by Steven Wilhelm, University of Tennessee; Brittany Zepernick, University of Tennessee and Robert Michael McKay, University of Windsor

How light shapes life underwater?

Winters on the Great Lakes are harsh. So harsh, in fact, that scientists who work there often focus on the summer months, when the tiny microbes at the base of the food chain are thought to be at their most productive.

However, new research is changing our understanding of these winter ecosystems, shedding light on a vibrant world of winter activity just beneath the ice.

Loss of winter ice is changing the Great Lakes food web

Scientists discovered in the early 2000s that communities of diatoms—tiny photosynthesizing algae—flourished in the light beneath the lake’s windswept ice. But it turns out that was only part of the story.

As winter ice disappears from the Great Lakes — reaching record lows in the winter of 2023-2024 — new analyses show that some diatoms appear to have another way to generate energy and survive in the dark, murky, ice-free waters into summer.

These microbes are critical to the health of the Great Lakes. They clean the water of pollutants and are the first step in the complex food web that supports a fishery that drives part of a regional economy. Changes here can have widespread effects on the ecology of the lakes and direct economic effects on surrounding communities.

Rising from the ice

Interest in life beneath the ice grew in 2007, when an international team of scientists aboard a Canadian Coast Guard icebreaker noticed something unusual as the ship sailed through the ice of Lake Erie.

When the ice broke, dark brown water seeped out of the lake, teeming with diatoms.

Sporadic studies of winter microbes have been conducted in the past, but limnologists – scientists who study lakes – have not had the tools to fully understand the microbes’ behavior until recently.

For the past five years, the U.S. Department of Energy’s Joint Genome Institute has supported a molecular biology project that has sequenced the RNA of all the microorganisms in samples collected from Lake Erie to investigate how these organisms have survived the winter months and may or may not have adapted to future climate scenarios. New observations about how diatoms may use light are now emerging from this effort.

Using proteins commonly found in animal eyes

Normally, we think of diatoms as organisms that use sunlight to convert carbon dioxide into living material through photosynthesis. They are ubiquitous in the summer in the Great Lakes, where they help fuel the lakes’ multibillion-dollar sport and commercial fisheries.

In the winter, diatoms can create energy from light filtering through wind-swept ice. However, when there is no ice in the winter, diatoms are mixed with lake water that can sometimes be best described as chocolate milk. Light has difficulty passing through this murky water, and the diatoms get less of the specific wavelengths of light that drive photosynthesis.

We collected samples during the winter of 2019-2020 to compare how diatom communities in open waters differed from those living under ice. We were surprised to find that when ice was absent, some diatoms used a different form of energy acquisition – driven by a pigment called rhodopsin.

Rhodopsins are light-sensitive proteins that are perhaps best known as a major component of animal eyes. In marine systems, these proteins were shown in 2001 to be involved in energy generation in bacterial cells, specifically in the production of adenosine triphosphate, or ATP. ATP is a chemical that organisms use as an energy source for many cellular processes, leading to the nickname “molecular currency” of living cells.

It appears that some diatoms in Lake Erie use this energy-generating mechanism to increase photosynthesis during the ice-free, low-light winter months.

Differences in the two processes can be important: Photosynthesis helps cells fix carbon to produce new biomass, as well as cellular energy in the form of ATP. In rhodopsins, ATP is produced, but there is no direct carbon fixation.

This means that cells can probably survive but not grow in these murky waters. But in biology, survival is everything: if an organism’s competitors don’t survive the harsh conditions but the organism does, there will be more nutrients when conditions improve. For this reason, the rhodopsins in these diatoms seem to be both a survival mechanism and a chance to survive in murky, ice-free winter conditions.

Watching life in the lake evolve as the climate changes

As Lake Erie and other temperate lakes to the north become warmer and ice-free, these data suggest that the diatoms that thrived in ice-covered lakes may be replaced by diatoms with rhodopsins during the winter months.

The consequences of this change are potentially manifold: Small changes at the base of the food web can affect fisheries. In addition, some diatoms are known to produce compounds that are toxic to wildlife and humans.

We currently have only guesses about how changes in algal species will affect fisheries, tourism, and coastal resource management in the long term. How algal communities change over time is a response to many factors, and light is just one of them. But the chance to see this change from the beginning creates a unique opportunity to understand the effect of a warming climate on the Great Lakes and similar lakes worldwide.

Steven Wilhelm, professor of microbiology, University of Tennessee; Brittany Zepernick, postdoctoral researcher in microbiology, University of Tennesseeand Robert Michael McKay, Director and Professor, Great Lakes Institute for Environmental Research, University of Windsor

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Published by Laura B.

I am a licensed librarian with over 30 years of experience as an environmental information professional. I am currently the Sustainability Information Curator for the Illinois Sustainable Technology Center and also manage the center’s strategic communications. View all posts by Laura B.