Light‐dark discrepancy of heterotrophic bacterial substrate uptake

The past decade has seen a wide application of the measurement of radioactively labelled organic substrate uptake by aquatic microorganisms, as reviewed in refs. 1-3. When determined over a range of near natural substrate concentrations, mathematical treatment of the uptake fractions for the various concentrations yields a maximum velocity of uptake termed heterotrophic potential, or Vmax [4,5], which is attributed nearly exclusively to the heterotrophic bacterial populations [1-5] . Correction for respiration can be made [6] to give a more quantitative estimation. Vmax values have been used relatively to establish the trophic status of a series of lakes [7,8], and absolutely in studies of nutrient cycling in aquatic ecosystems [e.g. 9-11 ] and indexing specific activity of heterotrophic bacteria in natural waters [ 12]. All these studies have employed dark incubation of the water samples, plus labelled substrate, for a period of time inversely proportional to the degree of eutrophication. It has been recently reported, however, that dark incubation results in an underestimation of heterotrophic potential, as this measures only the chemoheterotrophic component of total heterotrophy, the difference being accounted for by photoheterotrophy, i.e. the light-mediated uptake of organic compounds by photosynthetic organisms [13]. This paper presents results from a series of experiments employing postincubation size fractionations of surface water samples incubated in situ in both light and dark bottles. By this technique, the respective contributions from both the autotrophic and heterotrophic populations, in explaining the light-dark discrepancy of substrate uptake at near natural concentrations, may be examined.