CHLOROPHYLL AND CAROTENOID-PIGMENTS IN SOLAR SALTERN MICROBIAL MATS

The distributions of carotenoids, chlorophylls, and their degradation products have been studied in two microbial mat systems developed in t he calcite and calcite/gypsum evaporite domains of a solar saltern sys tem. Phormidium valderianum and Microcoleus chthonoplastes are the dom inant cyanobacterial species, respectively, and large amounts of Chlor oflexus-like bacteria occur in the carbonate/gypsum mat. In both syste ms, the major pigments are chlorophyll a, zeaxanthin, beta-carotene, a nd myxoxanthophyll, which originate from these mat-building cyanobacte ria. This common feature contrasts with differences in other pigments that are specific for each mat community. Thus, chlorophyll c and fuco xanthin, reflecting diatom inputs, are only found in the calcite mat, whereas the calcite/gypsum mat contains high concentrations of bacteri ochlorophylls c produced by the multicellular green filamentous bacter ia. In both cases, the depth concentration profiles (0-30 and 0-40 mm) show a relatively good preservation of the cyanobacterial carotenoids , zeaxanthin, beta-carotene, myxoxanthophyll, and echinenone. This con trasts with the extensive biodegradation of cyanobacterial remains obs erved microscopically. Fucoxanthin in the calcite mat is also transfor med at a faster rate than the cyanobacterial carotenoids. Chlorophyll a, the major pigment in both mats, exhibits different transformation p athways. In the calcite/gypsum mat, it is transformed via C-13(2) carb omethoxy defunctionalization prior to loss of the phytyl chain, leadin g to the formation of pyrophaeophytin a and, subsequently, pyrophaeoph orbide a. On the other hand, the occurrence of the enzyme chlorophylla se, attributed to diatoms in the calcite mat, gives rise to extensive phytyl hydrolysis, with the formation of chlorophyllide a, pyrophaeoph orbide a, and, in minor proportion, phaeophorbide a. Studies of the so urces of the photosynthetic pigments and of their transformation pathw ays in such simplified ecosystems provide a basis for the understandin g of the distribution patterns of these compounds in more complex aqua tic environments.