Toxic and non toxic microcystis colonies in natural populations can be differentiated on the basis of rRNA gene internal transcribed spacer diversity

Assessing and predicting blooms dynamics and toxin production by Microcystis requires analysis of toxic and non-toxic Microcystis strains in cultures and in natural populations. We show that genetic differentiation of Microcystis colonies based on rRNA-ITS sequences correlates with microcystin production. Hence, ecological studies of toxic and non-toxic cyanobacteria are now feasible through studies of rRNA-ITS genotypic diversity in isolated cultures or colonies, and in natural communities. Microcystis colonies (107) isolated from 13 European lakes, of which toxicity had been determined by MALDI-TOF, were grouped using rRNA-ITS DGGE. Based on DGGE analysis of amplified ITSa and ITSc fragments, yielding supplementary resolution (Janse et al., 2003), 59 classes could be distinguished. Microcystin-producing and non-producing colonies were separated into different classes. Sequences obtained from at least one representative strain from most classes were congruent with the classification based on ITS DGGE. Alignment of these colony sequences and published rRNA-ITS sequences from cultured Microcystis strains of known toxicity confirmed that microcystin production correlated with rRNA-ITS sequences. About 30% of the colonies that were analyzed gave rise to more than one bands in DGGE profiles, indicating that many colonies consisted of aggregated strains. Colonies sharing identical rRNA-ITS sequences were often assigned to different morphospecies, and colonies of one morphospecies often contained different rRNA-ITS sequences, confirming inconsistencies reported for morphological identification. Sequence diversity was not significantly correlated with geographical origin since identical sequences could originate from geographically distant lakes. DGGE community profiles from two Dutch lakes from which colonies had been isolated showed abundance of different strains at subsequent bloom stages. Although not all bands in the community profiles could be matched with isolated colonies, the profiles suggest that non-toxic colonies dominate later in the season and in scums.