Beneficial bacteria help these marine worms survive extreme cold

Antarctic marine worms survive with a little help from their bacterial friends.

Close relatives of earthworms, polychaetes are some of the most common animals on the ocean floor, but how these species survive the low temperatures of Antarctic waters has been a mystery. Other Antarctic species, like icefish, make their own antifreeze proteins, “but most Antarctic organisms do not produce these proteins,” says Cinzia Corinaldesi, a marine ecologist at Marche Polytechnic University in Ancona, Italy. (SN: 1/13/22).

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Specialized bacteria living inside three different species of Antarctic polychaetes make proteins that help the worms not freeze to death, Corinaldesi and colleagues report June 21 in Science Advances.

The finding illustrates how important microbes can be for their hosts, says Amy Apprill, a microbial ecologist at Woods Hole Oceanographic Institution in Massachusetts who wasn’t involved with the study. “Our knowledge of host-microbe interactions in the ocean is still incredibly limited.”

Corinaldesi’s colleagues traveled to three coastal areas of Antarctica in the Ross Sea and scooped up ocean sediment containing three common marine worm species — two that survive on scraps of dead organisms and one that is a predator. The water temperature at these sites, the team reports, was around –1° Celsius. In the lab back in Italy, Emanuela Buschi, now a researcher at Anton Dohrn Zoological Station in Fano, analyzed DNA from the worms to see what microbes called them home.

The most common bacteria found in the worms were Meiothermus silvanus and two types of Anoxybacillus, which were not found in a separate analysis of the sediment itself or within DNA collected from other related worms.

Bacterial proteins extracted from the polychaetes are known to be involved in cold tolerance. Two enzymatic proteins, for example, produce glycerol and proline, which are thought to protect against extreme cold “due to their ability to reduce the freezing point of internal liquids,” Corinaldesi says.

Though missing from today’s Antarctic ocean floor, Meiothermus bacteria have previously been found in frozen sediment from underneath the nearby Ross Ice Shelf, which “suggests a long-term connection between polychaetes and these bacteria,” Corinaldesi says. She and colleagues think that the bacteria are passed down from parent worms to their offspring.

Confirming that young wormlings get the helpful microbes from their parents would require analyzing the microbiome of the polychaetes at different stages of their lives, Apprill says. Newborns may also pick up the bacteria from other worms that they interact with.

The bacteria benefit from their partnership with the worms too, Corinaldesi says, as they receive a safe home in exchange for making protective proteins.

Corinaldesi is now continuing to explore the relationships between Antarctic organisms and their microbes, and how those relationships evolved. Studying Antarctic microbiomes, she says, “will allow us to understand many secrets of life adaptation to extreme conditions.”