Chesapeake Bay bacteria thrive in copper far above the EPA’s short-term limit

Bacteria grown on test panels in the Chesapeake Bay survived copper concentrations above 70 milligrams per liter, thousands of times higher than the 18 micrograms per liter that the U.S. Environmental Protection Agency sets as its short-term limit for copper spikes in seawater. The finding, from scientists at the U.S. Naval Research Laboratory and Duke University, was published on October 13, 2026 in Biointerphases. It suggests that marine microbes have adapted to the copper coatings that ships use against fouling.

Two weeks on a panel in the bay

The experiment was simple in design. The team placed pieces of PVC panel in the Chesapeake Bay, some with a copper-based coating and some uncoated, and left them for two weeks. They then extracted DNA, sequenced the genomes of the bacterial colonies that had formed, identified copper-resistance genes, and tested how much copper each colony could tolerate, according to the release from AIP Publishing.

The most striking result came from the controls. Bacteria from colonies that had not initially grown on the copper-coated surfaces turned out to be just as copper-resistant as the rest, and could thrive at concentrations above 70 milligrams per liter. In other words, tolerance was not confined to the survivors of the coating; it was widespread in the community.

The paper’s name refers to early-stage marine biofilms, and that detail frames the finding. A biofilm is the first slimy layer of microbes that coats a submerged surface, and in the first two weeks it forms the foundation on which larger organisms settle. If the pioneers of that layer shrug off copper, a coating that releases the metal to keep a hull clean may be fighting a community already equipped for it. The researchers sequenced the genomes of the colonies specifically to find out which copper-resistance genes were present, which is what ties the high tolerance to adaptation rather than to a one-off quirk of a single colony.

Seventy milligrams against eighteen micrograms

The units matter here. Seventy milligrams per liter is 70,000 micrograms, so the colonies tolerated roughly 3,900 times the 18 micrograms per liter figure the release cites as the EPA short-term seawater limit. The two numbers are not a like-for-like safety comparison, since the limit protects marine life from a spike while the bacteria were exposed to copper-based water treatments in testing, but the gap shows how far beyond ordinary exposure these colonies can go. Older EPA criteria documents carry still lower saltwater values; the agency’s 1984 copper criteria list a saltwater maximum concentration of 2.916 micrograms per liter.

Kenan Fears, an author of the study, said his team had observed increasing copper resistance in some of the ports it frequents over the past decade. He also noted that copper was not always in a water-soluble state: it only became so after oxygen accumulated in Earth’s atmosphere, a reminder that microbes have had a long evolutionary history with the metal. His observation about ports comes from his team’s own sampling experience, and the experiment described in the release covers a single location, the Chesapeake Bay, over a single two-week window. Those limits mean the result is best read as strong evidence that copper tolerance exists in coastal bacteria, not as a map of where it is most common or how fast it is spreading.

Why antifouling coatings are under pressure

Biofouling is the buildup of barnacles, algae and other marine life on a hull, and copper-based coatings are widely used to stop it. The authors treat the high resistance as a likely marker of evolutionary adaptation to those coatings. They also point to the regulatory side: “With increasing regulatory pressure on copper use in antifouling coatings, there is an urgent need for alternatives,” Fears said.

The researchers outline two directions. One is to target the molecular pathways that let bacteria express copper-resistance genes, weakening their survival in copper-rich settings. The other is to design coatings that favor microbial communities which either deter larger fouling organisms such as barnacles or tubeworms, or at least do not interfere with the release of biocidal compounds from the coating. Both ideas treat the bacteria as part of the solution rather than only the problem, since a coating that nudges the early microbial community in a useful direction could reduce the need to rely on copper alone. The release stops short of claiming that any such coating exists today; it frames them as avenues the new genetic data make possible.

The paper is titled “Extreme copper tolerance and genomic adaptation in early-stage marine biofilms.” Its authors are Kailey N. Richard, Sara M. Tuck, Sophie M. Colston, Maryssa A. Beasley, W. Judson Hervey, Kevin Le, Sean Brown, Katherine J. Franz and Fears. It appears in Biointerphases, an AVS journal published by AIP Publishing, under the DOI 10.1116/6.0005640, and the release is accompanied by an image credited to Fears.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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