THE EFFECT OF NUTRIENT LIMITATION AND ITS INTERACTION WITH LIGHT UPON THE PRODUCTS OF PHOTOSYNTHESIS IN MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

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Abstract
The metabolic fate of photosynthetically‐fixed CO2 was determined by labeling samples of Merismopedia tenuissima Lemmerman for 30 min with NaH14CO3 and analyzing its incorporation into low molecular weight compounds, polysaccharide and protein. In N‐ and P‐sufficient cultures, relative incorporation into protein increased as the irradiance used during the labeling period was decreased to 20 μE · m‐2 s‐1. This pattern was found for cells grown at irradiances of either 20 or 180 μE · m‐2· s‐1, although incorporation into protein was greater in cultures grown at the higher irradiance. In N‐limited continuous cultures, relative incorporation into protein was low, independent of growth rate, and the same for samples tested at 20 or 180 μE · m‐2· s‐1 irradiance. In contrast, 14C incorporation into protein by P‐limited cultures increased as growth rate increased, and at relative growth rates greater than 0.25, the incorporation was greater at 20 than at 180 μE · m‐2· s‐1. However, the total RNA content and maximum photosynthetic rate of the cultures was the same at all growth rates tested. The interaction between nutrient concentration and light intensity was studied by growing‐limited continuous cultures at the same dilution rate, but different irradiances. Relative incorporation into protein was highest in cultures grown at 20 μE · m‐2· s‐1, in which the relative growth rate was 0.4. These results suggest that photosynthetic carbon metabolism may respond to relative growth rate μ/μmax rather than to growth rate directly.