The presence of high microcystin concentrations in cyanobacterial bloo
ms additionally affects species diversity. Blooms with high toxin cont
ents can reach higher cell densities, which is also demonstrated by mi
crocystin cell contents. In vitro experiments show that microcystins i
nfluence phytoplankton proliferation. The action is strongly dependent
on the phytoplankten species tested and light conditions. We propose
that the environmental impact of different microcystins depends on the
ir enzymatic inhibition activity and thus could not be measured merely
on the basis of their toxicity to vertebrate species. Their role in h
eavy cyanobacterial bloom and scum formation is discussed, as well as
their impact on the massive proliferation of other species following t
oxic cyanobacterial bloom degradation.