Green Pages

Microbial Mats

The past and the future intertwine on Little Ambergris Cay.

By Rachel Craft ~ Photos By Usha Lingappa

Little Ambergris Cay, an uninhabited 1,600-acre island in the Turks & Caicos, is home to birds, turtles, rock iguanas, and more. But the majority of its denizens are hidden beneath the muddy surface of the island’s mangrove swamps. What could easily be mistaken for a thick layer of mud are in fact intertidal microbial mats—entire ecosystems made up of billions of bacteria and other micro-organisms. 

Entire ecosystems are hidden beneath the muddy surface of Little Ambergris Cay’s mangrove swamps.

Although their individual constituents can’t be seen with the naked eye, microbial mats boast a level of biodiversity comparable to that of the Amazon rainforest. They form at interfaces between land and water. They range from a few millimeters to tens of centimeters thick and contain several layers, each dominated by a different type of microbe. The topmost organisms feed on sunlight, while the next layer feeds on the top layer’s byproducts, and so on, forming a self-sufficient food chain within each mat. Most organisms secrete slime or filaments, which hold the mat’s tiny inhabitants together and provide structural integrity.

Microbial mats are some of the oldest living forms of life for which there is substantial fossil evidence. The oldest known microbial mat fossils are roughly 3.5 billion years old. To put that in perspective, dinosaurs appeared roughly 230 million years ago, and humans didn’t arrive on the scene until 2 million years ago. Because they’re so ancient, scientists think microbial mats may hold clues to the origins of life on Earth—and possibly other planets. 

That’s what brought Dr. Usha Lingappa, then a graduate student at Caltech studying geobiology, and her colleagues to the Turks & Caicos in 2017. While microbial mats have likely sprung up elsewhere in TCI, Little Ambergris Cay has the best conditions for mat growth. The small island consists of a bedrock rim surrounding a tidal lagoon, where rising and falling water levels promote mat formation. The lagoon is protected from strong waves that could wash mats away, and the lack of human activity and large animals minimizes damage to the mats. As a result, Little Ambergris Cay has “absolutely resplendent microbial mats,” Lingappa says. 

Lingappa’s team was interested in how photosynthetic microbes, like the ones in microbial mats, have shaped Earth’s environment over time. Some of their research focused on determining how similar current mats are to ancient mats, and using current mats to glean information on how ancient mats evolved. “What’s cool about mats is they give us a window into what life might have been like on Earth before the rise of plants and animals,” Lingappa says. 

Individual microbes—which were the dominant life-forms on Earth 3.5 billion years ago—are too small to be recognizable as fossils, but microbial mats are a different story. Their structures are large and complex enough to recognize as fossils billions of years later. This can be tricky, as some rock features are easily confused with microbial mats, but Lingappa says there are clues to look for. Rock layers tend to stay consistently parallel, while microbial mat layers will be thicker on topographical high points and thinner on low points. This is because microbial mat growth is dependent on sunlight. Sand grains trapped within layers can also give away a microbial mat fossil, because mats are sticky enough to trap sand grains that would normally roll off of rocks and other non-living surfaces.

Looking beneath the thick layer of mud, scientists discovered microbial mats underneath the cay’s tidal lagoon.

Some scientists are also using mats to narrow down the search for life on other planets. Because no one has found evidence of extraterrestrial life—yet—Lingappa says, “We don’t really know what we’re looking for.” One approach to this challenge is studying the most extreme forms of life on Earth, which are mostly microbial and can often be found in the form of mats.

Besides the intertidal mats flourishing on Little Ambergris Cay, there are many other types of mats on Earth, some living in extreme environments—like near underwater thermal vents, which are heated by magma and can reach over 700ºF, or in salt marshes that are too salty for other species. These mats give scientists an idea of what life might look like elsewhere, both past and present. Even if life does not exist on Mars today, it may have existed in the past. Scientists are studying traces of ancient microbial mats on Earth and looking for the same microscopic fossilized “fingerprints” on other planets. 

Besides shedding light on the past, microbial mats play a key role in Earth’s present. Chemically, they help cycle things like carbon, nitrogen, and sulfur and provide nutrients to surrounding ecosystems. Physically, they help hold down and stabilize the shoreline. Microbial mats, especially those in mangrove ecosystems, also contribute to carbon sequestration—drawing carbon dioxide from the air and storing it in the sediment.

Although Lingappa says this type of carbon sequestration isn’t a magic bullet for climate change, it is encouraging. Intertidal ecosystems—swamps, lagoons, and similar environments where microbial mats form—contribute disproportionately to carbon sequestration, meaning they can store a larger fraction of carbon from the air than the fraction of the earth’s surface area they take up. If these mats disappeared, Lingappa says, all the carbon stored within them would be suddenly released back into the atmosphere. 

That’s why Lingappa’s team also studied microbial mats’ resilience to environmental shifts. They went to Little Ambergris Cay in 2017 to investigate how changing sea levels might impact the microbial mats there. What they learned:  When they cut away slices of mat and transplanted them to different elevations, the mats continued to grow. Although the microbe types within transplanted mats stayed the same, the mats changed texture—from dry and leathery to wet and goopy or vice versa, depending on the water level at their new elevation. 

During this study, category-5 Hurricane Irma swept through the Islands, and the team’s outdoor research lab. Although Irma devastated many areas in the Caribbean, its silver lining was that it allowed Lingappa’s team to study firsthand how microbial mats responded to being flooded, covered in sediment, or uprooted and deposited elsewhere by severe storms. Irma decimated large swathes of Little Ambergris Cay’s microbial mats, but new mats grew rapidly over surfaces exposed or deposited by the hurricane. In fact, the mats grew much faster after the hurricane than they normally would while undisturbed, and Little Ambergris’s mat communities largely recovered from Irma within two years. 

This “slice” of a microbial mat shows the various layers, each dominated by a different type of microbe.

Mats’ rapid regrowth after being decimated by hurricanes or transplanted to different elevations suggest these lifeforms excel at adapting quickly to environmental disruption—which, due to climate change, is on the rise.

Lingappa says it’s encouraging to know that these mats could recover from storms or rising sea levels, but she stresses that climate change isn’t that simple. “One of the things that’s scary about climate change is that it’s going to cause a lot of different things to happen,” she says, including extreme weather events like hurricanes, rising sea levels, and changing temperature and chemistry in both seawater and the atmosphere. “While we can study one of these effects at a time, we really don’t know how they will interact . . . As we see these impacts happening, there are going to be surprises.” More research is needed to fully understand how mats will respond to the bigger picture of climate change. 

To this end, more studies are underway in TCI. Dr. Lizzy Trower and her team from the University of Colorado, Boulder, have been working on Little Ambergris Cay to better understand how microbial mats might help stabilize sediment after a hurricane. By trapping sediment delivered by storms, mats have the potential to help islands keep pace with rising sea levels. Trower’s team is also interested in microbial mats on other islands in the Turks & Caicos, and is exploring a potential collaboration with TCI Community College.

Lingappa says that while scientists have known about microbial mats for decades, they only recently got the DNA sequencing technology to start understanding mats’ inner workings. “There are so many different microbes in the mats,” she says, “and most of them, we don’t even know what they do.” Mat research in places like Little Ambergris Cay could reveal new information on Earth’s past—and its future.

Rachel Craft is a Colorado-based writer and recovered engineer who loves all things outdoors. You can learn more about her at www.racheldelaneycraft.com.



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