Abstract

This study determines organic-matter (OM) composition in the different color layers of a stratified hypersaline microbial mat and verifies the hypothesis that each layer includes a distinct group of lipids. The relation of precursor lipids from the microbial mat to the hydrocarbon composition in fossil records was also evaluated. To that end, the composition was studied of glycolipids (GLs), phospholipids (PLs), and “neutral” lipids (NLs, including hydrocarbons, n-alkanols, sterols, hopanols, free fatty acids, and wax esters) in four different color layers (A–D; depth intervals: up to 0.5 cm, 0.5–1.0 cm, 1.5–3.0 cm, and 3.0–6.0 cm, respectively) of a stratified hypersaline mat from the Vermelha Lagoon, Rio de Janeiro, Brazil.

Microscopic characterization revealed the presence of 16 cyanobacterial morphospecies, with predominance of Microcoleus chthonoplastes. The notable prevalence of saturated straight-chain fatty acids (FAs), n-16:0 and n-18:0 and their monounsaturated counterparts, n-16:1 and n-18:1 in all three lipid fractions (GLs, PLs, and NLs), associated with the domination of n-C₁₇ alkane and n-C_17:1 alkene among the hydrocarbons confirmed the main imprint of cyanobacteria. The composition of the studied lipid classes implies the contribution of sulfate-reducing bacteria such as Desulfomicrobium sp. strain, purple sulfur bacteria, as well as the possible input of Geobacter spp. and Desulfovibrio spp., particularly in the deeper layers.

The notable decrease in total extractable lipids (TELs) yield from layers A to D indicates that lipid synthesis is far more intense by photosynthesizing cyanobacteria than by anaerobic microorganisms. The content of PLs was uniform and low (< 5%) in all layers, implying their extremely quick degradation. GLs, followed by NLs, were the most abundant in all layers indicating the medium, which is characterized by carbon source excess and limited nitrogen source, which regulates microorganism growth. The upper layers, A (green) and B (reddish-brown) differ from those lower, C (dark brown greenish) and D (brown) according to the NLs/GLs ratio, which is higher in the former.

The lipid compositions reveal distinctions between the individual layers in the microbial mat. The observed layers clearly differ according to the amount of high-molecular-weight (C₂₂–C₃₁) n-alkanes and long-chain (C_21–C₃₀) n-alkanols, the content of phytol, bishomohopanol, tetrahymanol, C₂₇–C₂₉ sterols, the stanol/stenol ratio in the neutral lipid fraction, as well as the content of branched (iso and anteiso) FAs and w9/w7 FA ratio in the GLs fraction. The mentioned parameters imply a greater contribution of sulfate-reducing and purple sulfur bacteria to layer B, higher impact of photosynthetic red algae in upper layers A and B, the elevated contribution of marine ciliate species, feeding on bacteria to layers B and C, as well as the increment of anoxygenic phototrophic and heterotrophic bacteria to layer D. The greatest capability for the synthesis of hydrocarbons is observed in layer B.

The composition of lipid classes in the microbial mat showed a significant relationship with the most important biomarkers' fingerprints in the source rocks extracts and petroleum derived from the carbonate hypersaline environments, including the distribution of n-alkanes, a high abundance of phytane and gammacerane, as well as a distribution of C₂₇–C₂₉ regular steranes. Therefore, these results offer an insight into the transformation of microbial OM during the sedimentation processes in a hypersaline environment and its contribution to the fossil record.