ABIOTIC STRESS MODIFIES THE SYNTHESIS OF ALPHA-TOCOPHEROL AND BETA-CAROTENE IN PHYTOPLANKTON SPECIES

We performed laboratory experiments to investi-gate whether the synthesis of the antioxidants -tocopherol (vitamin E) and -carotene in phytoplankton depends on changes in abiotic factors. Cultures of Nodularia spumigena, Phaeodactylum tricornutum, Skeletonema costatum, Dunaliella tertiolecta, Prorocentrum cordatum, and Rhodomonas salina were incubated at different tempe-ratures, photon flux densities and salinities for 48h. We found that abiotic stress, within natural ecological ranges, affects the synthesis of the two antioxidants in different ways in different species. In most cases antioxidant production was stimulated by increased abiotic stress. In P.tricornutumKAC 37 and D.tertiolectaSCCAP K-0591, both good producers of this compound, -tocopherol accumulation was negatively affected by environmentally induced higher photosystem II efficiency (F-v/F-m). On the other hand, -carotene accumulation was positively affected by higher F-v/F-m in N.spumigena KAC 7, P.tricornutum KAC 37, D.tertiolecta SCCAP K-0591 and R.salina SCCAP K-0294. These different patterns in the synthesis of the two compounds may be explained by their different locations and functions in the cell. While -tocopherol is heavily involved in the protection of prevention of lipid peroxidation in membranes, -carotene performs immediate photo-oxidative protection in the antennae complex of photosystem II. Overall, our results suggest a high variability in the antioxidant pool of natural aquatic ecosystems, which can be subject to short-term temperature, photon flux density and salinity fluctuations. The antioxidant levels in natural phytoplankton communities depend on species composition, the physiological condition of the species, and their respective strategies to deal with reactive oxygen species. Since -tocopherol and -carotene, as well as many other nonenzymatic antioxidants, are exclusively produced by photo-synthetic organisms, and are required by higher trophic levels through dietary intake, regime shifts in the phytoplankton as a result of large-scale environmental changes, such as climate change, may have serious consequences for aquatic food webs.