In situ conditions and interactions between microbes and minerals in fine-grained marine sediments: A TEM microfabric perspective

Microbes, their exocellular secretions, and their impact on the mineralogy and microfabric of fine-grained continental margin sediments were investiga ted by transmission electron microscopy. Techniques were used that retained the in situ spatial relations of both bio-organic and mineralogical consti tuents. Photomicrographs were taken of characteristic mineral-microbe assoc iations in the first meter of burial at conditions ranging from aerobic to anaerobic. Single-celled prokaryotes, prokaryotic colonies, and eukaryotic organisms were observed as were motile, sessile, and predatory species. Bac terial cells dominate the assemblage. The most commonly observed mineral-bi ological interaction was the surrounding, or close association, of isolated heterotrophic bacterial cells by clay minerals. Almost without exception, the external surfaces of the bacteria were covered with secreted exocellula r slimes composed of cross-linked polysaccharide fibrils. These fibrils act to bind sediment grains into relatively robust microaggregates, roughly le ss than or equal to 25 mu m in diameter. These exocellular polymers can sig nificantly impact the interaction between microbes and minerals, as well as the chemical and physical transport of fluids and dissolved aqueous specie s through the sediment. Although pore water chemical profiles from the fiel d sites studied have dissolved Fe and Mn, no close association was found be tween the microbes imaged and precipitated metal oxyhydroxides or other aut higenic minerals, such as is commonly reported from laboratory cultures.