Relationships between bacteria and grain surfaces in intertidal sediments1

Seasonal changes in total bacterial numbers and their associated mucus coatings in surficial sediments were examined. Bacterial numbers followed the temperature cycle, with highest numbers in summer. The specific surface areas of the sediments were measured rather than inferred from other granulometric properties; bacterial numbers were proportional to surface areas only for sample suites collected at the same time. Bacteria inhabited shallow depressions on sand and silt grains; they were not found on grains smaller than about 10 pm or inside smaller pores like those on weathered feldspar grains. Mucus coatings also followed a seasonal cycle, increasing in abundance and coalescence from spring into summer. These coatings accumulated clay grains, suggesting that the relationship of bacteria to surface area may be due to bacterial control of surface area rather than the reverse. Organic carbon concentrations in grain size separates of these sediments increased with decreasing size until the fine silt fraction, and decreased in the clay fraction; it is not clear, however, whether this trend is a result or a cause of bacterial colonization patterns. Particle surfaces are important habitats for sedimentary bacteria. Adsorption of nutrients to solid surfaces and an accompanying increase in bacterial biomass was demonstrated in the early laboratory work of ZoBell(1943). Subsequent measurements of bacterial numbers or metabolic activities in sediments have exhibited a strong inverse correlation with sediment grain size, implying a surface area dependence (Dale 1974; Hargrave 1972). However, this surface area dependence of bacteria has not yet been verified with direct measures of surface area. The difficulties in relating measured surface areas to other granulometric parameters have been pointed out by Mayer and Rossi (1982). Microtopography of sediment grains has also been implicated as a factor influencing bacterial colonization on sediment grains. In an optical microscope study of sand grains, Meadows and Anderson (1966) found bacteria in patches with large ’ This work was supported by NSF ISP 8011448 and NOAA Sea Grant WLRF-45. Contribution 8302 of the Maine Benthic Oceanography Group. 2 Present address: Department of LMarine Science, University of South Florida, St. Petersburg 33701. 3 To whom reprint requests should be directed. areas of bare suface between them; these patches were often found in hollows and cracks. Weise and Rheinheimer (1978) found the density of bacterial cells to be determined by microtopography, with the highest densities on subrounded or subangular grains. Frankel (1977) found that subangular to subrounded grains of hornblende supported fewer bacteria than relatively smooth biotite grains. The separated folia of the biotite grains provided a more protected habitat and consequently supported a larger population. The degree of protection seems to influence the population size of colonizing microbiota. Surface morphology affected both the total biomass and the community structure of the microbiota (including bacteria, algae, and grazers) on silica grains in a running seawater experiment (Nickels et al. 1981). Total microbial biomass was higher on grains with more surface irregularities. Smooth grains of the same size supported a lower population of procaryotes and microalgae, with an increase of microeucaryotic grazers. Extracellular exudates of bacteria have become subjects of increasing interest due to their potential roles in affecting the sedimentological and trophic status of sediments. There is, however, a limited literature describing mucus coatings on