THE PATH OF CARBON IN PHOTOSYNTHESIS BY MARINE PHYTOPLANKTON

Using cultures of a number of different marine algae (diatoms Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin, chrysophyte Isochrysis galbana Parke, green flagellate Dunaliella tertiolecta Butcher, dinoflagellate Gonyaulax tamarensis Lebour) the short‐term, pattern of ¹⁴CO₂ assimilation has been investigated. In all except D. tertiolecta the labelling of amino acids and intermediates of the tricarboxylic acid (Krebs) cycle was significantly heavier than that of sugar phosphates. Over periods of 30–120 s labelling in amino acids and Krebs cycle intermediates accounted for 41–95% of the ¹⁴C fixed (depending on the alga). Over shorter times (< 10 s) the pattern in the 2 diatoms showed significant labelling of C₄ acids (and related com‐pounds) and little labelling of sugar phosphates. The reverse wits seen with D. tertiolecta. Also, in the 2 diatoms and in G. tamarensis significant inhibition of photosynthesis by oxygen could only be achieved with 100% oxygen; atmospheric levels having little effect. Parallel measurements of 2 carboxylating enzymes showed that ribulose‐1,5‐diphosphate carboxylase (RuDPCase) was significantly greater than phospho (enol)pyruvate carboxylase (PEPCase) activity only in the green flagellate. It is suggested that photosynthesis in marine diatoms depends on an active PEPCase utilizing bicarbonate as a substrate and that a less active RuDPCase utilizes CO₂. In D. tertiolecta the pattern more closely resembles that of a “Calvin (C₃)” plant. The dinoflagellate and the chrysophyte appeared to show a mixed C₃ and C₄ photosynthesis.