When scientists from the University of Calgary recovered samples of dormant, heat-loving bacteria from two locations thousands of kilometers apart in the Atlantic Ocean, they were on a mission to prove something: These two kinds of thermophilic bacteria came from the same place.
Scientists have long suspected that the endospores (a dormant form of bacteria that can survive harsh conditions the bacteria otherwise wouldn’t) at both sites, Svalbard and the Labrador Sea, might share the same warm, anoxic origins and genetic history.
To test the hypothesis, a team of scientists scooped up sediments from the seabed and cultivated the endospores within a lab, with the goal of extracting DNA from the samples.
“Because endospores are very resilient, you can’t extract DNA from them [in their dormant form]. So you have to grow them in the lab, make them happy, keep them growing.”
“Because endospores are very resilient, you can’t extract DNA from them [in their dormant form]. So you have to grow them in the lab, make them happy, keep them growing. Then you can extract the DNA,” explained Brielle Hrymoc, a Ph.D. student in geomicrobiology at the University of Calgary who led the research.
Previous studies of thermophilic endospores, or thermospores, in cold marine sediments—for instance, a 2014 study led by Alexander Loy, a microbiologist at the University of Vienna—typically looked at a small or partial piece of their genomes.
“At that time, our sequencing options were more limited compared to now, and we could mainly only look at individual genes, which have limited resolutions in differentiating between closely related species and strains,” said Loy, who was not involved in Hrymoc’s research.
In the current work, which Hrymoc will present on 12 December at AGU’s Annual Meeting 2024 in Washington, D.C., the researchers aimed to reconstruct the full genomes of individual bacteria. A full genomic sequence can provide a wealth of information about bacteria, including where they come from and how they are adapting to their surroundings.
“From these two sites that are more than 4,000 kilometers away from each other, we recover a very similar thermophile.”
The novel finding, according to Hrymoc, is that “from these two sites that are more than 4,000 kilometers away from each other, we recover a very similar thermophile.” This result, she said, “suggests that there is strong evolutionary conservation between the two.”
That two genomes fetched from different locations in the northwestern and northeastern Atlantic are so closely related means that something must be seeding these bacteria across the world’s oceans. The researchers suggest it could be the huge chain of underwater volcanoes known as the Mid-Atlantic Ridge.
The Role of Mid-Ocean Ridges
Mid-ocean ridges occur when tectonic plates diverge at the bottom of the ocean. When the plates separate, ocean water can seep into the cracks, where it becomes extremely hot (up to 400°C, or 750°F) and creates a subsurface flow. Eventually, this flow bursts back into the sea via hydrothermal vents and circulates globally.
“The amount of water that gets taken [in at mid-ocean ridges] is the same as if you took all of the rivers in the world that discharge into the ocean, so we’re talking about massive-scale dispersal of seawater,” explained Hrymoc. This process of water seeping into and spouting out of the crust is ongoing, similar to how the water in a swimming pool is continuously filtered and circulated.
Oil reservoirs, which expulse gas and fluid from deep sediment layers, can also disperse endospores, but at a much smaller scale than mid-ocean ridges. So the team focused on mid-ocean ridges in this work.
Tracer of Microbial Dispersal
In their active form, thermophiles located around the mid-ocean ridge play an important role in the biogeochemical cycle in the ocean. But when they’re dormant on the cold seabed, the endospores act as a genetic information-storing seed bank, which can be germinated under ideal conditions for research purposes.
Thermospores are an ideal subject for scientists studying the spatial distribution patterns of ocean microbes because they have two distinct features that other microbes do not have: the ability to travel long distances and the ability to be dormant for extended periods.
Their dormancy means they can’t move on their own, so any movement is directly attributable to external dispersing forces. It also means they can survive extreme conditions, so an area without thermospores usually means they couldn’t get to that area via those forces, not that they couldn’t survive living there. Hence, by sequencing the DNA of thermospores in various locations, researchers can isolate the effects of passive dispersal on microbe distribution from other effects, such as environmental selection.
“Learning about what contributes to the diversity of microbial communities in the environment is important, and we may be progressing to answering the questions now as the techniques and methods of studying them have also evolved,” remarked Loy.
—Miriam Bahagijo, Science Writer