Phytoplankton possess cell-surface oxidases that oxidize amino acids and primary amines to produce extracellular H 2 0 2 , NH4+, and a-keto acids (from amino acids) or aldehydes (from primary amines). The NH,+ is subsequently taken up and used for growth. A survey of marine phytoplankton shows that at least 3 genera possess cell-surface L-amino acid oxidases: Pleurochrysis and Pryrnnesium (prymnesiophytes), and Arnphidinium (a dinoflagellate). These enzymes are quite similar, with halfsaturation concentrations of ca 0.1 to 0.5 PM, but differ in their inhibition by the nonspecific protease proteinase K, and a detergent, lithium dodecyl sulfate.
The concentration of cadmium varies like that of a nutrient in the open ocean. Detailed studies of the marine diatom Thalassiosira wejssflogii have shown that cadmium can act a s an algal nutrient under conditions of zinc limitation. We show here that cadmium can also enhance the growth of a vanety of species, includ~ng a chlorophyte and some prymnesiophytes, when they are zinc limited. The replacement of zinc by cadmium occurs at environmentally relevant inorganic cadmium and zlnc concentrations. Very low concentrations of inorganic cadmium that are beneficial under conditions of moderate zinc-limitation become toxic in cultures severely limited by zinc. The role of cadmium as an algal nutnent is thus observable in a narrow, species-specific range of inorganic zinc and c a d n~i u n~ concentrations.KEY WORDS: Cadmium/zlnc replacement Trace metal limitation . Trace metal toxicity -Micronutrients Phytoplankton
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