Human impact on seafloor microbiomes may be 'bigger than anybody ever considered'

May 14, 2021

That shipwreck is providing habitat for sea creatures, but what else is it doing? (Pexels/Leonardo Lamas)

A yacht that sank almost 80 years ago has drastically altered the microbiome of its seafloor environment, raising questions about the role of artificial features, such as shipwrecks and oil rigs, in promoting habitat growth.

About 2,000 historic shipwrecks, having sunk over 50 years ago, lie at the bottom of the Gulf of Mexico, and their influence on microorganisms such as fish, crustaceans and other aquatic organisms is better-documented than the vessels' effect on seafloor microbiomes. With a study published April 22 in The ISME Journal, researchers sought to address this gap by analyzing the microbiotic diversity around the shipwreck Anona.

"The footprint of this one shipwreck is significantly larger than the footprint of natural habitat features," said lead author Leila Hamdan, an associate professor of marine microbial ecology at the University of Southern Mississippi. "And this entity has been on the seafloor for a significantly less amount of time than those natural habitat features, so our impact on seafloor microbiomes, because of the stuff that we put there, may be bigger than anybody ever considered."

Anona was a luxury yacht built in 1904 that had been pressed into cargo-carrying duty during World War II, according to Hamdan. Bound for the West Indies with potatoes aboard, it capsized in the Gulf of Mexico in 1944. All on board survived. Today it sits about 1,258 meters below the surface of the Gulf, roughly 115 miles south of Mississippi's Gulf Coast. 

Hamdan was able to begin studying Anona in 2014 while stuck on another ship waiting for a storm to pass before moving to shore. She sensed a unique opportunity to indulge her curiosity about how the island theory of biogeography applies to shipwrecks.

This theory, developed in the 1960s by biologists Robert MacArthur and E.O. Wilson, essentially states that, depending on the amount of resources present and distance from a population source, more species should appear the closer one moves toward an island, which could be any ecosystem surrounded by unsuitable habitat, such as a mountaintop or a literal island in the middle of the ocean. 

"We, as the scientific community, have made some assumptions that these rules apply to elephants and birds and all other things, so they should probably apply to microbes, but that's not always the case," Hamdan said in an interview with The Academic Times. "I wanted to know, do the rules around how many organisms live on island habitats or island-like habitats apply to microbes, and do those rules apply in the deep sea?"

To determine the bacterial diversity around Anona, Hamdan and her colleagues took 138 sediment samples from a range of points up to 1,000 meters away from the ship, analyzed the DNA that was present and performed statistical analyses to assess how the amount of diversity changed moving away from the wreck.

The island theory of biogeography suggests that diversity would decline farther away from the ship, but Hamdan and her colleagues saw that diversity increased up to the 200-meter mark, then fell off, indicating the presence of a transition zone, or ecotone, where ship and seafloor microbes meet. 

"Where those two places meet, suddenly you have all the players present in the same place at the same time," Hamdan said. "That actually helped us figure out where the boundary between the shipwreck habitat in the seafloor was, and that was the thing that helped us say how big of a footprint this has on the local environment."

While Hamdan and her colleagues did not study microbial formation on Anona, the ship is likely providing multiple opportunities for bacteria to grow into a biodiverse halo of microorganisms happy to have found such a "luxurious habitat," according to Hamdan. The materials that make up the ship, from steel, to iron, to wood — which is food for bacteria — will eventually leach into the substrate surrounding it, and this leaching may account for the high microbiotic biodiversity around the ship.

"We can't explain the microbe that's 200 meters out as eating the ship, but it's clearly responding to something or some condition that that ship created," Hamdan said. "Imagine you're a mile down and 100 miles off the coast, and suddenly there's something as rich as a food source as a potato. Someone's going to come along and eat that."

The Anona's potatoes are long gone. But Hamdan argued that the rest of the ship has effected an influence on its environment far more than natural seafloor features, such whale falls, seeps or hydrothermal vents — features that have existed across geological time periods. According to the study, given the large number of shipwrecks still yet to be studied or even found in the Gulf of Mexico, as well as its 45,000 miles of oil and gas pipeline, it is important to better understand how these artificial features are affecting microbiome diversity. 

"It is an impressive footprint, and if you keep adding more and more to the seafloor, at some point, the benefits of all of this biodiversity will have to reach a tipping point where that good thing is no longer a good thing," Hamdan said.

In 2019, Hamdan's group discovered two additional shipwrecks in the Gulf of Mexico, adding to a running database of 12 total shipwrecks the researchers intend to study in the future. In future research, one of Hamdan's main goals is to reproduce the results she has published here. 

"In 100 years from now, we might have 100% more stuff on the seabed than we do at this moment," Hamdan said. "And if we don't capture the baseline of that as soon as can, we're really never going to know what the natural seafloor looks like because we're changing it every day."

The study, "Deep-sea shipwrecks represent island-like ecosystems for marine microbiomes," published April 22 in The ISME Journal, was authored by Leila J. Hamdan, Justyna J. Hampel, Rachel D. Moseley, Rachel L. Mugge and Anirban Ray, University of Southern Mississippi; Jennifer L. Salerno, George Mason University; and Melanie Damour, Bureau of Ocean Energy Management.

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