Phormidium retzii is one of the most commonly encountered lotic taxa in North America and has been recorded from a wide range of habitats worldwide. The genetic variability of this cosmopolitan, freshwater cyanobacterium was assessed using gene sequences and random amplified polymorphic DNA (RAPD) markers. Strains consistent with the morphological species circumscription were utilized from British Columbia, Canada to Rio Claro, Costa Rica, and from Rhode Island, USA to Washington State, USA. In addition, some strains were from geographically close (<25 miles apart) sites. The 16S rRNA gene was sequenced for eight of the 12 strains. Sequence homology ranged from 90.95-98.71. The nine RAPD primers utilized yielded a total of133 distinct bands from the 12 strains. The strains were variously related and showed great variation in pairwise genetic distances (0.23-0.83). Given the lack of sequence similarity, P. retzii as presently circumscribed most likely represents several cryptic species not clearly distinguishable with light microscopy of morphological characters. This conclusion may explain the lack of correlation between geographic proximity and genetic similarity. To assess the level of environmentally induced phenotypic plasticity of this taxon, one strain was subjected to three different current velocity and nutrient regimes (low, medium, and high) employing six stream mesocosms. ANOVAs and Bonferroni multiple comparison tests were performed with cell volumes, ratios of cell length:width and percent cover data. Low nutrient treatments elicited significantly (p<0.05) greater cell volumes and percent cover than did high treatments. Furthermore, current velocity did not show any significant effect on cell morphology. As such, it appears that a single strain is capable of wide morphological variability in response to some environmental conditions. The results of these two studies have great implications in the utilization of cyanobacterial morphospecies concepts.

Key words: cyanobacteria, Oscillatoriales, Phormidium retzii, plasticity, RAPD