KINETICS OF EXTRACELLULAR RELEASE IN AXENIC ALGAE AND IN MIXED ALGAL‐BACTERIAL CULTURES: SIGNIFICANCE IN ESTIMATION OF TOTAL (GROSS) PHYTOPLANKTON EXCRETION RATES¹

Abstract

SUMMARY In axenic Chlorella pyrenoidosa Chick cultures, extracellular release was linear with time, but plateau‐type curves were obtained in cultures with added bacteria. Initial rates of excretion were identical in both, systems. Kinetics of extracellular release in axenic Anabaena flos‐aquae (Lyng.) Bréb. cultures were more complex than in Chlorella but the initial excretion rates were identical in axenic and mixed algal‐bacterial cultures. In lakewater, extracellular release kinetics resemble the pattern in mixed Chlorella‐bacteria cultures. An explanation is an initial lag in bacterial utilization of algal extracellular products. As a result, both in situ and in the laboratory, consecutive short, experiments give higher excretion rates than single long incubations. It is suggested that the former are close to total or gross extracellular release rates whereas the latter give net values, detecting only substances not, removed by heterotrophs.

Carbon exchange and 14C tracer methods in a nitrogen-limited diatom, Thalassiosira pseudonana

“…Using Equation 10, excretion estimated from a 6 h incubation would range from 69 to 100 % of true among steady-state cultures (with error largest at low growth rates), and 59 to 54 % for the perturbed cultures. For a 2 h incubation, as sometimes suggested (Nalewajko et al, 1976;Lancelot, 1979), the corresponding estimates would be 32 to 100 % for steady-state, and 23 to 26 % for perturbed, cultures. These errors are much more substantial than those in production estimates, and pose a difficult problem for field measurements.…”

“…Such utilization can be rapid enough to influence apparent excretion rates after several hours of incubation (Nalewajko et al, 1976;Lancelot, 1979;Billen et al, 1980). It has generally been assumed that excreted carbon equilibrates so quickly with added tracer that underestimates due to tracer disequilibrium would not be an important problem in short incubat i o n~, and there is experimental support for this view (Wiebe and Smith, 1977;Wolter, 1982).…”

Phytoplankton-bacteria interactions: an apparant paradox? Analysis of a model system with both competition and commensalism

“…Many studies attempting to evaluate bacterial consumption of algal EOC (extracellular organic carbon) in natural waters have been carried out (Derenbach & Williams 1974, Itturiaga & Hoppe 1977, Smith et al 1977, Wiebe & Smith 1977, Lancelot 1979, Larsson & Hagstrom 1979, Bell & Sakshaug 1980, Burney et al 1981, Chr6st 1981, Itturiaga 1981, Cole et al 1982, Wolter 1982, Riemann & Ssndergaard 1984. Such studies have also been carried out in enclosure experiments (Eberlein et al 1983) and in laboratory systems (Bell et al 1974, Nalewajko et al 1976, Bell 1983. When excreted compounds are of low mineral nutrient content, bacterial growth will require concomitant uptake of dissolved phosphate and/or nitrogen compounds for synthesis of bacterial biomass.…”

Gross Excretion Rates of Natural Marine Phytoplankton and Heterotrophic Uptake of Excreted Products in the Southern North Sea, as Determined by Short-Term Kinetics

“…Yet incubation time is very important for estimating gross phytoplankton excretion rates because of the presence of heterotrophic microorganisms. Heterotrophs can effectively modify the composition of phytoplankton extracellular compounds through rapid and selective utilization of some of the organic molecules produced during the incubation (Smith, 1974;Nalewajko et al, . Under these circumstances it is likely that some of the measured rates of extracellular release mentioned above are apparent or net values.…”