Investigating Lake Superior’s Sulfur Dynamics: Insights into Primitive Earth

by Henrik Andersen
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Lake Superior sulfur cycle

A recent study on the sulfur dynamics within Lake Superior provides new insights, mirroring the sulfur cycle of Earth’s oceans billions of years ago. This research emphasizes the importance of organic sulfur in understanding the early chemical environment of Earth and microbial evolution.

The study underscores the pivotal role of organic sulfur in biogeochemical cycles.

Geochemist Alexandra Phillips, deeply engaged with sulfur research, is exploring its cycle in the environment, focusing particularly on its role in ancient oceans. Her research benefits from the nutrient-depleted waters of Lake Superior, which act as a proxy for Earth’s oceans billions of years ago. Phillips, previously associated with UC Santa Barbara and the University of Minnesota, Duluth, and her team have unearthed a novel sulfur cycle in Lake Superior. Their work, detailed in Limnology and Oceanography, highlights the significance of organic sulfur in these cycles.

Exploring Sulfur Forms: Sulfate and Hydrogen Sulfide

Sulfate ions (SO4) dominate sulfur’s presence in nature, especially in seawater. At the bottom of oceans and lakes, where oxygen is scarce, certain microbes convert sulfate into hydrogen sulfide (H2S). This hydrogen sulfide can either be rapidly used by other microorganisms or remain trapped in sediments for extended periods. Genomic evidence suggests this microbial activity has been occurring for over 3 billion years.

Lake Superior’s low sulfate levels provide valuable insights into the biochemistry of Earth’s early oceans, a finding attributed to Alexandra Phillips.

However, the abundance of sulfate is believed to have increased approximately 2.7 to 2.4 billion years ago due to the oxygenation caused by cyanobacteria’s photosynthetic activities. This raises questions about the sulfate source for ancient microbes.

Phillips, a marine and climate scientist with expertise in oceanography, geochemistry, and geobiology, also focuses on the role of organic sulfur in aquatic systems and advocates for diversity in STEM through social media.

The Importance of Organic Sulfur

Phillips has shifted her focus to organic sulfur – sulfur bonded with carbon. In current oceans, organic sulfur is much less prevalent compared to sulfate, but in environments like Lake Superior, where sulfate is scarce, organic sulfur becomes more crucial.

Sergei Katsev, a professor at the University of Minnesota’s Large Lakes Observatory and the project’s senior scientist, emphasizes the need to understand processes in sulfate-depleted environments to comprehend early Earth’s conditions.

Lake Superior as a Proxy for Ancient Oceans

Lake Superior’s sulfate levels are significantly lower than those of modern oceans, resembling the chemical conditions of ancient oceans with minimal sulfate. Phillips investigates how sulfur cycling might have operated under these conditions, considering which microbes are involved in sulfate reduction, their preferred organic sulfur compounds, and the fate of the produced hydrogen sulfide.

In their fieldwork, Phillips and her team collected water and sediment samples from Lake Superior, focusing on oxygen-rich and oxygen-poor sites. They utilized shotgun metagenomics to identify sulfate-reducing microbes, finding numerous such organisms in sulfate-rich sediment layers.

Examining Microbial Preferences for Organic Sulfur

The researchers experimented with various organic sulfur forms to determine microbial preferences, discovering a preference for sulfo-lipids over sulfur amino acids. Despite the energy cost, this preference is less energy-intensive than the energy gained from converting sulfate to hydrogen sulfide.

Further, they found that sulfo-lipids, produced by other microbes, were more abundant in sediments.

Investigating the Fate of Hydrogen Sulfide

Phillips next explored what happens to the hydrogen sulfide produced in this cycle. In modern oceans, it can form pyrite through reaction with iron or produce organic sulfur compounds through reaction with organic molecules. The study revealed significant organic matter sulfurization in Lake Superior, indicating organic sulfur as both a source and a sink in the sulfur cycle.

A New Perspective on the Sulfur Cycle

This newly discovered cycle, involving transformations between organic sulfur, sulfate, and hydrogen sulfide, offers a fresh perspective on aquatic systems’ sulfur dynamics. Phillips emphasizes the need to consider organic sulfur as a central component in nutrient-poor environments like Lake Superior or the ancient ocean.

Morgan Raven, a biogeochemist at UC Santa Barbara and the study’s senior author, suggests that while this organic sulfur cycling is likely common in marine and freshwater sediments, it is overshadowed by the abundance of sulfate in the ocean. Lake Superior’s low-sulfate environment reveals the dynamic nature of the sedimentary organic sulfur cycle. Organic sulfur appears to not only be a microbial energy source but also plays a role in preserving organic carbon and molecular fossils. This understanding could shed light on the evolution of early sulfur-cycling microorganisms and their impact on Earth’s chemistry.

Phillips believes sulfur was integral to early life forms and their metabolic processes. A deeper understanding of the sulfur cycle could offer insights into the redox chemistry utilized by early life.

Reference: “Organic sulfur from source to sink in

Frequently Asked Questions (FAQs) about Lake Superior sulfur cycle

What is the main focus of the study on Lake Superior’s sulfur cycle?

The study focuses on understanding the sulfur cycle in Lake Superior, which provides insights into the sulfur dynamics of Earth’s ancient oceans. It emphasizes the role of organic sulfur in early microbial evolution and the primitive Earth’s chemistry.

How does Lake Superior’s environment contribute to this study?

Lake Superior’s nutrient-poor waters mimic the conditions of Earth’s early oceans, particularly in terms of low sulfate levels. This similarity allows researchers to draw parallels and gain insights into the sulfur cycling processes of ancient times.

Who is Alexandra Phillips and what is her role in this study?

Alexandra Phillips is a geochemist with expertise in oceanography, geochemistry, and geobiology. In this study, she leads the research on sulfur cycling in Lake Superior, exploring how this process might have occurred in Earth’s ancient oceans.

What new insights does this study bring to the understanding of Earth’s early oceans?

The study reveals a novel sulfur cycle in Lake Superior, underscoring the importance of organic sulfur in environments with low sulfate levels. This new perspective helps to understand the biogeochemical processes of Earth’s early oceans and the evolution of microbial life during that era.

How does the study of Lake Superior’s sulfur cycle impact our understanding of early microbial life?

By examining the sulfur cycle in an environment similar to ancient Earth’s oceans, the study provides insights into the types of biochemical processes early microbes might have used. It highlights the importance of organic sulfur in early microbial metabolisms and the evolution of life on Earth.

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