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Microbial Makeup of Beach Communities May Influence Resilience to Sewage Contamination

NEW YORK (GenomeWeb News) – While the bacterial community makeup of two distant beaches was highly similar, researchers led by Woods Hole Oceanographic Institution's Rebecca Gast found that there were also differences in diversity that may make one beach more resilient in the face of sewage contamination, as they reported in PLOS One yesterday.

Gast and her colleagues collected water, dry sand, and wet sand samples from an East Coast beach and a West Coast beach over the course of a few weeks, including during water-quality violations. By sequencing 16S hypervariable region ribosomal DNA, the researchers found that though the samples were similar overall, there were noticeable differences in composition.

All beaches contain some level of bacteria, but sometimes bacteria stem from sewage and may pose a health risk to beach-goers.

"Recreational beaches are important to the economies of many coastal towns, and understanding the causes and impacts of beach water quality violations is important in managing the problem," Gast said in a statement. "This work provides insight into environmental bacterial communities during putative contamination events, reflecting the potentially different sources of material."

Gast and colleagues collected water, wet sand, and dry sand from the Avalon Bay Beach in Catalina Island, Calif., and from the 333 Commercial Street beach in Provincetown, Mass. They extracted DNA from those 24 samples and sequenced them on the Roche Genome Sequencer GS-FLX. Some 8,050 sequence tags were randomly chosen to assess diversity among sample types and sites, and tags belonging to the orders Bacteroidales, Bifidobacteriales, and Clostridiales were extracted to assess fecal organism content.

The researchers also collected data regarding environmental factors like water temperature, tidal range, and recent rainfall, among other measures.

The Avalon beaches, the researchers noted, were more prone to water-quality violations.

From their set of samples, the researchers identified 39 phyla among the nearly 631,000 total and 2,350 unique bacterial sequence tags they recovered.

Overall, sand samples harbored greater bacterial community diversity than the water samples, and the Provincetown site had greater diversity than the Avalon site, the researchers reported.

No matter which beach the sample was from, though, proteobacteria, particularly those from the orders Alphaproteobacteria and Gammaproteobacteria, were found at high levels in water samples. Additionally, SAR11 sequence tags, belonging to Pelagibacter species were also common, comprising some 15 percent of sequence tags in Provincetown water samples and 25 percent of sequence tags in Avalon water samples. Bacteria from the phyla Bacteroidetes, especially the Flavobacteriaceae family, were also frequently found in water samples.

Similarly, most beach sand sequence tags from both spots belonged to the phyla Acidobacteria, Actinobacteria, Bacteroidetes, Proteobacteria, and Planctomycetes, which, the researchers said, is broadly similar to what's found in soil communities.

Despite that broad similarity, the researchers noted that there were significant differences in community structure based on sample type and sample location at the level of individual sequences. For instance, their analysis indicated localized differences in community composition based on the distribution of tags.

Part of those differences may be due to environmental variables at Provincetown, the researchers said, as sample temperature and tidal range influenced community structure. In an analysis of similarity of bacterial community composition, Provincetown samples from wet sand cluster closely together and dry sand samples cluster closely together. All Avalon sand samples, though, show great similarity whether or not they were dry or wet sand samples. This effect, the researchers said, could be due to the greater tidal range at Provincetown as compared to Avalon.

By turning to alternative fecal indicator bacteria — examining bacteria belonging to the orders Bacteroidales, Bifidobacteriales, and Clostridiales — the researchers found that water and sand samples from the same sites were more similar to each other than to other water or sand samples. This suggested to the researchers that there could be regional differences in how humans affect the environment.

Provincetown samples, they noted, appeared to be influenced by temperature, indicating that this subset of the total community may be influenced by temperature or that bacterial load co-varies with temperature.

"[A]lthough the species richness in samples from both sites was similar, the differences in community composition between samples collected at Avalon versus those collected at Provincetown may suggest greater ecological health and resiliency to contamination at Provincetown, where bacterial exceedance events occur less frequently and water quality is generally quite good," Gast and colleagues said.

They added that such community-based methods could be applied to a number of environmental samples and eventually may help trace the source of bacterial pollution at beaches.