PHENOTYPE VARIABILITY OF IDENTICAL GENOTYPES - THE NEED FOR A COMBINED APPROACH IN CYANOBACTERIAL TAXONOMY DEMONSTRATED ON MERISMOPEDIA-LIKE ISOLATES

Five Merismopedia-like cyanobacterial strains were collected from micr obial mats at Norderney Island, subcultured in the laboratory, and fin ally grown as unicyanobacterial cultures. As a sixth strain, Merismope dia glauca from the ''Sammlung von Algenkulturen'' at Gottingen (SAG) was used for comparisons. According to morphological and physiological characteristics initially observed in the field and during initial su bculturing, the five strains were assigned to the species Merismopedia glauca, Merismopedia punctata, or Merismopedia elegans. However, afte r prolonged maintenance under laboratory conditions, the formation of platelet-like colonies stopped, whereas cell sizes, production of extr acellular polymeric substances, and division patterns were stably main tained. These physiological and morphological parameters allowed us to divide the six strains into two clusters. This division was further s upported by the profiling of total cell protein and phycobilisomes usi ng SDS-PAGE. The nearly complete 16S rDNA sequence of three of the six isolates was determined. The comparative sequencing analysis revealed an almost 100% identity of these three Merismopedia-like strains. The evolutionary distance dendrogram constructed placed this Merismopedia cluster into a common line of descent with Synechocystis sp. strain P CC6906. Based on the analysis of common stretches of 1,050 nucleotides , the overall similarity between the sequence types of ''Merismopedia' ' and ''Synechocystis'' is 96-97%. The values of the different methods for taxonomic classification of unicyanobacterial strains, the relati onship of the cyanobacterial genera Merismopedia, Synechococcus, Synec hocystis, and Eucapsis sp., and the functional role of different Meris mopedia morphologies within microbial mats are discussed. It is sugges ted that all analyzed Merismopedia strains be combined into one specie s, namely Merismopedia punctata Meyen (1839).