Morphological adaptations of 3.22 Ga-old tufted microbial mats to Archean coastal habitats (Moodies Group, Barberton Greenstone Belt, South Africa)

Abstract

a b s t r a c tMicrobial life was well established and widespread by the Paleoarchean; however, the degree of evolutionary advancement such as microbial motility, intra-and inter-species interactions, phototropism, or oxygenic photosynthesis by that time remains highly debated. The 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB, South Africa) are Earth's oldest well-preserved siliciclastic tidal deposits. They exhibit a unique assemblage of microbial mats, providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings. The fossil mats are preserved as kerogenous laminations (0.5-1 mm thick) that can be traced laterally for ∼15 km in a ∼1000 m-thick succession of fine-to coarse-grained tidal sandstones and conglomerates. We here present a detailed stratigraphic and depositional facies analysis, documenting the association of the three principal mat morphotypes with specific environmental settings: (1) planar-type in coastal floodplain, (2) wavy-type in intertidal, and (3) tufted-type in upper inter-to supratidal facies. All mat types indicate a flourishing phototrophic biota; moreover, the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments.

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…In the case of the Moodies Group, recent stratigraphic and depositional facies analysis have documented the association of three principal mat morphotypes representing different environmental settings: (1) planar-type in coastal floodplains, (2) wavy-type in intertidal zones, and (3) tufted-type in upper inter-to supratidal zones. All mat types suggest formation by phototrophic biota, but the tufted morphology implies an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments (Homann et al, 2015).…”

Cyanobacterial evolution during the Precambrian

“…phototaxis). In addition, since the tufts of modern microbial mats are almost exclusively composed of vertically aligned clumps of cyanobacteria, tufted structures in the Archean, at least as far back as 3.2-2.7 Ga (Flannery & Walter 2012;Homann et al 2015), have been suggested to indicate the presence of cyanobacteria at that time. Coniform stromatolites have been reported from even older rocks (*3.4-3.5 Ga; Hofmann et al 1999;Allwood et al 2006;Van Kranendonk 2006); these forms were also likely heavily influenced by phototactic growth of microorganisms but these authors stop short of confidently ascribing them to cyanobacterial activity.…”

A Proposal for Formation of Archaean Stromatolites before the Advent of Oxygenic Photosynthesis

“…Hydrothermal activity would have produced marked variation in concentration of dissolved inorganic electron donors in Archaean euphotic zones such as littoral, coastal, and river environments where microbial mats were formed (Homann et al, 2015). Environments replete with ferrous iron or hydrogen sulfide would have changed repeatedly and reversibly to ones where the only available electron donors were organic – these locations would therefore have alternated in their provision for photosynthesis by Types I and II reaction centers.…”