Release of dissolved organic material from the cells of phytoplankton populations

Abstract

A series of in situ productivity experiments were carried out on four freshwater reservoirs. Carbon fixation into particulate matter was estimated from the $^{14}$C content of phytoplankton collected on membrane filters. Inorganic $^{14}$C was removed from the filtrate by air bubbling at pH 4 and aliquots of the resulting solution of extracellular organic $^{14}$C were dried on metal disks for radioactivity estimation. Extracellular release expressed as a percentage of the total carbon fixation increases with depth towards the bottom of the photic zone and in bright sunlight there may also be an increase near the surface. The surface increase in percentage extracellular release (p.e.r.) is associated with the inhibition of photosynthesis by high light intensities. The increase in p.e.r. with depth is associated with dark fixation of inorganic carbon. In the dark p.e.r. is always higher than in uninhibiting light. The inhibition of dark fixation by previous exposure to bright sunlight indicates that 90% or more of the fixation in dark bottles is by algae. Laboratory experiments with phytoplankton and with axenic cultures of Chlorella pyrenoidosa confirm these relationships. For the light bottles there is a highly significant correlation between p.e.r. at the depth of maximum photosynthesis and for the photic zone as a whole. From this relationship it is possible to estimate extracellular release per unit area from a determination at one depth only. When phytoplankton populations are diluted with filtered water p.e.r. increases with dilution, the increase being most marked when the population density is reduced by more than 90%. A similar relationship holds over a range of population densities in the in situ experiments. The population density of phytoplankton was estimated by pigment analysis-chlorophylls a, b and c, and plant carotenoids. With each of these pigments there is a highly significant inverse correlation between pigment concentration and the logarithm of p.e.r. The results indicate that less productive waters have the higher relative release of extracellular products. Productivity estimation methods that measure carbon fixation into particulate matter only may underestimate primary production by up to 30%. The increase in p.e.r. with decreasing population density suggests the operation of an equilibration system. The time-course curves of extracellular release give no clear indication of the nature of this equilibration. Nevertheless, it was established that one extracellular product (glycollic acid) is taken up by phytoplankton and/or bacteria in the water by a process of diffusion that is independent of light. The $^{14}$C-labelled extracellular products of three phytoplankton populations were separated by paper chromatography. All three were found to contain glycollic acid, polysaccharide, and various amino acids.