Polycyclic Aromatic Hydrocarbon Degradation in the Sea-Surface Microlayer at Coastal Antarctica

As much as 400 Tg of carbon from airborne semivolatile aromatic hydrocarbons is deposited to the oceans every year, the largest identified source of anthropogenic organic carbon to the ocean. Microbial degradation is a key sink of these pollutants in surface waters, but has received little attention in polar environments. We have challenged Antarctic microbial communities from the sea-surface microlayer (SML) and the subsurface layer (SSL) with polycyclic aromatic hydrocarbons (PAHs) at environmentally relevant concentrations. PAH degradation rates and the microbial responses at both taxonomical and functional levels were assessed. Evidence for faster removal rates was observed in the SML, with rates 2.6-fold higher than in the SSL. In the SML, the highest removal rates were observed for the more hydrophobic and particle-bound PAHs. After 24 h of PAHs exposure, particle-associated bacteria in the SML showed the highest number of significant changes in their composition. These included significant enrichments of several hydrocarbonoclastic bacteria, especially the fast-growing genera Pseudoalteromonas, which increased their relative abundances by eightfold. Simultaneous metatranscriptomic analysis showed that the free-living fraction of SML was the most active fraction, especially for members of the order Alteromonadales, which includes Pseudoalteromonas. Their key role in PAHs biodegradation in polar environments should be elucidated in further studies. This study highlights the relevant role of bacterial populations inhabiting the sea-surface microlayer, especially the particle-associated habitat, as relevant bioreactors for the removal of aromatic hydrocarbons in the oceans.

The Pseudoalteromonas multipartite genome: distribution and expression of pangene categories, and a hypothesis for the origin and evolution of the chromid

Bacterial genomes typically consist of one large chromosome, but can also include secondary replicons. These so-called multipartite genomes are scattered on the bacterial tree of life with the majority of cases belonging to Proteobacteria. Within the class gamma-proteobacteria, multipartite genomes are restricted to the two families Vibrionaceae and Pseudoalteromonadaceae. Whereas the genome of vibrios is well studied, information on the Pseudoalteromonadaceae genome is much scarcer. We have studied Pseudoalteromonadaceae with respect to the origin of the chromid, how pangene categories are distributed, how genes are expressed relative to their genomic location, and identified chromid hallmark genes. We calculated the Pseudoalteromonadaceae pangenome based on 25 complete genomes and found that core/softcore are significantly overrepresented in late replicating sectors of the chromid, regardless of how the chromid is replicated. On the chromosome, core/softcore and shell/cloud genes are only weakly overrepresented at the chromosomal replication origin and termination sequences, respectively. Gene expression is trending downwards with increasing distance from the chromosomal oriC, whereas the chromidal expression pattern is more complex. Moreover, we identified 78 chromid hallmark genes, and BLASTp searches suggest that the majority of them were acquired from the ancestral gene pool of Alteromonadales. Finally, our data strongly suggest that the chromid originates from a plasmid that was acquired in a relatively recent event. In summary, this study extends our knowledge on multipartite genomes, and helps us understand how and why secondary replicons are acquired, why they are maintained, and how they are shaped by evolution.

Taxonomic annotation errors incorrectly assign the family Pseudoalteromonadaceae to the order Vibrionales in Greengenes: implications for microbial community assessments

Next-generation sequencing has provided powerful tools to conduct microbial ecology studies. Analysis of community composition relies on annotated databases of curated sequences to provide taxonomic assignments; however, these databases occasionally have errors with implications for downstream analyses. Systemic taxonomic errors were discovered in Greengenes database (v13_5 and 13_8) related to orders Vibrionales and Alteromonadales. These orders have family level annotations that were erroneous at least one taxonomic level, e.g., 100% of sequences assigned to the Pseudoalteromonadaceae family were placed improperly in Vibrionales (rather than Alteromonadales) and >20% of these sequences were indeed Vibrio spp. but were improperly assigned to the Pseudoalteromonadaceae family (rather than to Vibrionaceae). Use of this database is common; we identified 68 peer-reviewed papers since 2013 that likely included erroneous annotations specifically associated with Vibrionales and Pseudoalteromonadaceae, with 20 explicitly stating the incorrect taxonomy. Erroneous assignments using these specific versions of Greengenes can lead to incorrect conclusions, especially in marine systems where these taxa are commonly encountered as conditionally rare organisms and potential pathogens.

A novel thymidylate synthase from the Vibrionales, Alteromonadales, Aeromonadales, and Pasteurellales (VAAP) clade with altered nucleotide and folate binding sites

Thymidylate synthase (TS, E.C. 2.1.1.45) is a crucial enzyme for de novo deoxythymidine monophosphate (dTMP) biosynthesis. The gene for this enzyme is thyA, which encodes the folate-dependent TS that converts deoxyuridine monophosphate group (dUMP) into (dTMP) using the cofactor 5,10-methylenetetrahydrofolate (mTHF) as a carbon donor. We identified the thyA gene in the genome of the Vibrio parahaemolyticus strain FIM-S1708+ that is innocuous to humans but pathogenic to crustaceans. Surprisingly, we found changes in the residues that bind the substrate dUMP and mTHF, previously postulated as invariant among all TSs known (Finer-Moore, Santi & Stroud, 2003). Interestingly, those amino acid changes were also found in a clade of microorganisms that contains Vibrionales, Alteromonadales, Aeromonadales, and Pasteurellales (VAAP) from the Gammaproteobacteria class. In this work, we studied the biochemical properties of recombinant TS from V. parahemolyticus FIM-S1708+ (VpTS) to address the natural changes in the TS amino acid sequence of the VAAP clade. Interestingly, the K_m for dUMP was 27.3 ± 4.3 µM, about one-fold larger compared to other TSs. The K_m for mTHF was 96.3 ± 18 µM, about three- to five-fold larger compared to other species, suggesting also loss of affinity. Thus, the catalytic efficiency was between one or two orders of magnitude smaller for both substrates. We used trimethoprim, a common antibiotic that targets both TS and DHFR for inhibition studies. The IC₅₀ values obtained were high compared to other results in the literature. Nonetheless, this molecule could be a lead for the design antibiotics towards pathogens from the VAAP clade. Overall, the experimental results also suggest that in the VAAP clade the nucleotide salvage pathway is important and should be investigated, since the de novo dTMP synthesis appears to be compromised by a less efficient thymidylate synthase.

Salsuginimonas clara gen. nov., sp. nov., a member of the family Pseudoalteromonadaceae isolated from a brackish river

A bacterial strain designated LSN-49^T was isolated from a brackish river in Taiwan and characterized using a polyphasic taxonomy approach. Cells of strain LSN-49^T were Gram-staining-negative, aerobic, poly-β-hydroxybutyrate accumulating, motile by means of a monopolar flagellum, non-spore forming, straight rods and formed shiny and translucent colonies. Growth occurred at 20-40 °C (optimum, 25-30 °C), at pH 6-10 (optimum, pH 7-8) and with 0-3 % (w/v) NaCl [optimum, 0-1 % (w/v)]. The predominant fatty acids were summed feature 3 (comprising C16 : 1ω7c and/or C16 : 1ω6c), C17 : 1ω8c and C16 : 0. The polar lipid profile consisted of a mixture of phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylcholine, (PC), two uncharacterized aminophospholipids (APL1 and APL2), one uncharacterized glycolipid (GL1), four uncharacterized phospholipids (PL1-PL4) and four uncharacterized lipids (L1-L4). The major polyamine was putrescine. The major isoprenoid quinone was Q-8 and the DNA G+C content was 51.0 mol%. The results of phylogenetic analyses based on 16S rRNA gene sequences indicated that LSN-49^T formed a distinct lineage with respect to closely related genera in the family Pseudoalteromonadaceae. LSN-49^T was most closely related to Pseudoalteromonas, Algicola and Psychrosphaera and showed 89.3-92.1 % sequence similarity with members of the family Pseudoalteromonadaceae with validly published names. On the basis of the genotypic and phenotypic data, LSN-49^T represents a novel genus and species of the family Pseudoalteromonadaceae, for which the name Salsuginimonas clara gen. nov., sp. nov. is proposed. The type strain is LSN-49^T (=BCRC 81005^T=LMG 29726^T=KCTC 52439^T).

Evolution of a global regulator: Lrp in four orders of γ-Proteobacteria

Background

Bacterial global regulators each regulate the expression of several hundred genes. In Escherichia coli, the top seven global regulators together control over half of all genes. Leucine-responsive regulatory protein (Lrp) is one of these top seven global regulators. Lrp orthologs are very widely distributed, among both Bacteria and Archaea. Surprisingly, even within the phylum γ-Proteobacteria (which includes E. coli), Lrp is a global regulator in some orders and a local regulator in others. This raises questions about the evolution of Lrp and, more broadly, of global regulators.

Results

We examined Lrp sequences from four bacterial orders of the γ-Proteobacteria using phylogenetic and Logo analyses. The orders studied were Enterobacteriales and Vibrionales, in which Lrp plays a global role in tested species; Pasteurellales, in which Lrp is a local regulator in the tested species; and Alteromonadales, an order closely related to the other three but in which Lrp has not yet been studied. For comparison, we analyzed the Lrp paralog AsnC, which in all tested cases is a local regulator. The Lrp and AsnC phylogenetic clusters each divided, as expected, into subclusters representing the Enterobacteriales, Vibrionales, and Pasteuralles. However the Alteromonadales did not yield coherent clusters for either Lrp or AsnC. Logo analysis revealed signatures associated with globally- vs. locally- acting Lrp orthologs, providing testable hypotheses for which portions of Lrp are responsible for a global vs. local role. These candidate regions include both ends of the Lrp polypeptide but not, interestingly, the highly-conserved helix-turn-helix motif responsible for DNA sequence specificity.

Conclusions

Lrp and AsnC have conserved sequence signatures that allow their unambiguous annotation, at least in γ-Proteobacteria. Among Lrp orthologs, specific residues correlated with global vs. local regulatory roles, and can now be tested to determine which are functionally relevant and which simply reflect divergence. In the Alteromonadales, it appears that there are different subgroups of Lrp orthologs, one of which may act globally while the other may act locally. These results suggest experiments to improve our understanding of the evolution of bacterial global regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0685-1) contains supplementary material, which is available to authorized users.