Highly heterogeneous temporal dynamics in the abundance and diversity of the emerging pathogens Arcobacter at an urban beach.

While the significance of Arcobacter in clinical settings grows, the ecological dynamics of potentially pathogenic Arcobacter in coastal marine environments remains unclear. In this study, we monitored the temporal dynamics of Arcobacter at an urban beach subject to significant stormwater input and wet weather sewer overflows (WWSO). Weekly monitoring of bacterial communities over 24 months using 16S rRNA amplicon sequencing revealed large, intermittent peaks in the relative abundance of Arcobacter. Quantitative PCR was subsequently employed to track absolute abundance of Arcobacter 23S rRNA gene copies, revealing peaks in abundance reaching up to 108 gene copies L-1, with these increases statistically correlated with stormwater and WWSO intrusion. Notably, peaks in Arcobacter abundance were poorly correlated with enterococci plate counts, and remained elevated for one week following heavy rainfall. Using oligotyping we discriminated single nucleotide variants (SNVs) within the Arcobacter population, revealing 10 distinct clusters of SNVs that we defined as Arcobacter "ecotypes", with each displaying distinct temporal dynamics. The most abundant ecotype during stormwater and modelled WWSO events displayed 16S rRNA sequence similarity to A. cryaerophilius, a species previously implicated in human illness. Our findings highlight the diverse environmental drivers of Arcobacter abundance within coastal settings and point to a potentially important, yet overlooked exposure risk of these potential pathogens to humans.

Urban beaches are environmental hotspots for antibiotic resistance following rainfall.

To reveal the occurrence and mechanisms for dispersal of antibiotic resistance (AbR) among the microbial assemblages inhabiting impacted coastal environments, we performed a weekly, two-year duration time-series study at two urban beaches between 2014 and 2016. We combined quantitative PCR and multiplex PCR/reverse line blot techniques to track patterns in the occurrence of 31 AbR genes, including genes that confer resistance to antibiotics that are critically important antimicrobials for human medicine. Patterns in the abundance of these genes were linked to specific microbial groups and environmental parameters by coupling qPCR and 16S rRNA amplicon sequencing data with network analysis. Up to 100-fold increases in the abundance of several AbR genes, including genes conferring resistance to quinolones, trimethoprim, sulfonamides, tetracycline, vancomycin and carbapenems, occurred following storm-water and modelled wet-weather sewer overflow events. The abundance of AbR genes strongly and significantly correlated with several potentially pathogenic bacterial OTUs regularly associated with wastewater infrastructure, such as Arcobacter, Acinetobacter, Aeromonas and Cloacibacterium. These high-resolution observations provide clear links between storm-water discharge and sewer overflow events and the occurrence of AbR in the coastal microbial assemblages inhabiting urban beaches, highlighting a direct mechanism for potentially significant AbR exposure risks to humans.

Diversity and Activity of Diazotrophs in Great Barrier Reef Surface Waters.

Discrepancies between bioavailable nitrogen (N) concentrations and phytoplankton growth rates in the oligotrophic waters of the Great Barrier Reef (GBR) suggest that undetermined N sources must play a significant role in supporting primary productivity. One such source could be biological dinitrogen (N2) fixation through the activity of "diazotrophic" bacterioplankton. Here, we investigated N2 fixation and diazotroph community composition over 10° S of latitude within GBR surface waters. Qualitative N2 fixation rates were found to be variable across the GBR but were relatively high in coastal, inner and outer GBR waters, reaching 68 nmol L-1 d-1. Diazotroph assemblages, identified by amplicon sequencing of the nifH gene, were dominated by the cyanobacterium Trichodesmium erythraeum, γ-proteobacteria from the Gamma A clade, and δ-proteobacterial phylotypes related to sulfate-reducing genera. However, diazotroph communities exhibited significant spatial heterogeneity, correlated with shifts in dissolved inorganic nutrient concentrations. Specifically, heterotrophic diazotrophs generally increased in relative abundance with increasing concentrations of phosphate and N, while Trichodesmium was proportionally more abundant when concentrations of these nutrients were low. This study provides the first in-depth characterization of diazotroph community composition and N2 fixation dynamics within the oligotrophic, N-limited surface waters of the GBR. Our observations highlight the need to re-evaluate N cycling dynamics within oligotrophic coral reef systems, to include diverse N2 fixing assemblages as a potentially significant source of dissolved N within the water column.