Analogues of ancient Earth oceans

Conditions on the surface of the ancient Earth are thought to have been drastically different than today. For much of the Archaean Eon, one of the earliest periods of Earth’s history (approximately 3.8-2.5 billion years ago), the Earth’s atmosphere was likely devoid of molecular oxygen and was rich (by today’s standards) in carbon dioxide and methane. The oceans for much of the Archaean Eon were rich in dissolved, ferrous iron, based on modern rock records, and were poor in sulfate and sulfide – quite different from the sulfate-rich and oxygenated oceans of today. This early period of Earth’s history is presumably when life would have arisen and early microbial metabolisms key to modern nutrient cycling would have evolved – things like photosynthesis, nitrogen fixation, and carbon and iron metabolisms. Early microbial ecosystems must have eventually exploited the open ocean, rich in iron – but how did these communities function? How did their anoxic and reducing ecosystem influence their evolution? And how did they, likewise, shape the conditions of the Earth? One limit to our ability to answer such questions comes from the huge ‘biogeochemical gap’ between our modern Earth and ancient oceans. Microbes and marco-scale life (like us) have adapted to take advantage of (or at least cope with) the high levels of molecular oxygen that are pervasive in many modern ecosystems, and aquatic environments analogous to the ancient oceans are hard to find. It’s challenging to test hypotheses of how ancient life may have functioned at an ecosystem scale, and it’s hard to seek out microorgansims that may have retained traits crucial to the success of early life.

In my research, I seek out novel ecosystems that can be used as analogues of ancient Earth oceans and probe these systems for the unusual microbial life they contain. Much of my graduate work focused on iron-rich Boreal Shield lakes at the International Institute for Sustainable Development Experimental Lakes Area (IISD-ELA; Ontario, Canada). We explored whether seasonally anoxic Boreal Shield lakes (that only develop O₂-free waters for part of each year, followed by seasonal mixing) could serve as a unique type of Archaean ocean analogue. These lakes are not permanently anoxic, but they are far more abundant globally than any other type of iron-rich and anoxic aquatic system currently known. We showed that these lakes can develop anaerobic microbial communities upon anoxia that are robust against seasonal mixing (Schiff et al., Sci Rep, 2017) and that these lakes harbour anoxygenic phototrophs in the Chlorobia class that have the genomic potential for iron oxidation, a poorly studied processes within this group that may have been highly relevant on ancient Earth (Tsuji et al., ISME J, 2020). We are continuing to sample diverse aquatic environments that can potentially serve as early Earth ocean analogues (leading to some great field work experiences!), and we are studying the microorganisms in these environments using a combination of environmental DNA/RNA sequencing and microbial enrichment cultivation. This work is shedding new light on poorly understood microbial metabolisms that could have helped shape the course of Earth’s history and is giving us a new window into the vast microbial diversity on our modern planet.