The subsurface of tidal-flat sediments as a model for the deep biosphere

Subsurface sediments of tidal flats and the marine deep biosphere share several features. Although on different scales in time and space, geochemical profiles and microbial successions follow the same trends. Microbial activities are governed by the availability of electron acceptors and the quality of electron donors. Two deep-biosphere sites and a shallow site from a German North Sea tidal flat were exemplarily chosen to compare geochemical settings and microbiological features. At all sites, microbial abundance was elevated at sulfate-methane transition zones. The known discrepancy between cultivation-based and molecular diversity assessments is observed, but similar microbial community compositions are found with each of the approaches at deep and shallow sites. These findings lead to the conclusion that we are presently unable to draw a cutting line between the shallow and the deep subsurface. Rather, there appears to exist only one subsurface biosphere with gradual differences. Therefore, tidal flats serve as an excellent model to perform microbiological experiments and to test novel techniques before applying them to much deeper and older samples.