Lake Cadagno: A Window to Earth's Ancient Past

Deep within the Swiss Alps lies Lake Cadagno, a fascinating meromictic lake captivating scientists. Its unique properties offer a glimpse into Earth's conditions before the Great Oxidation Event, a time when oxygen was scarce and anaerobic life thrived.

The Mystery of Meromictic Lakes

Lake Cadagno, formed over 10,000 years ago, is one of approximately 200 known meromictic lakes worldwide. What sets these lakes apart is their distinct layers of water that never fully mix. Lake Cadagno features two such layers:

• The Upper Layer: A typical freshwater environment, teeming with fish and safe for swimming.
• The Lower Layer: Beginning just 13 meters below the surface, this layer is a dense, sulfurous, oxygen-free pool inhospitable to multicellular life.

Why Don't the Layers Mix?

In most lakes, the water mixes, distributing oxygen throughout. However, in Lake Cadagno, the layers remain separate due to differences in chemical composition. Rainwater feeds both layers, but takes different paths:

1. The Top Layer: Rainwater flows directly into the lake from the surrounding granitic mountains.
2. The Bottom Layer: Rainwater seeps into the Piora Valley's dolomite, a porous rock rich in salts like sulfate. As the water travels through the dolomite, it loses oxygen and absorbs salts, becoming denser. This heavier, salt-rich water flows into the lake from sublacustrine springs, forming the anoxic bottom layer.

A Haven for Anaerobic Life

The oxygen-free lower layer of Lake Cadagno provides a sanctuary for anaerobic bacteria, organisms that perished during the Great Oxygen Catastrophe. These microorganisms thrive in the unique microenvironments created by the sublacustrine springs.

• These bacteria consume sulfate and release toxic sulfide.
• At the boundary between the layers, a blanket of Chromatium okenii, a pink-bodied photosynthesizing bacterium, utilizes sulfur in place of oxygen.

An Unexpected Food Chain

Despite the separation, the two layers are not entirely isolated. Chromatium okenii reside at the top of the bottom layer to maximize sunlight exposure. While they remain within the oxygen-free zone, zooplankton from the upper layer venture down to feed on them, then return to the oxygenated waters. This relationship forms the base of the upper layer's food chain, supporting the lake's abundant fish population.

Modeling Earth's Ancient Past

Lake Cadagno's unique anaerobic ecosystem offers scientists a valuable opportunity to study conditions resembling those before the Great Oxygen Catastrophe. For instance, the Chromatium okenii blanket increases the water's density, causing it to sink. The microorganisms then swim upwards, creating a subtle mixing of water known as bioconvection. This ancient phenomenon may provide insights into how early life developed the ability to swim.

Unlocking the Secrets of Early Life

Lake Cadagno continues to be a source of scientific discovery, offering clues about the evolution of life on Earth and the processes that shaped our planet.