Community and pigment structure of Arctic cyanobacterial assemblages: the occurrence and distribution of UV-absorbing compounds

Three groups of cyanobacterial communities were widely distributed in the benthic environment of lakes, ponds and streams on Ellesmere Island and Cornwallis Island in the Canadian High Arctic: (1) sheets or spherical colonies of Nostoc (up to 20 mm diameter); (2) biofilms up to 7 mm thick, dominated almost exclusively by Oscillatoria; (3) microbial mats up to 8 mm thick containing several taxa, particularly Scytonema and Phormidium. The abundance of heterocystous genera (communities 1 and 3) implies that N2 fixation plays an important role in the nitrogen economy of these ecosystems. Most of the communities were rich in pigments absorbing in the UV-blue end of the spectrum, such as scytonemin and mycosporine-like amino acids. Spectroradiometric analyses of sections of the communities showed that short wavelength radiation did not reach the bottom layer where phycobiliprotein-rich cells were located. This lower community experienced low irradiance in the photosynthetically active radiation band (400–700 nm), restricted to the wavelengths of the yellow-red waveband (550–650 nm). The surface screening of high energy wavelengths may confer an adaptive advantage to these communities which grow under continuous light during the polar summer.