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Small, free-living bacteria have been considered to be either dormant or heterotrophically active and adapted for efficient survival on low concentrations of UDOM (utiiisable dissolved organic material) by way of starvation-survival. They were, in fact, shown to be heterotrophically active in the water column of a subtidal reef in Natal (South Africa), an environment containing UDOM derived from accumulated plant litter. They are more prevalent over the sand substratum between such reefs, where plant litter is transient and the concentration of organic matter (5.4 mg 1-' TOC) is only 30 % of that found on the reef studied. Simple uptake experiments conducted in both summer and winter, using a labelled algal extract (LAE) as substrate, showed the bacteria over the sand to be as active as those over the reef. In most cases, uptake of LAE commenced immediately after addition and progressed Linearly; short-term uptake site saturation possibly occurred in the few remaining cases. Results appear to conform to the starvation-survival theory. The abundance of small, free-living bacteria in the aquatic environment is well documented, recent noteworthy reviews on their role in the marine environment being those of Azam et al. (1983) and Lucas (1986). In some earlier work it was suggested that these bacteria were dormant (Wangersky 1977, Stevenson 1978), but other research clearly showed that they account for considerable heterotrophic activity in many environments (Williams 1970, 1981, Berman 1975, Azam & Hodson 1977, Cole & Likens 1979, Fuhrman 1981, Schleyer 1981). Extensive work of such a nature was carried out on a nearshore, subtropical, subtidal reef in Natal, South Africa (Schleyer 1980, 1981, 1984). These studies supported the latter finding and were later extended to the expansive and less productive sand substratum interspersing Natal's reef systems (Schleyer & Roberts unpubl.). Due to water exchange, the bacterial populations are common to both environments and were found to be almost uniform in number (2 X 106 ml-l), community structure and heterotrophic activity (mean V,,, values of 293 mg C m-3 d-' and 244 mg C m-3 d-' O Inter-Research/Printed in F. R. Germany were obtained over the reef and sand respectively; Schleyer 1984, Schleyer & Roberts unpubl.). The only major difference between the populations is a proportionately greater number of free-living bacteria over the sand (92.8 %; Schleyer & Roberts unpubl.) than over the reefs (79.0 %; Schleyer 1981). However, the populations are spatially and temporally well separated and are distinct in terms of their food availability; bacteria on the reefs have available a rich source of fine particulate and dissolved organic carbon (FPOC and DOC), derived mainly from accumulated plant detritus (Schleyer 19811, in contrast to the sand substratum over which plant litter is merely transient in its passage. Total organic carbon (TOC) levels of only 30 % (F = 5.4 mg I-') of those found on the experimental reef were measured over the sand. The sand environment is thus nutrient-depleted compared to that of the reefs and it was suspected that the 'deprived' bacteria may manifest dormancy. For this reason it was decided that the free-living bacteria over the sand substratum provided a good opportunity to conduct a simple test for dormancy. It had already been established that the incubation of samples with tracer quantities of a labelled algal extract (LAE) elicited 'normal' rates of uptake over a l h period (Schleyer & Roberts unpubl.), so the question posed was whether an induction period was required before uptake commenced. The linearity of progressive uptake of LAE was thus monitored for this purpose. Methods. In 2 separate experiments, conducted in summer and winter, water samples were collected in the surf and backline at beaches off Addington and Sunkist in Durban, South Africa. The former site is roughly 200 m from the nearest reef and is fairly sheltered from wave action by the Durban Harbour breakwaters. The latter is 2.5 km from a groyne, the nearest reef-like feature, and is an exposed, high-energy 102 Mar Ecol. Prog. Ser. 53: 101-103, 1989 As Abs Abb Ss Sbs Sbb

Microbial activity in water overlying the near-shore sand substratum in Natal, South Africa