By Jacob Valdivieso Ojeda and Miguel Ángel Huerta Díaz 

The planet that we live on and call Earth was formed from the condensation of interstellar dust and gases about 4.6 billion years ago (Sagan, 2001). Ancient rock fossils indicate that life originated soon after (in geological terms), about 3.7 billion years ago, in shallow seas and oceans of the primitive Earth (Nutman et al., 2016). 

At that time, the environmental conditions on planet Earth were very different from those we experience today: the atmosphere and primitive ocean lacked dissolved oxygen and were rich in CO2, ammonia, methane and, generally speaking, gases that are harmful to most living beings today. 

Microorganisms were the dominant forms of life and triggered an astonishing and irreversible change in the Earth’s atmosphere: they began to produce molecular oxygen (O2), which, in time, would become an important part of the atmosphere. 

Microbial mats and their characteristics

Today, it is possible to find modern representatives of these ancient communities of microorganisms in the form of so-called microbial mats, which can be defined as accumulations of autotrophic microorganisms, which use light to capture carbon (e.g. photosynthetic cyanobacteria and diatoms), and heterotrophic microorganisms that use the carbon from autotrophic ones to make their own cells (e.g. sulfate- and methanogen-reducing bacteria). These different communities of microorganisms are located on laminated layers that range in size from millimeters to centimeters (Figure 1). 

Figure 1. Hypersaline microbial mat in Laguna Figueroa. The different layers of colors are made up of different groups of microorganisms.

In the beginning, these communities were abundant and present in almost all environments; however, their number decreased considerably about 680 million years ago, mainly as a result of the formation of the ozone layer, which allowed organisms to evolve that began to feed off them (Briggs & Crowther, 1990). One consequence of this process is that microbial mats today are only found in extreme environments with extreme temperature, salinity or light intensity. In other words, their habitat is restricted primarily to hypersaline environments (Van Gemerden et al., 1989).

Hypersaline environments are places where seawater evaporates at such high rates that water salinity may be increased from 33g/L (the current value for ocean water) to values reaching 250 g/L. Hypersaline environments abound in the Baja California peninsula, with one example being the lagoon named Laguna Figueroa on the west coast, 6 miles north of the San Quintín Bay (Figure 2).

Figure 2. Location and general characteristics of Laguna Figueroa in Baja California, Mexico. The microbial mats are mostly found in the orange flooding zone. 

A sand barrier about 12 miles long (Horodyski et al., 1977; Margulis et al., 1980) shapes Laguna Figueroa and protects and sets it apart from the adjacent Pacific Ocean. Its main source of water is the ocean, and this water is filtered through the dunes during high tide, which creates permanent or temporary hypersaline water wells. The lagoon is made up of a narrow strip of marshland and a large evaporite flat, where the microbial mats are located (Figure 3).

Figure 3. Hypersaline microbial mats in Laguna Figueroa with orange-pink coloration alongside halophilic plants. Salt crusts may be observed in the driest areas. At the top is the sand barrier that separates the lagoon from the ocean.

This lagoon was first studied in the late 1970s and early 1980s, mainly from evolutionary, ecological and geological perspectives (Margulis et al., 1980). However, despite being a unique environment with similar evolutionary significance to Cuatro Ciénegas (Coahuila, Mexico) or Shark Bay (Australia), it has remained practically forgotten and ignored by the scientific community in Mexico.

Why study the cycles of elements in microbial mats in Laguna Figueroa?

In the Oceanological Research Institute (IIO) of the Autonomous University of Baja California (UABC), in Mexico, we have spent several years studying the biogeochemical cycles of elements in microbial mats in hypersaline environments. This type of study offers an invaluable opportunity to use microbial mats as models to reconstruct and interpret their role in the evolution of life and the changes they may have caused in the development of the primitive Earth.

As a result of these studies, we have discovered that hypersaline microbial mats, such as those found in Laguna Figueroa, exhibit interesting characteristics. They are made up of a large accumulation of microorganisms, which together are able to trap large amounts of organic carbon, and they contribute to capturing part of the carbon that is found freely in the atmosphere as CO2, thus helping to mitigate the greenhouse effect caused by anthropogenic emissions of the gas.

In addition, microbial mats have a great capacity to produce O2, to an extent (up to 1000 µM min-1; Revsbech et al., 1983; Canfield & Des Marais, 1991) comparable to that of a rainforest, but on a millimeter scale for microbial mats. This is why mats played a significant role in the transformation of the chemical makeup of the Earth’s atmosphere and facilitated the evolution of new life forms. 

Although evaporation zones currently cover only a relatively small proportion of the Earth’s surface (15%; Warren, 2008), throughout the planet’s geological history, particularly during the Permian, these areas (including microbial mats) were substantially more abundant (Figure 4), up to five times the current level (75%; Warren, 2008). 

Figure 4. Paleolatitudinal distribution of evaporite deposits from the Permian to the present day. The inset graph shows the current latitudinal distribution of the surface area of the Earth covered by desert. Taken from Warren (2008). 

The aforementioned biogeochemical processes, such as the mats’ and hypersaline sediments’ ability to trap CO2, mean that these organisms may have a significant and global impact on the cycles of elements. Therefore, they may have played a more important role in the geological history of the Earth, influencing ocean chemistry and the biogeochemical cycles of elements, in addition to regulating the temperature of the atmosphere and, as a result, the Earth’s climate.

Microbial mats as analogs of possible ancient microbial life on Mars

One of the main reasons why, in the IIO, we continue to study the biogeochemical cycles of elements in modern microbial mats is the search for biosignals. We know that mats can produce biosignals that leave a trace in the fossil record in rocks (Riding, 2000; Arp et al., 2001). These biosignals may serve as proxies in modern environments in the same way fossils do, and this provides a geological record of an organism’s activity (Sageman & Lyons, 2005). 

In other words, biosignals (e.g. trace metals, isotopes) represent codified information on biodiversity, trophic associations and environmental conditions in the past. Additionally, they are the mark of biogeochemical processes in elements in microbial mats. For these reasons, studying them can help us to differentiate between elements that are or were used by mats, and those found naturally within the Earth’s crust.   

Thus, in the right conditions, the degree of accumulation or enrichment of certain elements may function as a biosignal. One of the most interesting discoveries we have made by studying the cycles of elements in mats – and which has drawn most attention from the scientific community – is that microbial mats tend to accumulate large quantities of some trace metals like molybdenum and cadmium. We have measured up to 300 times as much molybdenum in the mat as in the Earth’s crust (Valdivieso-Ojeda et al., 2014). This peculiar characteristic may serve as a biosignal of the presence of these groups of microorganisms in ancient environments on Earth – and could also be applied in ancient hypersaline areas of Mars (and even other planets), discovered by NASA’s Rover missions (Litchfield, 1998). 

The basic idea behind the study of microbial mats is that a biological indicator on Earth could be used to better interpret the characteristics of other planets. This was the case with the meteorite ALH84001 – it was necessary to search for and use biosignals that could determine the presence or absence of extraterrestrial microorganisms before it hit planet Earth (Figure 5). This explains why studying the distribution of metals in microbial mats as a biosignal may be a tool in the search for microbial life on other planets. 

Figure 5. Photographs of the meteorite ALH84001, which is a dark-brown diogenite weighing 1.931 kg. Discovered on December 27th, 1984 by a U.S. Smithsonian Institute expedition in Antarctica. (A) Outer part of the meteor and scale. (B) Micrography of the inside of the meteorite. Formations similar to microorganisms are observed. Credit: NASA/JPL.

Preserving our heritage

We hope that studying the biogeochemical processes in mats in hypersaline environments such as Laguna Figueroa will help to draw attention to the importance of these systems in regulating the Earth’s climate, as possible producers of biofuels, as a key part of the biogeochemical cycles of different elements, and as models in the search for extraterrestrial microbial life. 

We also hope that our research will further their preservation, care and study, given that Laguna Figueroa (and other hypersaline zones in Mexico) constitutes a 5,000-year-old hypersaline environment of a significance and evolutionary value matched by few places in the world, and which could be lost in a matter of years or months if it is not properly valued. 

References

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