Abstract
To test the applicability of substrate uptake kinetics for assessing rates of microbial heterotrophic activity in the sea, the conformity between accepted notations for pure enzyme systems as opposed to natural populations is examined. Minimum rates for the in situ uptake of organic carbon are calculated using 14C‐labeled glucose as a substrate. To provide an estimate of naturally occurring substrate, a complementary bioassay procedure is applied to complete the analysis. The isolation and the uptake characteristics of the bioassay organism, which resembles Achromobacter aquamarinus, and its capability of discerning glucose at concentrations as low as 4 × 10−8 m are discussed. Although the reliability of kinetic constants cannot be assured in multisample situations because of the possibility of inherent sample differences, we have applied the complete analysis to coastal and offshore waters to examine the magnitude and internal consistency of the results. The results showed clearly the influence of nutrient‐poor Amazon River water in offshore regions of the South Atlantic Ocean and suggest the possibility of combining uptake measurements of natural populations with those of pure cultures of a test organism. The necessity for an improved direct analysis for more reliable estimates of the natural substrate concentration remains.
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