nitrogen-fixing bloom covered 50% of the total phytoplankton nitrogen demands. Although picocyanobacteria may represent 70-80% of the total biomass, they contributed only 50-60% to primary production. Aphanizomenon and Nodularia possess gas vesicles that provide them with buoyancy. The numerical analyses have shown that these organisms may benefit greatly from this.

It was demonstrated that after a mixing event Aphanizomenon increased its photosynthetic rate 3-fold by floating up. It was furthermore shown that the daily integrated light is the essential trigger for the development of cyanobacterial blooms in the Baltic Sea and not temperature. It was demonstrated that the picocyanobacteria were predominantly limited by nitrogen. The diazotrophic cyanobacteria are naturally not limited by nitrogen. Surprisingly, it was found that these organisms were limited by iron. Also the picocyanobacteria seemed to be limited by iron, preventing them from using nitrate. During the 1999 cruise no nutrient limitation was detected but the relative low insolation limited primary production. BASIC offers an explanation for the limited distribution of nitrogen-fixing, heterocystous cyanobacteria in marine environments. Rather than high salinity it seems that the high level of sulfate poses a problem for these organisms. Some cyanobacteria produce toxins. Considerable amounts of the hepatoxin nodularin were detected. This toxin occurred only in species of Nodularia. Aphanizomenon and the picocyanobacteria appeared not to be toxic. Nodularin was toxic for various species of zooplankton.

The population genetics of Nodularia was studied by analyzing in total 15,000 single filaments of Nodularia, sampled from various geographic locations of the Baltic Sea, which has produced an enormous database. Two important conclusions were drawn from these studies. The first is that the distinguishing of different species of Nodularia on the basis of morphological characteristics is not justified. The second major conclusion is that between the Nodularia active genetic exchange occurs.
The Baltic Sea is one of the largest bodies of brackish water in the world. It is surrounded by 9 different countries and the only connection to the North Sea is via the narrow Öresund. The exchange of Baltic Sea waters is therefore limited while the level of wet deposition from the surrounding land masses and river runoff's are considerable and hence, so is eutrophication. Eutrophication in the Baltic Sea has led to mass occurrence ("blooms") of certain plankton, a well studied phenomenon that can be predicted on the basis of nutrient dynamics. However, a notable exception is the regular mass-occurrence of blooms of cyanobacteria (blue-green algae). Although blooms of cyanobacteria are well-known from freshwater lakes, they are almost lacking in the marine environment, with the exception of the Baltic Sea and tropical oceans. The cyanobacterial blooms in the Baltic Sea are commonly diazotrophic, i.e. the organisms fix atmospheric nitrogen. Indeed, preliminary studies suggest that such blooms may account for 15-25% of the nitrogen input into the Baltic Sea. These diazotrophic cyanobacteria possess gas vesicles which enable them to float to the surface where they may accumulate as dense scums. Cyanobacterial blooms typically occur in the summer but it has proved difficult to predict when and where they will develop as the blooms have an extremely patchy distribution. Also an important environmental concern is the toxicity of these blooms.
Although less obvious than surface accumulations of the larger cyanobacteria, the biomass of picoplanktonic cyanobacteria is even greater but far less studied. The proposal BASIC fits into Area 1.2 Biospheric Processes (1.2.2) and particularly to the parts 'The functioning of ecosystems' and Biodiversity and environmental change' of the ENVIRONMENT and CLIMATE prograrnme and it will contribute to the ELOISE (European Land-Ocean Interaction Studies) science plan. The objectives of BASIC are derived from the results of the project 'Development and Fate of Cyanobacterial Blooms in the Baltic' (EV5V-CT94-0404). The overall objective of the BASIC project proposed here, is to achieve a better understanding of the initiation and the temporal and spatial distribution of cyanobacterial water blooms in the Baltic Sea. More specifically it will aim at (i) developing an integrated model for photosynthesis, nitrogen fixation and toxin production, (ii) assessing the limiting factors for growth, photosynthesis and nitrogen fixation, (iii) identifying cyanobacterial toxins and toxin producers and to determine their ecological role, (iv) investigating the transfer of nitrogen from diazotrophic to non-diazotrophic cyanobacteria and last, at (v) examining the genetic structure of the cyanobacterial population in relation to the physiological capacities of the system..

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