Characterization of microbial mat from Lagoa Vermelha-RJ: textural, mineralogical, and isotopic Insights

Microbial mats forming microbialites are stratified communities, distributed according to the gradient of light, oxygen, and physicochemical nutrients, coexisting with different types of metabolism. These communities are restricted to regions where extreme environmental conditions limit the development of complex organisms. In addition to environmental characteristics such as salinity, pH, temperature fluctuations, and chemical composition of the medium, the rate of carbonate precipitation has been associated with the phosphate (PO₄³⁻) cycle ^[1,2]. To characterize the relationship between microorganisms and carbonate precipitates, we conducted textural, mineralogical (using X-ray diffractometer - XRD), and carbon isotopic (δ¹³C) analyses on three different layers of a microbial mat from Lagoa Vermelha, Rio de Janeiro, cultivated in laboratory conditions. After removing the precipitates, the samples were centrifuged at 4,000 rpm for 5 minutes. Then, the organic fractions were treated with hydrochloric acid (HCl), followed by washes with deionized water and drying in a muffle furnace. The organic matter in the top layer, with gelatinous texture and green and orange coloration, showed the most negative δ¹³C values (-21.52‰), and the corresponding precipitates from this layer had δ¹³C values of -5.72‰. These precipitates were identified as aragonite (CaCO₃) and magnesian calcite (Ca,Mg(CO₃)). The basal layer showed a characteristic similar to the top layer: the gray and gelatinous organic matter had a δ¹³C value of -18.37‰, and the precipitates had values of -3.95‰, also composed of Ca,Mg(CO₃), and CaCO₃, but with quartz (SiO₂). This correlation between organic layers and their respective precipitates is expected, as microorganisms preferentially use lighter carbon than heavier carbon, resulting in the crystallization of carbonates with light carbon signatures ^[2,3]. However, the intermediate layer, which is white, gelatinous, and rigid, showed a significant negative δ¹³C value of -16.35‰, while the precipitates had heavier δ¹³C values of -0.40‰ compared to the other precipitates from the basal and top layers. However, carbonate apatite (Ca₉Na_0,2(PO₄)_6,3F_1,0(OH)_1,1), apatite (Ca₅(PO₄)₃), halite (NaCl), and SiO₂ were also observed. Therefore, the hypothesis is considered that PO₄³⁻ captured Ca, forming Ca₉Na_0,2(PO₄)_6,3F_1,0(OH)_1,1 and Ca₅(PO₄)₃, thus inhibiting the precipitation of calcium and magnesium carbonate. Studies show that the potential origin of PO₄³⁻ may be linked to the decomposition of organic matter ^[2]. However, if this were the source, we would expect a lighter δ¹³C signature, since organic matter enriches this isotope in the environment. Therefore, the hypothesis is also considered that the input of PO₄³⁻ may be related to some factor of environmental change, such as human influence. In conclusion, our study on microbial mats from Lagoa Vermelha, cultivated in laboratory, provides insights into the complex dynamics of microorganisms and carbonate precipitates. The identification of the PO₄³⁻ suggests a possible role in inhibiting the precipitation of calcium and magnesium carbonate. While previous studies suggest that PO₄³⁻ may be related to the decomposition of organic matter, our results indicate a potential association of this ion with environmental changes, which may be influenced by human activity. Therefore, further investigations are needed to fully understand the exact mechanisms and potential anthropogenic impacts on these environmental processes.

References:[1] Dupraz, et al. 2009. Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews, 96(3), 141-162. [2] House, 1987. Inhibition of calcite crystal growth by inorganic phosphate. Journal of Colloid Interface Science, 119, 505-511. [3] Sumner, 2001. Microbial influences on local carbon isotopic ratios and their preservation in carbonate. Astrobiology, 1(1), 57-70.