CARBON FIXATION IN CULTURED MARINE BENTHIC DIATOMS.

The enzyme activity of ribulose 1.5-bisphosphate carboxylase-oxygenase (RuBisCO) and phosphoenolpyruvate carboxylase (PEPC) was measured in four speciesof marine benthic diatoms isolated from subtidal sediments of Graveline Bayou, Mississippi. Enzyme activities were measured in cultures of Amphora micrometra Giffen, A. tenerrima Aleem and Hustedi, Nitzschia fontifuga Cholnoky, and Nitzschia vermicularis Grunow that were grown at light levels supporting μ[SUBmax] and at light-timing irradiances. All four species exhibited similar RuBisCO: PEP ratios (range = 1-1.8) at μ[SUBmax] the lowest ratio (0.4) was observed in A. micrometra. Reduced light levels increased PEPC relative to that measured at μ[SUBmax] in two species. Two-dimensional paper chromatography was used to determine the first products of carbon fixation in A. micrometra. After a 15 s incorporation period, the first product of photosynthetic carbon fixation was 3-phosphoglycerate even though this alga had a PEPC activity that was three times higher than that of RuBisCO. After 30 s, over 50% of the recovered radioactivity was still in this compound. Stable carbon isotope analyses of a mixture of the four pinnate diatoms also suggest the predominant carbon fixation pathway in these benthic diatoms was similar to C3 plants. [ABSTRACT FROM AUTHOR]