Different sensitivity of selected planktonic eukaryotic and prokaryotic phototrophs to dissolved humic substances: potential modes of action and ecological implications

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In freshwaters ecosystems, humic substances (HS) comprise the majority of organic carbon and are taken up in parts by many, but obviously not all freshwater phototrophs. Internalized HS lead to oxidative stress and/or interact directly within the photosynthetic electron chain. In this regard, prokaryotes appear to be more sensitive than eukaryotes. To elucidate potential underlying modes of action, uptake/depuration of HS, growth rates, and photosynthetic performance of selected coccal green algae and cyanobacteria shall be studied upon exposure to HS. The general ability to uptake and depurate molecules of 0.5 kDa will be checked with the rhodamine B assay. To test the assumption that quinoid building blocks in the HS are the effective structures for the reduction of photosynthetic oxygen release, one differently quinoid- or phenol-enriched HS with enhanced redox properties will be prepared and applied. As oxidative stress parameters, H2O2, TOSC, activity of antioxidant enzymes and substrate contents as well as the expression of certain stress genes will be determined. Growth rates and photosynthetic activity will be related to the oxidative stress status. The potential acquisition of a multiple stress stress tolerance will be tested by solt-tolerance of phototrophs pre-exposed to HS. To assess the mode of action of internalized HS with the photosynthetic electron transport, different fluorescence and optical techniques will be applied to identify the putative targets.