Many alpine streams inhabit conspicuous epilithic biofilms on pebbles and rocks that are formed by members of the cyanobacterial genus Chamaesiphon (Synechococcales). In the Austrian Alps, some Chamaesiphon species can even overgrow up to 70% of the surface of river rocks, and hence they must play an important but still unstudied ecological role in the organic matter flux. Since photo-biological traits have not been investigated so far, photosynthetic features, pigments, and UV-sunscreen compounds were studied in three Chamaesiphon morphospecies (C. geitleri, C. polonicus, C. starmachii). These species form conspicuously differently colored spots on cobbles and boulders in the alpine streams. While C. polonicus typically forms red crusts on flat pebble conglomerate, C. geitleri and C. starmachii are characterized by dark brown and black biofilms in the field, respectively. Photosynthesis-irradiance (PE) curves indicate that all three Chamaesiphon species have different light requirements for photosynthesis, with C. starmachii and C. polonicus preferring high and low photon fluence rates, respectively, while C. geitleri takes a position in between. This low-light requirement of C. polonicus is also reflected in ca. ten-times lower chlorophyll a, zeaxanthin, and ß-carotene concentrations, as well as in a lack of the UV-sunscreen scytonemin. All Chamaesiphon morphospecies exhibit the mycosporine-like amino acid porphyra-334. The physiological and biochemical data indicate strong intraspecific differences in photosynthetic activity and pigment patterns, which explain well the distinct preferences of the three studied Chamaesiphon morphospecies for sun-exposed or shaded habitats.
Keywords: Alpine stream; Light requirements; Photosynthesis; Pigments; Porphyra-334; Scytonemin.