Palaeoecology of a billion‐year‐old non‐marine cyanobacterium from the Torridon Group and Nonesuch Formation

Abstract

A new chroococcalean cyanobacterium is described from approximately 1‐billion‐year‐old non‐marine deposits of the Torridonian Group of Scotland and the Nonesuch Formation of Michigan, USA. Individual cells of the new microfossil, Eohalothece lacustrina gen. et sp. nov., are associated with benthic microbial biofilms, but the majority of samples are recovered in palynological preparations in the form of large, apparently planktonic colonies, similar to extant species of Microcystis. In the Torridonian, Eohalothece is associated with phosphatic nodules, and we have developed a novel hypothesis linking Eohalothece to phosphate deposition in ancient freshwater settings. Extant cyanobacteria can be prolific producers of extracellular microcystins, which are non‐ribosomal polypeptide phosphatase inhibitors. Microcystins may have promoted the retention and concentration of sedimentary organic phosphate prior to mineralization of francolite and nodule formation. This has a further implication that the Torridonian lakes were nitrogen limited as the release of microcystins is enhanced under such conditions today. The abundance and wide distribution of Eohalothece lacustrina attests to the importance of cyanobacteria as oxygen‐producing photoautotrophs in lacustrine ecosystems at the time of the Mesoproterozoic–Neoproterozoic transition.

Early photosynthetic eukaryotes inhabited low-salinity habitats

“…Lacustrine and floodplain environments are known from Archean successions, and stromatolites and 12 C-depleted organic matter in these rocks document early microbial ecosystems driven by photosynthesis (85). Significantly, shales associated with alluvial and fluviatile sandstones in the Late Mesoproterozoic successions from Scotland and midcontinent North America (86,87) contain abundant and modestly diverse microfossils reasonably interpreted as eukaryotic (88). The taxonomic affinities of these fossils are unclear, and photosynthetic microorganisms may or may not be present in the assemblage.…”

Matworld – the biogeochemical effects of early life on land

“…More direct support for such mats is found in a 2.06-1.88 Ga dryland system (Makgabeng Formation) which shows 'roll-up' and other sedimentological structures indicative of microbial mats (Eriksson et al, 2000;Simpson et al, 2013). By 1.2-1.0 Ga, microbially induced sedimentary structures are preserved from a wet environment subject to periodic drying (Torridonian succession) (Prave, 2002;Strother & Wellman, 2016). This succession has also yielded eukaryote fossils from freshwater and subaerially exposed habitats, including multicellular forms (Strother et al, 2011).…”

Evaluating evidence from the Torridonian Supergroup (Scotland, UK) for eukaryotic life on land in the Proterozoic

“…Such inhibitors prevent release of inorganic phosphate from organic compounds, so potentially contributing to phosphate build-up around organics at the bottom of the Torridon lakes. However, as noted by Strother & Wellman (2016), such an explanation is incomplete because phosphates are lacking from the Nonesuch Formation in which Eohalothece is also found.…”

“…13d; Callow 2011). Strother and Wellman (2016) put forward a hypothesis to explain some of these reticulated structures that invokes interaction of raindrops with EPS binding of the sediment, rather than simply wrinkling of microbial mats.…”