Microbial transport, survival, and succession in a sequence of buried sediments

Two chronosequences of unsaturated, buried loess sediments, ranging in age from <10,000 years to >1 million years, were investigated to reconstruct pa tterns of microbial ecological succession that have occurred since sediment burial. The relative importance of microbial transport and survival to suc cession was inferred from sediment ages, porewater ages, patterns of abunda nce (measured by direct counts, counts of culturable cells, and total phosp holipid fatty acids), activities (measured by radiotracer and enzyme assays ), and community composition (measured by phospholipid fatty acid patterns and Biolog substrate usage). Core samples were collected at two sites 40 km apart in the Palouse region of eastern Washington State, near the towns of Washtucna and Winona. The Washtucna site was flooded multiple times during the Pleistocene by glacial outburst floods; the Winona site elevation is a bove flood stage. Sediments at the Washtucna site were collected from near surface to 14.9 m depth, where the sediment age was similar to 250 Ita and the porewater age was 3700 years; sample intervals at the Winona site range d from near surface to 38 m (sediment age: similar to 1 Ma; porewater age: 1200 years). Microbial abundance and activities declined with depth at both sites; however, even the deepest, oldest sediments showed evidence of viab le microorganisms. Same-age sediments had equal quantities of microorganism s, but different community types. Differences in community makeup between t he two sites can be attributed to differences in groundwater recharge and p aleoflooding. Estimates of the microbial community age can be constrained b y porewater and sediment ages. In the shallower sediments (<9 m at Washtucn a, <12 m at Winona), the microbial communities are likely similar in age to the groundwater; thus, microbial succession has been influenced by recent transport of microorganisms from the surface. In the deeper sediments, the populations may be considerably older than the porewater ages, since microb ial transport is severely restricted in unsaturated sediments. This is part icularly true at the Winona site, which was never flooded.