Climate Change

Microalgae are adapting to warming climate, declining nutrient levels in seas with novel strategy

Study finds microalgae are firing up a light-responsive protein to use sunlight for growth

By Rohini Krishnamurthy
Published: Wednesday 18 October 2023
Photo: iStock

Microalgae, which form the base of the food chain in the ocean and capture carbon dioxide from the atmosphere, appear to rely on a unique strategy to cope with global warming, according to a new study. The findings are published in the journal Nature Microbiology.

As climate change reduces the availability of nutrients in the sea, marine microalgae or eukaryotic phytoplankton fire up a protein called rhodopsin. It is related to the protein in the human eye responsible for vision in dim light. This light-responsive protein is helping the microalgae flourish with the help of sunlight in place of traditional chlorophyll. 

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Microbial rhodopsins, per the study, are proposed to be major light capturers in the ocean. Estimates suggested they may absorb as much light as chlorophyll-based photosynthesis in the sea, which also captures light to generate energy and food. However, their biological role in these organisms was unclear before the study.

“Global warming is increasing drought on land and the same thing happens in the ocean: The warmer the surface water gets, the lower are the nutrients in these surface water layers,” Thomas Mock, professor of marine microbiology in University of East Anglia’s School of Environmental Sciences and the study’s author, said in a statement.

There is less mixing between the surface waters and nutrient-rich deeper waters as the oceans warm. So nutrients become scarce at the surface, impacting the primary producers such as microalgae that are present in the top layer.

Algae starve and, therefore, produce less food and capture less carbon dioxide from the atmosphere, Mock explained. 

Read more: Study finds fish surprisingly resistant to marine heatwaves

In these areas,  the capacity of algae to make food and take up carbon dioxide should be much more reduced, the researchers said. This is akin to reduced crop yield on land if iron- and nitrogen-rich fertilisers are scarce. 

To understand the role of rhodopsins, researchers cloned them in the lab and confirmed that they capture light to generate energy (Adenosine triphosphate or ATP — the energy currency of all cells).

They also tested the abundances of rhodopsin transcripts (a molecule of ribonucleic acid or RNA that contains genetic information copied from deoxyribonucleic acid or DNA). Rhodopsins were found to be more concentrated in low latitudes, where there is less mixing of ocean waters and lower concentrations of nutrients, including dissolved iron.

“For algae to produce food and to remove carbon dioxide from the atmosphere, they need sunlight,” Mock noted. To harness sunlight, the microalgae require a lot of iron. However, 35 per cent of the surface of the ocean does not have enough iron to support the growth of algae, he explained.

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This is particularly relevant for the Southern Ocean, which is the largest iron-limited aquatic ecosystem. But they are home to the largest populations of consumers such as krill, fish, penguins and whales, which depend on primary producers such as microalgae.

These findings, according to the authors, have the potential to reduce the negative effects of changing environmental conditions, such as ocean warming and even the reduction in the productivity of crops. 

The same mechanism could be deployed to enhance the activity of microbes that cannot use light, such as yeast. “We can modify them so that they can use light for growth, which is desirable in biotechnology, such as the production of insulin, antibiotics, enzymes, antivirals and even biofuel,” Mock added.

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