Controls on microbial activity and tidal flat evolution in Shark Bay, Western Australia

Abstract

Microbial deposits at Shark Bay constitute a diverse living microbial carbonate system, developed in a semi-arid, highly evaporative marine setting. Three tidal flats located in different embayments within the World Heritage area were investigated in order to compare microbial deposits and their Holocene evolution. The stressing conditions in the intertidal-subtidal environment have produced a microbial ecosystem that is trapping, binding and biologically inducing CaCO 3 precipitation, producing laminated stromatolites (tufted, smooth and colloform), non-laminated thrombolitic forms (pustular) and cryptomicrobial non-laminated forms (microbial pavement). A general shallowing-upwards sedimentary cycle was recognized and correlated with Holocene sea-level variations, where microbial deposits constitute the younger (2360 years BP) and shallower sedimentary veneer. In addition, sediments have been documented with evidence of exposure during the Holocene, from 1040 to 940 14 C years BP, when sea-level was apparently lower than present. Filamentous bacteria constitute the dominant group in the blister, tufted and smooth mat types, and coccus bacteria dominate the pustular, colloform and microbial pavement deposit types. In the subtidal environment within colloform and pavement structures, microbial communities coexist with organisms such as bivalves, serpulids, diatoms, green algae (Acetabularia), crustaceans, foraminifera and micro-gastropods, which are responsible for exoskeleton supply and extensive bioturbation. The internal fabric of the microbial deposits is laminated, sub-laminar, scalloped, irregular or clotted, depending on the amount of fine-grained carbonate and the natural ability of microbial communities to trap and bind particles or induce carbonate precipitation. Nilemah tidal flat contains the thickest (1Á3 m) and best-developed microbial sedimentary system; its deposition pre-dated the Rocky Point and Garden Point tidal flats, with the most positive isotope values for d 13 C and d 18 O, reflecting strong microbial activity in a highly evaporative environment. There is an evolutionary series preserved within the tidal flats reflecting relative ages and degree of salinity elevation.

Unravelling core microbial metabolisms in the hypersaline microbial mats of Shark Bay using high-throughput metagenomics

“…Hamelin Pool in Western Australia spans an area of 1220 km 2 with an average tidal range of~60 cm and is well protected by carbonate banks, which confine sea water inflow. In addition, Nilemah contains some of the most well-developed microbial sedimentary systems in Shark bay (Jahnert et al, 2013).…”

“…Shark Bay offers a more extreme modern analogue, as it is hypersaline (460 psu salinity) compared with Highbourne Cay that has standard marine salinity (33 to 35 psu) Burns et al, 2004). Shark Bay in Western Australia harbours modern analogues consisting of several morphotypes, including lithifying and non-lithifying microbial mats (smooth and pustular, respectively), and mature columnar stromatolites Jahnert et al, 2013). Previous polyphasic approaches combining bacterial culture-dependent and independent approaches by our group and others reported a high diversity of bacterial and haloarchaeal OTUs in Shark Bay stromatolites and microbial mats (Burns et al, 2004;Papineau et al, 2005;Allen et al, 2009;Goh et al, 2009).…”

Origin and significance of lamination in Lower Cretaceous stromatolites and proposal for a quantitative approach

“…These hydrochemical fluctuations together with the hydrodynamical changes typical of tidal environments could alternately promote grain trapping and mat calcification in Leza coarse-grained stromatolites. However, biotic changes (e.g., alternation of mats of differing microbial composition at the stromatolite surface) cannot be ruled out, especially since mats of varied biotic compositions can alternate at the surface of present-day coarse-grained carbonate examples (Bowlin et al, 2012;Jahnert and Collins, 2013).…”

Lipid Biomarker and Isotopic Study of Community Distribution and Biomarker Preservation in a Laminated Microbial Mat from Shark Bay, Western Australia

“…Modern microbial mats, including laminated lithifying smooth mats, have been reported in the World Heritage listed Shark Bay, Western Australia [17,18]. Previous studies on microbial mats from Hamelin Pool have revealed a high biodiversity in both smooth and pustular mats [19][20][21].…”