Biogeochemical cycling and microbial diversity in the thrombolitic microbialites of Highborne Cay, Bahamas

Abstract

Thrombolites are unlaminated carbonate build-ups that are formed via the metabolic activities of complex microbial mat communities. The thrombolitic mats of Highborne Cay, Bahamas develop in close proximity (1-2 m) to accreting laminated stromatolites, providing an ideal opportunity for biogeochemical and molecular comparisons of these two distinctive microbialite ecosystems. In this study, we provide the first comprehensive characterization of the biogeochemical activities and microbial diversity of the Highborne Cay thrombolitic mats. Morphological and molecular analyses reveal two dominant mat types associated with the thrombolite deposits, both of which are dominated by bacteria from the taxa Cyanobacteria and Alphaproteobacteria. Diel cycling of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were measured in all thrombolitic mat types. DO production varied between thrombolitic types and one morphotype, referred to in this study as 'button mats', produced the highest levels among all mat types, including the adjacent stromatolites. Characterization of thrombolite bacterial communities revealed a high bacterial diversity, roughly equivalent to that of the nearby stromatolites, and a low eukaryotic diversity. Extensive phylogenetic overlap between thrombolitic and stromatolitic microbial communities was observed, although thrombolite-specific cyanobacterial populations were detected. In particular, the button mats were dominated by a calcified, filamentous cyanobacterium identified via morphology and 16S rRNA gene sequencing as Dichothrix sp. The distinctive microbial communities and chemical cycling patterns within the thrombolitic mats provide novel insight into the biogeochemical processes related to the lithifying mats in this system, and provide data relevant to understanding microbially induced carbonate biomineralization.

Gene Sequencing-Based Analysis of Microbial-Mat Morphotypes, Caicos Platform, British West Indies

“…These anoxygenic photoheterotrophic organisms likely inhabit the base of the photic zone in the mat, below the Cyanobacteria, where they can utilize organic compounds from the breakdown of biomass from primary producers while also making use of light energy to generate ATP (Imhoff 1995;Overmann and Garcia-Pichel 2013). A high proportion of Rhodobacteriales Alphaproteobacteria were also found in thrombolitic microbial mats in Highborne Cay, Bahamas, by genetic sequencing (Myshrall et al 2010) and lipid analysis .…”

Metabolic potential of lithifying cyanobacteria-dominated thrombolitic mats

“…Despite recent studies regarding the viral and microbial diversity, sedimentology, and biogeochemistry of the Bahamian thrombolitic mats (Desnues et al 2008;Planavsky and Ginsburg 2009;Myshrall et al 2010;and Mobberley et al 2012;Edgcomb et al 2013) there remains an overall lack of understanding of the molecular mechanisms that underlie and regulate the metabolisms associated with these communities. In this study, we survey the metabolic potential of thrombolitic mats by sequencing the metagenome using massively parallel DNA sequencing.…”

“…Together, these groups of organisms generate steep vertical chemical gradients within the thrombolites with large diel fluctuations. For example, O 2 levels throughout the diel cycle can go from undetectable to several-fold supersaturated in the top 1-5 mm of the lithifying mat surface (Myshrall et al 2010). The dominant organisms responsible for this O 2 -rich layer are photosynthetic cyanobacteria, which have been shown to be a driving force in the biogeochemical cycling in thrombolites (Myshrall et al 2010).…”

“…1b) comprised of a diverse microbial consortium, which is thought to drive the precipitation of extracellular carbonate through their various metabolic activities Myshrall et al 2010;Mobberley et al 2012). There have been several recent studies that have characterized the taphonomy of the thrombolite carbonate structures (Planavsky and Ginsburg 2009), the surrounding oolitic sand grains (Edgcomb et al 2013), as well as the biogeochemistry and microbial diversity of the thrombolite-forming microbial mats, from here on referred to as thrombolitic mats (Desnues et al 2008;Myshrall et al 2010;Mobberley et al 2012). These previous analyses have revealed several key functional groups of organisms associated with the mats and include: phototrophs, aerobic heterotrophic bacteria, sulfate-reducing bacteria, sulfide-oxidizing bacteria, and fermentative bacteria.…”

Effects of Elevated Carbon Dioxide and Salinity on the Microbial Diversity in Lithifying Microbial Mats

“…20 μm [31,32]. A Unisense PA2000 picoammeter (Unisense, Arhus, Denmark) with customized variable polarization was used to register the oxygen signal.…”