Functional profiling of coastal microbial mats from the Yucatan Peninsula: methane, nitrogen and sulfur metabolism

Beneath the surface of coastal lagoons, where fresh water meets the sea, lie thin, living carpets of microbes that act as the planet's invisible engines. These microbial mats are not merely slime; they are complex, layered cities where billions of organisms work in tight coordination to recycle the fundamental elements of life: carbon, nitrogen, and sulfur. In these microscopic neighborhoods, the chemistry of the environment shifts dramatically over just a few millimeters. Sunlight fuels the top layers, creating oxygen, while the deeper layers become dark and oxygen-free, allowing different teams of microbes to break down organic matter and release gases. Among the most critical of these processes are the creation and consumption of methane, a potent greenhouse gas, and the transformation of sulfur and nitrogen, which dictate how nutrients flow through an ecosystem. Understanding how these tiny communities function is essential for grasping how coastal environments regulate the climate and support the larger world of plants and animals.

In the Yucatan Peninsula, a region defined by its limestone landscape and extensive mangrove forests, researchers have turned their attention to these microbial mats to understand their hidden metabolic lives. While scientists have long known that these mats exist along the coast, often forming floating islands, flat crusts, or bumpy, pustular mounds, the specific genetic instructions that allow them to process methane, sulfur, and nitrogen remained a mystery. A team of scientists set out to decode this genetic potential by collecting samples from three distinct locations: the floating mats of Sisal, the flat and bumpy mats of Progreso, and the flat mats of Ría Lagartos. By using a technique called shotgun sequencing, which reads the collective DNA of all the microbes in a sample at once, the researchers could reconstruct the metabolic capabilities of these communities without needing to grow them in a lab. Their goal was to determine which chemical pathways were active and how the different types of mats might influence the release of greenhouse gases and the cycling of nutrients.

The study revealed that these mats are bustling with activity, dominated by specific groups of microbes that drive the methane cycle. The researchers found that the microbes responsible for creating methane, known as methanogens, were primarily a group called Methanosarcina. These organisms prefer to make methane by breaking down acetate, a simple organic compound, rather than using other methods. Interestingly, the mats also contained a rich community of methane-eating bacteria, or methanotrophs, which are even more abundant than the methane producers. The most common of these eaters was a genus called Methylobacterium. However, the balance between making and eating methane varied significantly depending on the type of mat and its location. The floating mats found in Sisal, which are associated with degraded mangrove areas, showed the highest potential for producing methane. In contrast, the flat mats from Progreso and Ría Lagartos were far more effective at consuming methane, acting as a filter that prevents the gas from escaping into the atmosphere. This suggests that the physical structure of the mat and the salinity of the water play a crucial role in whether a coastal ecosystem becomes a source of greenhouse gases or a sink that helps mitigate them.

Sulfur metabolism, another vital process in these salty environments, also showed distinct patterns across the different sites. The researchers identified a diverse array of bacteria capable of processing sulfur, with the most abundant groups being Desulfosarcina and Pseudodesulfovibrio. In most of the mats, the microbes were primarily using sulfur to build their own cellular structures, a process known as assimilatory reduction. However, the flat mats from Ría Lagartos were different; driven by the high salt content of their environment, these mats displayed a much higher potential for dissimilatory sulfate reduction, where sulfur is used to generate energy, and for re-oxidizing the resulting sulfide back into sulfate. This indicates a highly efficient, rapid sulfur cycle in the hypersaline conditions of Ría Lagartos. Meanwhile, the floating mats from Sisal showed a unique capacity for a sulfur relay system, which likely helps the community detoxify harmful sulfur compounds, a necessary adaptation for surviving in the fluctuating conditions of a degraded mangrove zone.

Nitrogen, the nutrient essential for plant growth, was found to be the most abundant metabolic theme in these mats, with nitrogen-related microbes making up nearly a quarter of the total community. The dominant players in this cycle were bacteria from the genera Streptomyces and Pseudomonas, which are versatile organisms capable of transforming nitrogen in multiple ways. The study found that the mats in Progreso, particularly those in restored mangrove areas, had a strong capacity for nitrogen fixation, the process of pulling nitrogen from the air to make it available for plants. This suggests these mats are actively enriching the soil. Conversely, the mats in Sisal and Ría Lagartos showed a much higher potential for denitrification, a process that converts nitrogen compounds back into gases that leave the ecosystem. This pathway can release nitrous oxide, another powerful greenhouse gas, implying that while these mats are vital for nutrient cycling, they may also contribute to atmospheric emissions in ways that differ from the more stable mats in Progreso.

Ultimately, this research paints a picture of functional diversity where the same basic microbial tools are used differently depending on the environment. The study confirms that while these mats share a core set of microbes capable of handling methane, sulfur, and nitrogen, their specific metabolic strategies are finely tuned to their local conditions. The flat mats of Ría Lagartos and Progreso appear to act as robust filters, consuming methane and retaining nitrogen, whereas the floating mats of Sisal are more likely to produce methane and release nitrogen gases. These findings highlight that the health and structure of mangrove ecosystems directly influence the chemical behavior of the microbial mats beneath them. By understanding these genetic potentials, scientists can better predict how coastal environments will respond to changes in salinity and vegetation, offering a clearer path toward managing these critical ecosystems to minimize greenhouse gas emissions and support biodiversity.