Carotenoid composition of terrestrial Cyanobacteria: response to natural light conditions in open rock habitats in Venezuela

Carotenoid contents of terrestrial cyanobacteria sampled from rock surfaces with different exposure to sunlight were studied in the Guyana-shield regi on of Venezuela. At the collecting locations of two table mountains Roraima -tepui (2700 m). Auyan-tepui (2000 m) and an inselberg near Puerto Ayacucho (80 m), the most frequent species were Stigonema ocellatum, Stigonema horm oides and Scytonema crassum, respectively. Usually, these species were asso ciated with the unicellular cyanobacterium Gloeocapsa sanguinea. Their caro tenoid and chlorophyll a contents were determined by high-performance liqui d chromatography. In general, carotenoids consisted almost equally of myxol glycosides and of beta -carotene derivatives, such as beta -carotene, echin enone and canthaxanthin, On a chlorophyll a basis, myxolglycoside content i ncreased in full exposure to solar irradiation. The relative proportions of the different beta -carotene derivatives appeared to be even more strongly influenced by irradiance, Whereas their total amount was increased at full exposure, the content of beta -carotene decreased and that of canthaxanthi n increased. Maximal ratios of canthaxanthin/beta -carotene of around 5 wer e observed in the most sun-exposed samples. We conclude that acclimation of the carotenoid content to irradiance, especially the adjustment or the rat io canthaxanthin/beta -carotene, is a response found in many different terr estrial cyanobacterial species, However. when a natural population was arti ficially shaded for 6 days, short-term acclimation in pigment content was n ot detected, The results support a photoprotective function of canthaxanthi n in terrestrial cyanobacteria under natural environmental conditions.