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Autotrophic-Heterotrophic Interactions in Cyanobacterial Aggregates

Final Report Summary - AHICA (Autotrophic-Heterotrophic Interactions in Cyanobacterial Aggregates)

The aim of the project was to analyze and quantify autotrophic and heterotrophic processes in 0.1 to 5 mm large cynobacterial colonies and aggregates of the Baltic Sea. During our studies, we combined microsensor technique with nanoscale secondary ion mass spectrometry (nanoSIMS). Using microsensors, the O2 fluxes and difussivities within cyanobacterial colonies can be studied at a micrometer scale. The nanoSIMS technique is a novel technique, which reveals elemental and isotopic surface composition after measurement of isotopic composition at a spatial resolution of 50 nm within the organisms, e.g. a single bacterium. Combination of this technique with stable isotope tracer incubations has enabled measurements of N2 and C fixation on a single-cell level in mixed field-populations of phytoplankton and bacteria. Hence, using this technique we can now ascribe N2 and C fixation to individual cells of different (micro-) organisms in the Baltic Sea. Using stable isotope tracers combined with nanoSIMS, fluorometry, and microsensors, we have studied gross and net photosynthesis, carbon net assimilation and respiration as well as N2 fixation and ammonium release in two dominating colony-formning cyanobacteria of the Baltic Sea, Aphanisamenon sp.and Nodularia spumigena.

Carbon and nitrogen budgets of Aphanisamenon sp. established from our measurement have shown that this cynobacterium presumably dominate N2 fixation in the Baltic Sea, due to its high cell-specific C- and N-fixation rates, its high abundance and long growth season. It is shown that volume-specific productivities of colonies of this cyanobacterium belong to the highest measured in aquatic systems and that this cyanobacterium is a key player for biogeochemical nitrogen fluxes, through N2 fixation and ammonium release, in the Baltic Sea where N2 fixation rates are in the order of 300 ktons N year-1. However, even at low cellular abundance Nodularia spumigena can contribute significantly to total C- and N- fluxes in the Baltic Sea due to its large cell and colony size and high N2-fixation rates during light as well as during darkness. A fast transfer of C- and N- fixed by Aphanisamenon sp. as well as by Nodularia spumigena to their associated bacteria and eukaryotic organisms was demonstrated. A high release of ammonium (circa 33 % of gross N2 fixation) in both Aphanisamenon sp. as well as Nodularia spumigena occurred during N2 fixation, and colonies are microenviroments enriched in ammonium to be used by other (micro-) organisms when the bulk inorganic nitrogen concentrations are low in the Baltic Sea.

These findings have important consequences for ecosystems with high abundance of large filamentous cyanobacteria like Aphanisamenon sp.and Nodularia spumigena as these nitrogen fixers also support other phytoplankton and bacteria at the base of the food web with nitrogen. In the Baltic Sea, the growth season of filamentous cyanobacteria is 2 - 3 months each summer. Hence, these cyanobacteria have substantial impact of the Baltic Sea ecosystem. The project has been followed up by new funds from Stockholm University (Baltic Adaptive Ecosystem Management (BEAM)) and by the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning to continue this novel and detailed research of C- and N biogechmical fluxes in the Baltic Sea.