THE SEDIMENTARY ECOLOGY OF BENTHIC CYANOBACTERIA IN MODERN AND ARCHEAN TIDAL FLATS

Commonly, algal mats are regarded as archaic life forms, which today thrive especially in extreme, restricted environments. In contrast, wide tidal flats form along the passive continental margins of our oceans in course of the Holocene transgression. Here, algal mats are widely distributed and constitute one of Earth’s largest ecosystem.

Microbial influences on tidal sediments are generally understood as biogeochemical processes generating stromatolites. However, also the interaction of algal mats with physical dynamic parameters plays an important role, and in sandy settings gives rise to biogenic structures.

The presentation discusses with the example of three types of tidal flats, how cyanobacteria respond to physical sedimentary processes: (i) Fishermans Island (Virginia, USA), a microtidal setting, (ii) Mellum Island, North Sea, a mesotidal setting, and (iii) Island of Noirmoutier, France, a macrotidal setting.

At all study sites, different types of algal mats establish from the low to the high water lines of the tidal flats. Thin, endobenthic algal mats composed by the highly mobile Oscillatoria limosa develop in the intertidal zone. This mat type stabilizes its sandy substrate 3 – 5 times compared to sterile sand. If buried, this cyanobacterium escapes quickly, and reestablishes a mat layer within a few hours. Thick, epibenthic algal mats are formed by Microcoleaus chthonoplastes, a cyanobacterium well adapted to the long periods of subaerial exposure in the lower supratidal zone. This mat type stabilizes its substrate up to 12 magnitudes. In tidal channels or close to the low water line, biofilm-coated sand grains are swirled around by constant turbulence. The photoautotrophic cyanobacteria escape the lethal burial, because the microbial-mineral aggregates stay longer in suspension than non-colonized grains.

We can quantify the response by benthic cyanobacteria to the hydraulic conditions in laboratory experiments, and by field studies using a portable flume chamber. The results suggest that we have to modify the quantification of physical sediment dynamics, if we study natural environments. With respect to Earth history, the quantification of those sedimentary processes allows us to rise the hypothesis, that cyanobacteria have been at least 2.9 Ga around.