Azure pools rich in magnesium and calcium carbonate but low in phosphorus provide an ideal habitat for ancient bacterial reefs at Cuatro Cinegas, in theChihuahuan Desert of Mexico.
By Rodrigo Prez Ortega Jun. 30, 2020 , 3:40 PM
Valeria Souza Saldvar never planned to devote her life to a remote and ancient oasis more than 1000 kilometers north of her laboratory in Mexico City. But a call in early 1999 changed that.
Its one of the best cold calls Ive ever made, says James Elser, a limnologist at the University of Montana. He had picked up the phone to invite Souza Saldvar to join a NASA-funded astrobiology project in Cuatro Cinegasa butterfly-shaped basin with colorful pools, or pozas, in the middle of Mexicos Chihuahuan Desert.
Neither Souza Saldvar, a microbial ecologist at the National Autonomous University of Mexico, University City, nor her ecologist husband and research partner Luis Eguiarte Fruns, also at UNAM, had ever visited Cuatro Cinegas. That first trip convinced them to completely change their research plans. Looking at those mountains and the water, I fell in love, Souza Saldvar says.
The landscapemore than 300 turquoise-blue pozas scattered across 800 square kilometers, among marshes and majestic mountainswasnt the only draw. The waters, whose chemistry resembled that of Earths ancient seas, teemed with microbes; unusual bacterial mats and formations called stromatolites carpeted the shallows. When Souza Saldvar first cultured the organisms from the pozas, The amount of microbes was enormous, as was the diversity of colors and colony sizes, she recalls. For her, this remote microbial hot spot was an irresistible mystery.
Since then, work by Souza Saldvar, Eguiarte Fruns, and a widening circle of collaborators in Mexico and the United States has shown that Cuatro Cinegaswhich means four marshes in Spanishis one of the most biodiverse places on the planet. Theres nowhere that has so much ancient diversity of microorganisms, says Michael Travisano, an evolutionary ecologist at University of Minnesota, Twin Cities, who has collaborated with the Mexican researchers since 2001. Among the most recent additions to that menagerie are hundreds of species of archaea, the ancient microbes that may have given rise to eukaryotesorganisms with complex, nucleated cells.
At the Pozas Azules ranch in Cuatro Cinegas, about 100spring-fed pools dapple the desert. Each has a unique microbial and mineral composition.
The diversity includes strains with unusual adaptations, such as the ability to build their lipid membranes with sulfur instead of the usual phosphorus, which is scarce in the waters of thepozas. It includes potential sources of new compounds for medicine and agriculture. And it poses a question that has occupied Souza Saldvar and Eguiarte Fruns for the past 20 years: How did this Noahs Ark of ancient microbes arise? Its a dream for every biologist to know the origin of diversification, Souza Saldvar says.
But her dream might be short-lived. Since the 1970s, farmers have intensively drained water from thepozasand rivers to irrigate nearby fields of alfalfa, grown for cattle fodder, gradually drying the improbable oasis. Souza Saldvar has galvanized a conservation effort that has slowed the drainage; in the coming weeks, a canal that removes 100 million cubic meters of Cuatro Cinegass water annually is scheduled to close. In the meantime, the researchers have been trying to describe as much as they can, as fast as they can, before their belovedpozasdry up and the precious microscopic life that has survived undisturbed for millions of years dies off.
Cuatro Cinegasservedas a stopping point for hunter-gatherers for thousands of years. To date, 50 archaeological sites with cave paintingssome dating to 2275 B.C.E.have been found in mountain cavesaround the basin. Much later, the region made a mark on history when Venustiano Carranza, born in a village at the basins margin, became a leader of the Mexican Revolution and president of Mexico from 1917 to 1920. Nowadays, the village is called Cuatro Cinegas de Carranza after him.
But in the 1960s, Cuatro Cinegas started to become famous for its biodiversity, as biologists began to describe new species of snails, fish, turtles, and plants found in the pools and marshesand often nowhere else.
Wendell Minck Minckley, a renowned ichthyologist at Arizona State University (ASU), Tempe, was first lured to Cuatro Cinegas after learning thatthe worlds only aquatic box turtle(Terrapene coahuila) lived there. Over the years, Minckley made frequent trips to thepozas, describing their snails and fish (Herichthys minckleyi, a cichlid, bears his name) while making connections with the local people.
In the Cuatro Cinegas Basin, ringed with mountains and desert, an aquifer feeds hundreds of pools and marshes. But canals tapping water for agriculture threaten the wetlands and the biodiversity they host.
(MAP) N. DESAI/SCIENCE; (DATA) E. MAMER AND T. NEWTON/NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES; VALERIA SOUZA SALDVAR; NATIONAL COMMISSION OF NATURAL PROTECTED AREAS MEXICO
Minckley also noticed peculiar, rocky structures in the pools. They were stromatolites, biological structures normally found as fossils dating back as much as 3.5 billion years. Colonies of photosynthesizing bacteria, which boosted early Earths oxygen, created the layered formations by depositing carbonates and trapping sediment in ancient, shallow seas. But these stromatolites were alive. Also found in other extreme environments such as Australias warm, salty Shark Bay, living stromatolites are sort of a window into early Earth, Elser says. Thepozasalso nurture bacterial mats, a soft form of stromatolites normally found deep in the ocean.
As early as the 1970s, Minckley realized the pools and their diversity were under threat: Local farmers were carving canals to tap their water. Thanks in part to his lobbying, the Mexican government in 1994 designated an 85,000-hectare protected area. But the drainage continued. Minckley knew that Cuatro Cinegas was going to die, Souza Saldvar says. He thought NASA might be its salvation.
In 1998, NASA established its Astrobiology Institute, a network of researchers studying life in extreme environments that might resemble conditions on other planets. Minckley saw an ideal astrobiology study site in the waters of thepozas, with their seemingly inhospitable chemistry and living stromatolites. But he was no expert on extreme environments, so he enlisted Elser, who specializes in how water chemistry affects ecosystems and also works at ASU. After they submitted a 1998 proposal to fund the project, however, NASA said they should add experts on microbiology and evolutionand those experts had to be Mexican to help secure permits to obtain samples. Based on colleagues suggestions, Elser called Souza Saldvar and Eguiarte Fruns, newly minted professors at UNAM. They joined, and NASAapproved the 3-year project.
Stromatolites, reeflike colonies of carbonate-secreting cyanobacteria, abounded in Precambrian seasand thrive at Cuatro Cinegas.
With two children in tow, the couple met Minckley and Elser at Cuatro Cinegas. Next to the turquoise-blue waters of La Becerrapoza, Minckley told them he believed the ecosystem was a glimpse of deep time. Do you see these miniature snails in my hand? Souza Saldvar recalls him saying. I just scooped them from the springhead, but their direct ancestors were eating sulfur bacteria in hydrothermal vents 220 million years ago in the bottom of the ancient Pacific.
Based on the water chemistrylow in phosphorus, iron, and nitrogenand the presence of living stromatolites, Minckley believed Cuatro Cinegas re-created the marine conditions found worldwide millions of years ago. He challenged the two researchers to explore its mysteriesand to protect itspozas. Only you, as Mexicans, can save them from the extinction caused by humans, Souza Saldvar recalls him saying.
Minckleydied2 years later, in 2001.
To inventory the full diversityof microbes at Cuatro Cinegas and trace their relationships, Souza Saldvar needed to study their DNA. To do so, scientists normally take microbial samples from a site and grow them in a lab. But many bacteria and archaea are difficult to culture, and only a few groups at the time had successfully analyzed DNA isolated directly from the environment. High magnesium levels in the water and slime from the microbes made isolating DNA from thepozasespecially difficult.
But Souza Saldvar and her students Ana Escalante and Laura Espinosa Asuar made a start. In 2006, they reported in theProceedings of the National Academy of Sciencesthat they had found 38 distinct groups of microbesfour times as many as in a typical salt marshcorresponding to 10 major lineages of bacteria and one of archaea. Half the bacterial groupswere most closely related to marine microbes. Almost 10% of the groups resembled ones that live on hydrothermal ventsfissures deep in the ocean where microbes thrive despite extreme heat and mineral concentrations.
As Minckley had suspected, Cuatro Cinegas had somehow preserved ancient marine life forms deep in the desert, more than 500 kilometers from the Gulf of Mexico, at a site where the last seas retreated some 20 million years ago.
Valeria Souza Saldvar and Luis Eguiarte Fruns (top) have spent 20 years studying biodiversity at Cuatro Cinegas, where they have found thousands of new species in living structures like a bacterial mat (bottom).
The deep time aspect [of Cuatro Cinegas] is very surprising, Travisano says. It is a true lost world, preserved by the hostile water chemistry, he and the Mexican team argued in a 2018 paper ineLife. Millions of years ago, they proposed, ancient marine ancestors found their way to the place,adapted to the extreme environment, and didnt change much.
Thepozasthemselves are not particularly ancient. The springs that nurture them are fed by deep aquifers in Sierra San Marcos y Pinos, filled with water accumulated during the last ice ages, Eguiarte Fruns says. Now, the water seeps to the surface because of an active fault beneath the basin. It rises through ancient marine sediments, picking up its unusual chemistry along the way. Somehow, the ancient microbespersisted and diversifiedin a succession of springs that must have appeared and vanished throughout geologic time. As in an ancient clock, Souza Saldvar says, all the original mechanisms are still working together to sustain unusual life.
To Frederick Cohan, a microbial ecologist at Wesleyan University who is not part of the Cuatro Cinegas project, the fact that many of the microbes are related to marine species and not species found inland is compelling. I think its saying those organisms are anciently there.
When the researcherslooked at the stromatolites, theyfound even more diversity. Samples from one site, Pozas Azules II, yielded more than 58,000 distinct microbial sequences, predominantly from bacterianot a direct count of species, but an indicator of biodiversity. In the Ro Mezquites, a stream that flows through the northern part of the basin and recharges several pools, they identified 30,000 sequences, mostly from cyanobacteria. More than 1000 sequences from Pozas Azules II appeared to be from archaea, the researchers reported inEnvironmental Microbiologyin 2009. The stromatolites also teemed with bacteria-infecting virusesstrains that wereunique to each pooland resembled marine viruses.
Studying the microbes hasnt been easy. There are thousands and thousands of new bacteria that we cant grow in culture, Souza Saldvar says. They could, however, identify some startling adaptations to the extreme conditions. In one bacterium found only in El Churince, a system of lagoons andpozason the western part of the basin, researchers sequenced the smallest genome ever found in its genus,Bacillus. The work, led by Gabriela Olmedo lvarez, a genetic engineer at Center for Research and Advanced Studies of the National Polytechnic Institute, Irapuato, also showed that the microbeB. coahuilensiscould synthesize membrane sulfolipids. This meant that, like some plants and cyanobacteria, it could use sulfur from the environmentinstead of phosphorusto form its cell membranes.
Shallow, mineral-rich pools and lagoons, with conditions like those in ancient oceans, are hot spots of microbial diversity. Floating mats at Cuatro Cinegas teem with the primordial microbes known as archaea, leading researchers to call them archaean domes.
(GRAPHIC) N. DESAI/SCIENCE; (DATA) GARCIA-MALDONADO ET AL., EXTREMOPHILES, DOI 10.1007/S00792-018-1047-2; CENTENO ET AL., MICOBIOLOY ECOLOGY, DOI: 10.1111/J.1574-6941.2012.01447
It likely stole these genes from a cyanobacterium, Olmedo lvarez says, enabling it to cope with scarce phosphorus, a condition thought to have prevailed in Earths earliest oceans. The microbes small genome may also have helped it thrive, as it required less phosphorus to build its DNA. Olmedo lvarez thinks the organism may offer a glimpse of the stratagems used by early microbes to adapt to their new environment.
Were just starting to understand the depth of diversity, says Olmedo lvarez, who found thatB. coahuilensisis itself starting tosplit into strainswith variations in phosphorus metabolism.
The low phosphorus conditions found in Cuatro Cinegas not only promoted local adaptations, but alsoaccelerated microbial diversification, Souza Saldvar and Elser argued in a perspective published in 2008 inNature Reviews Microbiology. Bacteria normally share bits of DNA with their neighbors in a process called horizontal gene transfer, which blurs the divisions between strains. But in Cuatro Cinegas, the microbeshungry for phosphorusessentially consume free DNA rather than incorporating it into their genomes. They will eat the DNA to get the phosphorus, Elser says.
Besides offering insights into evolution, Cuatro Cinegass microbial diversity may hold practical payoffs. Cuatro Cinegas is one of the richest places on the planet for genetic resources, Souza Saldvar says. For example, most modern antibiotics are derived from actinobacteria, which are abundant in thepozas. Susana De la Torre Zavala, a biotechnologist at the Autonomous University of Nuevo Len (UANL), University City, is searching for potential antibiotics in a library of 350 actinobacteria from the basin. Her team has also found that an extract from a microalga living in the poolsshows anticancer activity.
Agriculture, too, could benefit, Olmedo lvarez says. By 2050, the reservoirs of phosphorus that help sustain global harvests could become scarce, and the microbesability to concentrate the element from different sourcescould hold solutions. Were understanding Cuatro Cinegas, but were also understanding basic principles of ecological interactions that have an application in medicine and agriculture, she says.
As the scientific storyof Cuatro Cinegas unfolded, its fate has hung in the balance, with Souza Saldvar fighting a long series of battles over its water with local farmers and landowners, dairy companies, and politicians. Her weapons have been her rising scientific profile and a tireless outreach to the public, especially young people.
Souza Saldvar has drawn fireduring a 2013 microbiology congress, police had to protect her from protesting localsbut she has won a series of victories. In 2007, the daughter of the CEO of LALA, a giant dairy consortium with roots in the state of Coahuila, told her father she wouldnt speak to him because he was killing Cuatro Cinegas, Souza Saldvar says. The executive promptly scheduled a meeting with the scientist. You need to change your cows diet, Souza Saldvar says she told him, refusing to accept a courtesy yogurt he offered. Ill accept your yogurt when you do so. He promised not only to stop buying the regions alfalfa, but also to invest in environmental education projects for local children.
Two years later, she won an unusual ally, the powerful Mexican billionaire Carlos Slim. His foundation collaborated with the World Wildlife Fund (WWF) to buy the land surrounding El Churince in the western basin, and to provide researchers with a 5-year, 18 million Mexican peso ($1.4 million) grant to study Souza Saldvars favoritepoza. This allowed them to set up the infrastructure to perform long-term experiments. But it did not save the water.
Endemic fishes and turtles first drew scientists to Cuatro Cinegas, where they stumbled on its less visible microbial riches.
In 2010, Mexicos National Water Commission (CONAGUA) set out to replace the open, leaky canals, which lose 75% of the drained water, with less wasteful enclosed conduits. But the project was abandoned midwaymost likely because of corruptionand the old canals were never closed. As Cuatro Cinegas continued to dry up, the researchers raced to study El Churince, finding 5167 distinct species of bacteria and archaea in the last remaining pool. A close inspection of the genomes ofBacillusbacteria from one single square kilometer increased the known diversity of the group by more than 20%. By comparing DNA sequences, the team traced theBacillusdiversity to two ancient ancestors, one dating back 680 million years, the other 160 million years. Those dates coincide with the breakup of the supercontinents Rodinia and Pangaea, respectively, and the team thinks theoceans that formed during those convulsions carried the ancestral microbesto what is now the Cuatro Cinegas Basin, where they have persisted ever since.
Cohan says thats plausible.Bacillusfrom elsewhere fail to thrive in Cuatro Cinegas, most likely because they are outcompeted by the local microbes and cant adapt to the extreme conditions. And theBacillusspecies from Cuatro Cinegas are not found anywhere else in the world. Its just bizarre, Cohan says, but it makes thepozasso much more valuable and worth saving. Its kind of a paleontological microbial park.
In 2016, El Churince dried up just after the funding from the WWFCarlos Slim Foundation ended. The researchers felt devastated. Souza Saldvar says it was painful to see turtle shells lying on the now-barren soil. Its really sad, Olmedo lvarez says. Its gone.
On the eastern sideof the basin, things are looking brighter. In 2000, the conservation nongovernmental organization Pronatura Noreste acquired the Pozas Azules ranch: 2721 hectares hosting about 100pozas. Pronatura eventually gained rights to the water as well, enabling it to close canals draining thepozasin the ranch. Farmers are now encouraged to adopt water-sparing drip irrigation, and some are growing nopalan edible cactus popular in Mexican cuisinewhich requires much less water than alfalfa.
The researchers have focused their recent studies on Pozas Azules. In 2019, after an unusual spring rain, the team noticed alien-looking structures in the shallow waters of a site near Pozas Azules II: white microbial mats buoyed by gas. The gas appeared to be largely methane, and a genetic analysis showed the mats were teeming with archaea230 distinct species,they report in a preprint. That makes the spot the most diverse place of archaea that we know of, De la Torre Zavala says.
Now, the team hopes to analyze samples from the structures, which it calls archaean domes, in search of the elusive Asgard archaea, organisms previously found only in the deep ocean and thought tohold clues to the evolution of simple microbesinto complex eukaryotes. Although some in her team are skeptical, Souza Saldvar is convinced they will find them. Valerias usually right, De la Torre Zavala says.
Shaped and seeded with life by ancient seas, the Cuatro Cinegas Basin lies at the foot of the distant Sierra San Marcos. The white dunes bordering the basin are made of gypsum, a legacy of a Jurassic ocean.
Such prospects have added to Souza Saldvars determination to preserve Cuatro Cinegas, and she is enlisting young people for support. In every field trip since 2004, her team has spent time with students from the local high school, showing them how to use a microscope and take simple environmental measurements, and teaching them about sustainable agriculture. In 2011, with funding from the LALA Foundation and the WWFCarlos Slim Foundation, the scientists set up a college-level molecular biology lab at the school, which is now ranked among the best rural high schools in Mexico.
Hctor Arocha Garza is one of its graduates. Inspired by the secrets of Cuatro Cinegas, he pursued a Ph.D. in biotechnology at UANL with De la Torre Zavala, then returned to his hometown. My heart was in Cuatro Cinegas, he says. Now, hes leading the scientific branch of a privately fundedmegaproject called Cuatro Cinegas 2040that aims to build a science museum and make Cuatro Cinegas a scientific tourism destination, while supporting education and medical care for the villages young people.
The effort comes at a critical moment. More than 90% of the marshes are gone, and somepozasand lagoons are dry. But this year, CONAGUA committed toregulating water usageand closing illegal wells, and Pronatura Noreste will close the Saca Salada Canal, which drains the Ro Mezquites, as soon as the COVID-19 pandemic permits.
Those developments, and stories like Arocha Garzas, give Souza Saldvar hope for the future of Cuatro Cinegas. It has been a very complicated, long, and difficult process, she says. But now, she wrote in a recent book, There is a revolution occurring in this oasis: Science is the tool and kids are the drivers.
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Pools in the Mexican desert are a window into Earth's early life - Science Magazine
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