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, under­estimates 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.

Factors affecting phytoplankton extracellular release in the Southern Bight of the North Sea

“…In addition, for each phytoflagellate population, a specific inverse relation holds between PER and mineral nitrogen concentration. This means that when nutrients decrease during a bloom because of phytoplankton uptake, higher quantities of photoassimilated carbon are released into the surrounding medium in agreement with Fogg's hypothesis (Fogg et al, 1965) and other studies (Ignatiades and Fogg, 1973;Iturriaga and Hoppe, 1977;Sharp, 1977;Wolter, 1982). None of these authors, however, reported a continuous relationship, probably because of heterogeneity in phytoplanktonic species composition.…”

“…None of these authors, however, reported a continuous relationship, probably because of heterogeneity in phytoplanktonic species composition. As a result of such different physiological behaviour of the 3 main groups of phytoplankton succeeding each other in the Southern Bight of the North Sea, the highest PER values are not found in the most oligotrophic waters as suggested by earlier works (Fogg et al, 1965;Anderson and Zeutschel, 1970;Thomas, 1971;Berman and Holm-Hansen, 1974). Outside the bloom period, PER values are very similar in oligotrophic and eutrophic waters (Fig.…”

“…The data show that the percentage of extracellular release does not depend on light intensity in the Southern Bight of the North Sea. This result agrees perfectly with that reported by Smith et al (1977), but contrasts with most works which indicate that phytoplankton extracellular release increases relatively when primary production is limited by either low or high light intensities (Fogg et al, 1965;Hellebust, 1965;Watt, 1966;Anderson and Zeutschel, 1970;Berman and Holm-Hansen, 1974;Mague et al, 1980;Watanabe, 1980). Following these authors, PER increase at high light intensities corresponds to the release of larger amounts of glycollate produced by increased photorespiration.…”

“…Fogg et al (1965) suggested that photo-assimilated carbon is released into the surrounding medium instead of being used for growth when depletion of nutrients prevents protein synthesis. According to this hypothesis, higher PER would be found in oligotrophic than in eutrophic waters.…”

International Helgoland Symposium: Convener's report on the Informal Session on biomass and productivity of microorganisms in planktonic ecosystems

“…In regard to the way nutrients are derived from phytoplankton for bacterioplankton, Mommaerts commented that the values for phytoplankton excretion are decreasing through the years. The first values at the time of Fogg & Watt (Fogg et al, 1965) were up to 50 ~ of production while the most recent values from Williams and Yentsch (1976) were from 0 to 6 ~ He wondered if excretion per se exists at all. He stated that on the other hand he has indirect evidence of light lysis by phytoplankton.…”