Abstract
Abstract. Microbial mats are self-sustaining benthic ecosystems composed of highly diverse microbial communities. It has been proposed that microbial mats were widespread in Proterozoic marine environments, prior to the emergence of bioturbating organisms at the Precambrian–Cambrian transition. One characteristic feature of Precambrian biomarker records is that steranes are typically absent or occur in very low concentrations. This has been explained by low eukaryotic source inputs, or degradation of primary produced sterols in benthic microbial mats (“mat-seal effect”). To better understand the preservational pathways of sterols in microbial mats, we analyzed freely extractable and carbonate-bound lipid fractions as well as decalcified extraction residues in different layers of a recent calcifying mat (∼1500 years) from the hypersaline Lake 2 on the island of Kiritimati, central Pacific. A variety of C27–C29 sterols and distinctive C31 4α-methylsterols (4α-methylgorgosterol and 4α-methylgorgostanol, biomarkers for dinoflagellates) were detected in freely extractable and carbonate-bound lipid pools. These sterols most likely originated from organisms living in the water column and the upper mat layers. This autochthonous biomass experienced progressive microbial transformation and degradation in the microbial mat, as reflected by a significant drop in total sterol concentrations, up to 98 %, in the deeper layers, and a concomitant decrease in total organic carbon. Carbonate-bound sterols were generally low in abundance compared to the freely extractable portion, suggesting that incorporation into the mineral matrix does not play a major role in the preservation of eukaryotic sterols in this mat. Likewise, pyrolysis of extraction residues suggested that sequestration of steroid carbon skeletons into insoluble organic matter was low compared to hopanoids. Taken together, our findings argue for a major mat-seal effect affecting the distribution and preservation of steroids in the mat studied. This result markedly differs from recent findings made for another microbial mat growing in the nearby hypersaline Lake 22 on the same island, where sterols showed no systematic decrease with depth. The observed discrepancies in the taphonomic pathways of sterols in microbial mats from Kiritimati may be linked to multiple biotic and abiotic factors including salinity and periods of subaerial exposure, implying that caution has to be exercised in the interpretation of sterol distributions in modern and ancient microbial mat settings.
Highlights
Sterols are commonly used as biological markers for specific classes of organisms (Atwood et al, 2014; Brocks and Summons, 2004; Rampen et al, 2009; Volkman, 1986, 2005)
The preservation of primary eukaryotic sterols and their progressive alteration were studied in a ca. 1500-year-old microbial mat from the hypersaline Lake 2 on Kiritimati
Separate analysis of decalcified samples revealed that no significant “trapping” of sterols into the mineral matrix occurred in this mat
Summary
Sterols are commonly used as biological markers for specific classes of organisms (Atwood et al, 2014; Brocks and Summons, 2004; Rampen et al, 2009; Volkman, 1986, 2005). Y. Shen et al.: Sterol preservation in hypersaline microbial mats cal history of over 3 billion years, indicating that microbial mats probably represented the earliest complex ecosystems on Earth (Reitner and Thiel, 2011). EPSs are rich in acidic groups that bind cations such as Ca2+, thereby inducing a strong inhibitory effect on the precipitation of common minerals formed within microbial mats, such as CaCO3 (Arp et al, 1999; Dupraz et al, 2009; Ionescu et al, 2015). Previous studies indicate that early sequestration into a mineral matrix may promote the preservation of organic compounds (Summons et al, 2013; Smrzka et al, 2017; Thiel et al, 1999). Microbial mats possibly provide an enhanced chance for OM to survive in the geosphere if carbonate or other mineral precipitation occurs therein
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