Extracellular products of phytoplankton photosynthesis

Abstract

Following exposure in situ for periods of 3 to 24 h of samples of lake or sea waters to which $^{14}$C-bicarbonate had been added, radioactivity was found in dissolved organic matter in the water as well as in the cells of phytoplankton. The amount in the water was between 7 and 50% of the total carbon fixed in the photic zone of the water column. This production of extracellular $^{14}$C-labelled organic matter was found under a wide variety of conditions and with many different types of phytoplankton community. It thus seems likely that the widely used method, in which fixation of $^{14}$C in particulate matter only is determined, under-estimates total primary productivity. The labelled organic substances in the water are probably liberated by intact photosynthesizing cells rather than by breakage of cells during filtration. Glycollic acid is likely to be one of the principal substances concerned. Over a wide range of light intensities liberation of extracellular products by a given phytoplankton population was proportional to the amount of carbon fixed in the cells, except that it tended to be relatively greater at low light intensities (< 1 kilolux) and at light intensities high enough to inhibit photosynthesis (> 50 kilolux), when as much as 95% of the total organic $^{14}$C might be extracellular. Population density, period of exposure to $^{14}$C-bicarbonate and species differences also affect the extent of excretion. The implications of extensive liberation by phytoplankton of extracellular products of photosynthesis for our understanding of the trophic relationships in aquatic habitats seem to be considerable.