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AHICA — Result In Brief

Project ID: 219976
Funded under: FP7-PEOPLE

The role of blue-green algae in the Baltic ecosystem

Freshwater biologists and limnologists funded by the EU studied large colonies of cyanobacteria to determine their role in inputting carbon and nitrogen from the atmosphere into the aquatic environment.
The role of blue-green algae in the Baltic ecosystem
The 'Autotrophic-heterotrophic interactions in cyanobacterial aggregates' (AHICA) project investigated carbon and nitrogen fluxes in collections of large nitrogen-fixing cyanobacteria in the Baltic Sea. Cyanobacteria (commonly called blue-green algae) are photosynthesising bacteria that produce oxygen as a by-product.

Researchers combined micro-sensor techniques with nano-scale secondary ion mass spectrometry (nanoSIMs) to measure carbon and nitrogen fixation at the single-cell level. Scientists also employed nanoSIMs together with stable isotope tracers and micro-sensors and measured fluorescence.

The aim was to investigate photosynthesis, carbon assimilation, respiration, nitrogen gas (N2) fixation and the release of ammonium in two dominant colony-forming cyanobacteria in the Baltic Sea. The two species selected for study were Aphanisamenon sp. and Nodularia spumigena.

Aphanisamenon sp. was found to dominate N2 fixation in the Baltic Sea as a result of its high cell-specific carbon- and nitrogen-fixation rates together with its high abundance and long growing season. It was revealed that free oxygen production by colonies of Aphanisamenon sp. is amongst the highest measured in aquatic systems. This plays a major role in biogeochemical fluxes in the Baltic through N2 fixation and ammonium release.

The large cell and colony size of Nodularia spumigena enable it to make a significant contribution to the total carbon and nitrogen fluxes in the Baltic, even at low levels. High levels of ammonium in both Aphanisamenon sp. as well as Nodularia spumigena were released during N2 fixation. The colonies are therefore microenvironments enriched in ammonium that are useful to other microorganisms when the bulk inorganic nitrogen concentrations are low in the Baltic Sea.

Results indicate that nitrogen-fixing cyanobacteria play an important role in supporting phytoplankton and bacteria at the base of the food web. In the Baltic Sea, the growing season of filamentous cyanobacteria is two to three months each summer; therefore, these organisms have a significant impact on the Baltic Sea ecosystem.

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