Effects of temperature, irradiance, and daylength on the marine diatom Leptocylindrus danicus Cleve. II. Excretion

Rates of excretion of labelled dissolved organic carbon (DOC) by the marine diatom Leptocylindrus danicus Cleve were measured in axenic batch culture under 49 combinations of temperature (5, 10, 15, 20°C), daylength (15:9, 12:12, 9:15 LD), and irradiance (at least four irradiances per daylength). Carbon-specific excretion rates were a linear function of irradiance independent of temperature. DOC release by cells growing under 15:9 and 12:12 LD photoperiods were similar while excretion was significantly depressed under 9:15 LD. Excretion rates were positively correlated with 14C uptake rates and negatively correlated with chlorophyll a content. It is suggested that excretion may be closely tied to the photosynthetic capabilities of the cell. DOC losses were 1 to 3%, 2 to 11%. and 3 to 20% of total 14C incorporation at 15 to 20°C, 10°C, and 5°C, respectively.