Original paper

Current velocity and nutrient level effects on the morphology of Phormidium retzii (Cyanobacteria) in artificial stream mesocosms

Casamatta, Dale A.; Vis, Morgan L.


Environmentally induced morphological variability has long been recognized as problematic in the cyanobacteria. Phormidium retzii (C. A. Agardh) Gomont is a cosmopolitan cyanobacterium whose species description is one of the broadest in the Oscillatoriales, suggesting substantial phenotypic plasticity. In order to assess the degree of plasticity, a unialgal strain of P. cf. retzii was subjected to three current velocities (low, medium and high) and two nutrient levels (low and high) in six artificial stream mesocosms. P. cf. retzii filaments were allowed to colonize glass slides in each treatment. At the end of five weeks, percent cover of the slides and morphological measurements (cell length, width, volume and cell length: width ratio) were analyzed using analysis of variance (ANOVA) followed by Bonferroni multiple comparison tests. Cell length: width ratios were not significantly different across all treatments, while cell volumes were significantly affected by nutrient level (p < 0.001). low nutrient conditions elicited greater cell volumes and percent cover (p < 0.05) than did high, but current velocity was not significant. cell length and width were unaffected by current velocity, but both were significantly affected by nutrients, with low nutrient conditions showing significantly greater P. cf. retzii cell length (p < 0.05) and width (p < 0.01). Overall, our study showed no change in qualitative features, but a difference in cell size primarily due to nutrient levels. The morphological plasticity elicited by these environmental factors was much less than the range given in the species description.


algaecurrent velocitymorphologyphenotypic plasticityphormidium cf. retzii