ALGAE AS COMPETITORS FOR GLUCOSE WITH HETEROTROPHIC BACTERIA¹

Abstract

Dissolved organic carbon (DOC) constitutes the bulk of organic carbon in aquatic environments. The importance of DOC utilization by mixotrophic algae is unclear since heterotrophic bacteria are regarded as more efficient users. We tested the hypothesis that algae decrease the DOC concentration in the light to lower levels than in darkness resulting in competitive exclusion of heterotrophic bacteria according to the mechanistic competition theory. We investigated (a) the uptake kinetics of glucose as a model substrate by two cultured algae and mixed bacteria populations, (b) the competition for glucose between algae and bacteria in chemostats, (c) the effect of discontinuous glucose supply in chemostats, and (d) the minimum glucose concentrations achieved in cultures of algae and bacteria. Bacteria showed higher specific-glucose-uptake rates than algae. In chemostats, algae became extinct in the dark and coexisted in the light where they decreased bacteria to lower densities. Discontinuous glucose supply promoted the algae compared to continuous substrate addition. Several algae consumed glucose to lower concentrations in the dark than in the light and showed lower or equal residual glucose concentrations than bacteria. Residual concentrations were not related to allometric traits (cell volume) and photosynthetic potential (chl content). Overall, the hypothesis was not supported, and mechanisms of competition for DOC obviously differed from those for particulate prey. However, since some algae showed lower or equal residual glucose concentrations than bacteria, algal dark uptake of DOC may be important in deep layers of many waters.

Utilisation of leucine by several phytoplankton species

“…In the experiments presented here, no alga generated lower substrate levels in the light than in the dark, so that hypothesis (1) was not supported. This was also found in consumption experiments with glucose (Kamjunke et al, 2008). Furthermore, cyanobacteria and chlorophytes in our consumption experiments did not decrease substrate concentrations to lower levels than the heterotrophic bacteria.…”

Sunlight Effects on the Osmotrophic Uptake of DMSP-Sulfur and Leucine by Polar Phytoplankton

“…However, some studies have shown that several phytoplankton species do actively take up substrates at low concentrations so that they may, in fact, be competitive with bacteria [9], [10]. Among the organic substrates algae are able to use are pyruvate, acetate, lactate, ethanol, saturated fatty acids, glycolate, glycerol, hexoses, urea, and amino acids (e.g.…”

Temperature affects the response of heterotrophic bacteria and mixotrophic algae to enhanced concentrations of soil extract

“…The molar C:P ratios of the medium were rather low (48 and 82) indicating a carbon limitation of bacteria, and also the higher bacterial biomass in the chemostats at 20°C compared to that at 15°C renders a phosphorus limitation unlikely. The equilibrium biomass of Chlorella in the chemostats was about 1,000-fold lower than in other experiments with glucose as carbon source and a plenty phosphorus supply (1,500 lg P l -1 ; Kamjunke et al, 2008) which was probably due to the lower phosphorus concentration. Furthermore, algal biomass was also much lower than bacterial biomass (about factor 50-100).…”