Thursday, 1 May 2014

Two billion year old microbial ecosystems

Phosphorus is a key element for life---as phosphate (PO4) it helps form the structural framework of information storing DNA and RNA, and has crucial functions in the energy and support systems of organisms (such as ATP). Phosphorous occurs naturally in a number of minerals (e.g. apatite, which is calcium phosphate) and is even present in meteorites, but it is via a variety of weathering and microbial mechanisms that phosphorous-rich deposits (phosphorites) are formed. To do so microbially requires a bacterial consortium, an ecosystem driven by various microbes generating a complementary suite of reduction-oxidation processes specific to the organic and electron-donor substrates in a particular environmental setting.

Fig. 1. Fossilised sulphur-oxidising bacteria preserved
in 2 billion year old rocks, Karelia NW Russia (photo:
Aivo Lepland, Norwegian Geological Survey).
This is where Dr Tony Prave in the Department of Earth and Environmental Sciences and his colleagues have contributed to understanding the linkages between the processes associated with the biologically mediated phosphorous cycle and the formation of the earliest global phosphorites. The international team of geologists and biogeochemists, led by Dr Aivo Lepland of the Norwegian Geological Survey, focussed their efforts on rocks found in Karelia, NW Russia. These rocks are two billion years old and the unit of interest is the Zaonega Formation, a succession of organic-rich rocks containing phosphorite beds that was deposited during a period in Earth history when free di-oxygen was becoming abundant in the atmosphere and shallow portions of the oceans (the 2.3 billion year old event termed the Great Oxidation Event).

Cylindrical apatite particles indicative of a biogenic
origin and typically attributed to methanotrophic archaea
(photo: Aivo Lepland, Norwegian Geological Survey).
What Tony and co-workers documented within Karelian phosphorites is the presence of a fossilised microbial consortium, which in their interpretation consisted of sulphur bacteria (Fig. 1) associated with a fabric of cylinders composed of the phosphate-bearing mineral apatite (Fig. 2). The consistent size and shape of the apatite crystals is identical to those formed by methanotrophic archaea that in modern microbial ecosystems co-occur commonly with sulphur oxidisers. In effect, the team documented the establishment of a microbial ecosystem similar to those found in modern marine settings in which sulphur-oxidisers and methanotrophs co-habit---the remarkable aspect is that this ecosystem is more than two billion years old. Such geochemical and biological fingerprints provide a key example to constrain and understand the conditions under which life evolved not only on Earth but also potentially on other planets.

Potential influence of sulphur bacteria on Palaeoproterozoic phosphogenesis, Nature Geoscience