The ultimate system to understand our planet

Billions of years ago, Earth was home to extreme environments, including intense UV radiation, frequent volcanic eruptions, and very high concentrations of carbon dioxide in the atmosphere. Yet, under these conditions, biofilms formed and within these dense, slimy conditions, exciting things were happening.

Department of Earth Sciences and Department of Marine Sciences Professor Pieter Visscher has several recent papers exploring those exciting things, including the evolution of our earliest ancestors and processes that made life on planet Earth possible. Visscher says these findings emphasize our need to rethink how we look at the evolution of life on Earth, because slime played, and continues to play, a huge role.

Biofilms are communities of microorganisms, like bacteria, but also others like protists and viruses. They are home to the oldest forms of life on Earth, and Visscher says they have changed somewhat over time, but they are very likely still built the same today as they were three and a half billion years ago.

The microorganisms within the community all perform different functions; for example, modern biofilms are composed of long, cable-like filaments of blue-green algae, also known as cyanobacteria, that give the biofilm structural support, but they also secrete the exopolymeric substance (EPS) that gives the film a honeycomb structure and its slimy qualities.

The slime is critical, Visscher explains, because it helps biofilms sequester elements like calcium, magnesium, and even cadmium and arsenic. This process is how biofilms form the characteristic cement-like layers of these microbe-created structures, called microbialites.

As the microbialite accumulates more layers over time, they are deposited into the fossil record in structures called stromatolites. Evidence of these ancient biofilms is found in places like the Pilbara, Australia, and the Barberton, South Africa.

Modern (living) analogs of these exist in Shark Bay, Australia; Highborne Cay, Bahamas; Fayetteville Green Lakes in New York; and the Atacama Desert in Chile. Visscher performs many of his research studies at these sites, analyzing fresh and fossilized biofilms.

“The slime matrix plays an important role in the formation of these minerals,” Visscher says.

To investigate how important biofilms are to the global carbon cycle, Visscher and colleagues studied “whiting events,” which happen in the ocean and lakes when massive amounts of calcium carbonate are produced. They studied not only the water column but also the sediments.

Their findings were recently published in Communications Earth & Environment. They found that the most important processes to precipitate the carbonate minerals take place in the sediments, not in the water column.

“These events are important in the global carbon cycle because that calcium carbonate sinks, and eventually a lot of it ends up buried at the bottom,” Visscher says. “That’s how our planet became more livable, because that process was important to remove that carbon from the atmosphere.

“But what we discovered is the phytoplankton and a lot of them are cyanobacteria, in the water; they produce slime as well, but they do not necessarily produce carbonate minerals. Instead, their slime acts as a sponge for the calcium carbonate and then sinks to the bottom.

“Slime sequesters a lot of calcium, and acts like a conveyor belt going deeper, and we see in sediments that are 2,000 to 3,000 years old that the sediment microbes are still precipitating carbonate minerals actively there. This process has and continues to drive our climate for four billion years.”

Visscher says these findings about the role of biofilms show that we need to think about the long-term carbon cycle differently. They also provide clues about the evolution of our earliest ancestors. Organisms—such as humans—whose cells contain a nucleus are called eukaryotes, and our earliest eukaryotic ancestors may have developed thanks to slime.

“Our conclusions provide a clue for their evolution; microbialites and stromatolites are the oldest evidence of life on Earth. There’s always the discussion whether life started at hydrothermal vents or, as Charles Darwin said, a warm little pool, maybe an intertidal pool, or perhaps from geysers filling a lake. There is more evidence that this happened in the lake environment and indeed, the systems that we study in the modern environment appear there in very simplistic forms.”

In these dense biofilm communities, where the organisms are in close contact with one another, shielded from potentially extreme conditions, the slimy environment presents new evolutionary opportunities. Visscher and colleagues decided to look at freshwater and marine microbialites.

These findings were recently published in npj Biofilms and Microbiomes and though they found that the structures were similar in many ways, the communities were distinct, which may have influenced evolution differently in those systems.

To dig into this deeper, Visscher says another project he is currently working on is research on a type of archaea called Lokiarchaeota. These organisms were originally found near hydrothermal vents, but Visscher and colleagues started looking for them in the microbial mats in Shark Bay, where they found Lokiarchaeota in abundance.

“The idea is that the Lokiarchaeota and an alpha-proteobacterium together made the first eukaryotic cell, and this is the origin story that more and more people believe. The important thing is to find both of them in the stromatolites and in microbial mats and we thought to look at modern microbialites first.”

Visscher says that in modern mats, they found that within the well-organized stromatolite layer, sometimes they observe what is described as a “clotted” structure, or thrombolites. This messy appearance is due to the presence of protists—a kind of eukaryote—that come through and eat the structure-providing cyanobacteria. This helps piece together the evolutionary puzzle, says Visscher,

“The idea is that these protists could also play an important role as molecular indicators in evolutionary history. The oldest thrombolites, until recently, were believed to be about 1.9 billion years old. They found some in Australia that may be 2.3 billion years old.”

These findings by Visscher and his colleagues show the important roles slime played in the evolution of life on planet Earth. Biofilms were places where certain conditions were created that made life possible.

For example, biofilms existed even before oxygen, says Visscher. Before oxygen was freely available in the atmosphere, life depended on other elements like iron or arsenic for metabolic functions necessary for life. Studying the layers of ancient biofilms has allowed Visscher and others to piece together the conditions of early Earth and how life evolved under those extremes, which they detailed in another recent publication in the journal Geobiology.

“These biofilms essentially invented oxygen, and obviously the invention of oxygenic photosynthesis is very important for us, but how did these processes work before oxygen? This is where arsenic was very important,” says Visscher. “By following this through time, you can get some evidence of how these systems may have worked.”

There is still much to learn from biofilms, new and ancient. However, one thing is clear, by studying these systems, it seems we may owe a lot to slime, Visscher says.

“These biofilms are just remarkable. They may be the ultimate system to study to understand our planet.”