Abstract
Summary Two populations of the cyanobacterium Calothrix sp. found in Yellowstone thermal spring outflows differ greatly in their contents of scytonemin, a UV‐screening pigment, and in their photosynthetic carbon assimilation rates. Clonal isolates from both populations were used to investigate these phenotypic differences. Identical partial 16S rDNA sequences (∼900 bp) suggest a very close relationship between the two Calothrix populations and indicate that environmental differences may, in part, explain the field observations. The effects of native spring water on scytonemin synthesis and photosynthesis were tested during experiments using plated cells. Results show differences in the spring water environment were at least partly responsible for the differences in scytonemin content observed in the field. Furthermore, spring water effects on photosynthetic performance suggest adaptation in these strains to their spring of origin. Controlled experiments performed using cultures grown in artificial liquid medium showed no significant difference in photosynthetic carbon uptake between strains. However, significant differences were detected in their ability to synthesize scytonemin indicating genetic differences between populations. These findings suggest that both genetic and environmental differences are responsible for the naturally occurring variation in scytonemin content and photosynthetic ability in these two closely related populations.