CHANGES IN THE PATTERNS OF BIOSYNTHESIS AND COMPOSITION OF AMINO-ACIDS IN A MARINE PHYTOPLANKTER EXPOSED TO ULTRAVIOLET-B RADIATION - NITROGEN LIMITATION IMPLICATED

The impact of ultraviolet-B radiation (UVBR) on the biosynthesis and t he composition of intracellular dissolved free and combined amino acid s was examined in nitrate-replete and nitrate-deficient cultures of Te traselmis sp. Several similarities were observed in the response of Te traselmis sp. to nitrogen deficiency and exposure to UVBR, in support of a view that UVBR affects amino acid synthesis in phytoplankton via its inhibitory effects on nitrogen assimilation into the cells. At lev els that still permitted the uptake of carbon into the cells, both nit rogen deficiency and UVBR exposure resulted in a reduction in the over all rates of carbon incorporated into amino acids, an increase in the absolute concentrations of amino acids within the intracellular dissol ved free amino acid (INDFAA) pool and a decrease in the total cellular amino acid (TCAA) pool. An examination of the patterns of carbon assi milation into individual amino acids in cells exposed to UVBR revealed similarities with the patterns in cells subject to nitrogen deficienc y. The most conspicuous changes from the controls included an increase d incorporation of C-13 into glutamic' acid (glutamic acid + glutamine ) and aspartic acid and a marked reduction into alanine and valine. Ch anges in the concentrations of amino acids within the INDFAA and TCAA pools were also similar in nitrate-deficient and UVBR-exposed cells an d resembled the carbon assimilation patterns, These results strongly s uggest that UVBR-induced changes in the biosynthesis and composition o f amino acids are probably via its suppression of nitrogen assimilatio n into the cells.