EFFECTS OF BACTERIAL EXOPOLYMER ADHESION ON THE ENTRAINMENT OF SAND

Flow velocity required to erode a bed of acid-washed sand is increased by intergranular adhesion resulting from growth of the benthic marine bacterium Alteromonas atlantica. In general, we find that either pure exopolymer alone or exopolymer generated during in situ growth increases erosion resistance of fine quartz sand. Moreover, the degree of erosion resistance increases in proportion to the concentration of exopolymer-component uronic acids, which in turn is dependent on relative nitrogen content of peptone-based growth media. Specifically, we observe that approximately 100 nmol of exopolymer or 1.5 nmol of component uronic acids generated by in situ bacterial growth under nitrogen-rich conditions per gram of dry sediment can effectively double seawater-flume flow velocity required for initiation of transport of otherwise noncohesive, 125-177-mu-m quartz grains. This maximal effect corresponds to an estimated adhesive force that exceeds submerged particle weight by an order of magnitude and exceeds particle-specific uronic acids weight by seven orders of magnitude. Rapid analysis by gas chromatography/mass spectrometry of purified exopolymer obtained from A. atlantica cultures shows it to be a polysacharide containing approximately 20% uronic acids by weight. Maximal exopolymer and component uronic acid concentrations generated in these experiments are an order of magnitude less than values previously reported for estuarine sediments. These results are the first to correlate specific exopolymer components with sediment erosion resistance and further demonstrate the importance of microbial exudates in sediment binding believed to occur in a wide range of marine environments.