An international research team with participation from Würzburg has discovered how algae compensate for nutrient deficiencies. Their discovery could help counteract the negative effects of climate change.
One of the building blocks of ocean life can adapt to cope with the effects of climate change, according to new research led by the University of East Anglia (UEA). The discovery holds promises for biotechnology developments that could counter the negative effects of changing environmental conditions, such as ocean warming and even the reduction in the productivity of crops.
Corresponding authors of the study, now published in the journal Nature Microbiology, are Thomas Mock, Professor of Marine Microbiology in the School of Environmental Sciences at UEA, and his former PhD student Dr. Jan Strauss. At Julius-Maximilians-Universität Würzburg (JMU), Professor Georg Nagel and Dr. Shiqiang Gao from the Department of Neurophysiology at the Institute of Physiology were involved.
Base of the largest food web on Earth
Looking at eukaryotic phytoplankton, also referred to as microalgae, found over large parts of the ocean, the international team led by UEA's Prof Thomas Mock discovered the algae have found a way to cope with nutrient starvation, which is predicted to increase due to warming waters. This is good news for the food chain - marine microalgae are the base of the largest food web on Earth including krill, fish, penguins, and whales - as well as pulling CO2 from the atmosphere and producing oxygen.
"For algae to produce food and to remove CO2 from the atmosphere, they need sunlight," Prof Mock says. The dilemma, though, is that the cellular machinery for using sunlight requires a lot of iron. However, 35 percent of the surface of the ocean does not have enough iron to support the growth of algae. "In these areas algal productivity should be much more reduced, similar to land plants such as crops that lack iron- and nitrogen-rich fertiliser, meaning crops will not grow that well," explains Mock.
"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 because of reduced mixing that usually adds nutrients from the deeper ocean. Hence, algae are supposed to starve and therefore produce less food and take up less CO2 from the atmosphere."
Without iron and sunlight, it will not work
The research team discovered that algae have found a way to cope with nutrient starvation, by evolving an additional cellular machinery that allows them to use sunlight for growth without the need for iron.
Dr Strauss, who continued the research project together with Prof Mock after graduating from UEA while working as a postdoctoral scientist at the European Molecular Biology Laboratory (EMBL) in Hamburg and GEOMAR, Helmholtz Centre for Ocean Research Kiel, Germany, explains: "Some groups of microalgae can circumvent photosynthesis by using a light-driven proton pump to fuel growth".
Instead of being reliant on photosynthetic proteins that require iron (to generate ATP, the energy currency of all cells), algae use a light-responsive membrane protein that is related to one in human eyes: rhodopsins. These proteins do not require iron and one specific group of them pumps protons through membranes, which enables synthesis of ATP, which is a main function of photosynthesis in all photosynthetic organisms.
Basic research at the University of Würzburg
This is where Georg Nagel and Shiqiang Gao - two experts in the field of rhodopsin research - come in. During the collaborative work at Prof. Georg Nagel's lab, Dr. Shiqiang Gao cloned these diatom rhodopsins and confirmed their effective proton pump capabilities, even at low temperatures, using electrophysiological methods after heterologous expression in frog oocytes.
"Initially this work was very frustrating, as expression was very low in the plasma membrane," explains Dr. Gao. "The low expression in the plasma membrane of frog oocytes is most likely due to naturally occurring expression in chloroplasts, the powerhouse of the algae" speculates Prof. Nagel. He continues: "Fortunately Dr. Gao did not give up and found a way to dramatically increase expression which made possible a thorough investigation of this important protein".
Prof Mock added: "This is the reason why they still can thrive in these nutrient-poor surface oceans, and it is therefore also likely they will be able to cope with global warming as they are preconditioned.
"Although the ocean is getting warmer and therefore the surface layers are more and more deprived of nutrients, algae will be able to thrive better than predicted."
Of potential use for biotechnology
Potentially, the discovery could be used to enhance the productivity of crops, which also require iron for growth, Prof Mock said.
"This is universal for all primary producers. This machinery can also be used in biotechnology to enhance the productivity 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 (e.g., production of insulin, antibiotics, enzymes, antivirals, even biofuel)."
The team's work is particularly relevant for the Southern Ocean, which is both the largest iron-limited aquatic ecosystem and among the most productive, supporting the largest populations of algae consumers.
Prof Mock said: "No other habitat on Earth is more important than our oceans for the survival of humans and life in general."
Original publication
„Plastid-localized xanthorhodopsin increases diatom biomass and ecosystem productivity in iron-limited surface ocean", Nature Microbiology. DOI: 10.1038/s41564-023-01498-5. https://www.nature.com/articles/s41564-023-01498-5
