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Exponential phase cultures of the marine diatom Chaetoceros simplex (clone Bbsm) were cultured at 20°C and 636 PE m-2 S-' to determine the rates at which photosynthesis and partitioning of carbon were altered in response to short-term fluctuations In light intensity. Carbon-specific uptake rates equalled cell specific growth rates for a minimum of 3 h at hlgh light intensity (636 FE m-2 S-'). Low molecular weight (LMW) compounds were linearly labelled for up to 4 h under constant high light and accounted for nearly 70 % of total I4C fixed. Chloroform-soluble (lipid) and TCA-soluble (polysaccharide) fractions were uniformly labelled for only 2 h, accounting for 9 to 15 % of total l4C fixed. Under low light intensity (30 pE m-2 S-'), the proportion of I4C decreased in the LMW, chloroform-soluble and TCA-soluble fractions, but increased in particulate materials relative to that occurring at high light. There was an increase in the carbon fixation rates of LMW, chloroform-soluble, and particulate fractions when these cells were re-exposed to high light; carbon partitioning into the TCA-soluble fraction was initially suppressed, but recovered within 30 to 60 min. After 2 h of alternating 30 min exposures to high and low light, cells lost the ability to maintain a carbon fixation rate at 636 pE m-2 S-' equivalent to that found in control cultures. Carbon fixation into LMW, chloroform-soluble, and TCAsoluble fractions under fluctuating irradiance occurred only at high light, while protein labeling was unaffected. Based on these metabolic patterns, it appears that short-term (15 to 30 min) light fluctuations in situ may have a major influence on the pattern of carbon partitioning in marine phytoplankton.

Photosynthate partitioning in cultured marine phytoplankton: metabolic patterns in a marine diatom under constant and variable light intensities