Brown algae accumulate significant amounts of unique phenolic compounds, phlorotannins. These metabolites,
represented by a complex mixture of phloroglucinol (1,3,5-trihydroxybenzene) oligomers and polymers, have multiple
physiological functions in algal thalli. The most extensively studied function of phlorotannins is their contribution to the
chemical protection of macroalgae against diverse deleterious microorganisms, biofoulers and grazers. It is known that
different brown algal species produce specific phlorotannin molecules, which have unequal biological activity. In the
present study, we are considering effects of different phlorotannin extracts on a set of model unicellular microorganisms.
Phlorotannin extracts were isolated from seven brown algal species (Desmarestia aculeata, Pylaiella littoralis,
Chordaria flagelliformis, Chorda filum, Fucus vesiculosus, Pelvetia canaliculata, and F. serratus). The molecular
composition of the extracts was determined using HPLC-MS analysis. Microalgae Chlamydomonas reinhardtii P.A.
Dangeard (strain CC-124), Chlorella vulgaris Beijer. (strain BIN), Euglena gracilis Klebs (strain Z), gram-negative
bacteria Escherichia coli (strain КА769) and yeast Saccharomyces cerevisiae (strain LAN 201-ura3Δ) were chosen as
model objects for testing the phlorotannin toxicity.