Release of extracellular organic carbon during a diatom bloom in Lake Mossö: molecular weight fractionation

SUMMARY. Carbon fixation and release of extracellular organic carbon (EOC) during a diatom bloom were measured by the ¹⁴C—technique. Release in light was always a small fraction (1.5-6.0%) of total carbon fixation. Maximum values were found during the stationary phase of growth. A larger (15–50%) fraction of the inorganic carbon assimilated in the dark was released, although the absolute values were lower than in the light.
Gel permeation chromatography showed that low molecular weight products (<700 daltons) dominated EOC released in the light. A shift toward more complex molecules (> 10,000 daltons) was found in the stationary phase. EOC released in the dark had almost equal amounts of low (< 700 daltons), intermediate (˜5,000 daltons) and high (< 10,000 daltons) molecular weight products.
It is shown that with time the molecular weight composition of EOC shifted towards a predominance of larger molecules.