High potential for the biosynthesis of neutral lipid storage compounds in chronically-polluted subantarctic sediments

Abstract

Microorganisms in intertidal sediments of Ushuaia Bay (Tierra del Fuego, Argentina) are adapted to extreme conditions, including low temperatures, high UV-B radiation levels and the presence of various environmental pollutants. Due to tidal cycles, these organisms are also exposed to periods of drought, as well as rapid changes in temperature, salinity and nutrient availability. Members of a limited number of phyla are known to accumulate wax esters (WE) and triacylglycerol (TAG) as an adaptation response to stressful environmental conditions similar to those present in intertidal sediments of Ushuaia Bay. The goal of this work was to study the abundance and diversity of bacteria with the potential to biosynthesize these neutral lipid storage compounds in intertidal sediments polluted with aliphatic and polycyclic aromatic hydrocarbons, retrieved near a pier of a fuel storage facility. Homolog sequences of the key enzyme for WE and TAG biosynthesis, the wax ester synthase/acyl-CoA diacylglycerol acyltransferase (WS/DGAT), were identified in a metagenomic dataset from sediments of this site. Out of the 682,972 protein coding sequences of the dataset, 166 contained the wax ester synthase-like Acyl-CoA acyltransferase pfam domain commonly used to identify this enzyme (PF03007, E-value ≤10−5), the 74% of them full-length. A WS/DGAT C-terminal domain (PF06974) was also detected in the majority of the sequences. The relative abundance of WS/DGAT homolog sequences in the dataset was 1.42 ± 0.18 times the number of sequences of single-copy genes coding for ribosomal proteins (average ± standard deviation of 12 genes), suggesting a high prevalence of WE/TAG biosynthesis potential in the microbial community. Sequences were highly diverse, as 108 and 44 clusters were recovered using distance thresholds of 80% and 40% identity at the amino acid level, respectively. Furthermore, 64% of the putative enzymes shared low to moderate identity values with WS/DGAT homologs identified in bacterial genomes, indicating the presence of novel organisms with WE/TAG biosynthesis potential in the sediments. The taxonomic assignment of scaffolds containing WS/DGAT homologs (1 to 43.4 Kb, N50 = 35 Kb) indicated that members of the Actinobacteria (46 %), Proteobacteria (33 %), Bacteroidetes (3 %) and Acidobacteria (1 %) phyla could be the origin of the majority of the scaffolds, while 17% of them could only be assigned to Bacteria. These results suggest the presence of phylogenetically diverse and abundant microbial populations with the potential to biosynthesize neutral lipid storage compounds in intertidal sediments of this polluted site. This study is the starting point for more in-depth analyses of these metagenomic fragments, in order to increase our understanding of the mechanisms used by these diverse bacterial populations to adapt to environmental stressors in this extreme environment.