Effects of bacterial communities on the sensitivity of the phytoplankton Stephanodiscus minutulus and Desmodesmus armatus to tannic acid

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We investigated whether bacteria affect the sensitivity of Stephanodiscus minutulus (diatom) and Desmodesmus armatus (green alga) to tannic acid (TA). TA is a water soluble secondary plant metabolite which is released by submerged macrophytes and acts as an allelochemical because of its polyphenolic nature. Sensitivity of S. minutulus and D. armatus to TA changed in the presence of specific bacterial communities and bacterial isolates growing on TA. Without bacteria, growth of S. minutulus was inhibited by TA whereas the presence of bacteria pre-treated with high TA concentra- tions led to increased algal growth. Addition of sterile TA degradation products of bacterial isolates reduced inhibition of D. armatus. Differences in algal growth after TA addition were accompanied by changes in bacterial community composition (BCC) detected by DGGE and subsequent sequencing. This indicates that phytoplankton sensitivity to TA can be mediated by accompanying bacteria due to changes in BCC, which in turn are influenced by pre-treatment (different TA concentrations) and mode of addition of allelochemicals (pulsed or semi-continuous). Our study demonstrates that effects of allelochemicals on algal growth can be substantially changed by interference with micro- organisms, which has not previously received attention.