The role of light-driven proton pumps in sustaining oceanic primary production

Microbial rhodopsins are photoreceptor proteins that convert light into biological signals or energy. Proteins of the xanthorhodopsin family are common in eukaryotic photosynthetic plankton including diatoms, which contribute ca. 45% of annual oceanic primary production and therefore underpin significant ecosystem services such as providing food for heterotrophic marine organisms and carbon sequestration mitigating adverse effects of global warming caused by human activity. In diatoms, xanthorhodopsin represent an alternative energy system to support growth and biomass under unfavourable growth conditions such as limitation by essential nutrients (e.g. dissolved iron). Hence, since xanthorhodopsins play a key role in enhancing oceanic primary production under adverse environmental conditions, they can be considered a 'climate-change coping mechanism', especially as nutrient limitations are expected to become more prevalent globally in a warming surface ocean. Unfortunately, in 2023, the oceans have broken heat records five years in a row, and on the latest UN climate summit, COP28, there was no commitment to phase out fossil fuels. Instead, the summit president declared that UAE would double oil and gas output this decade. Hence, it is likely the oceans will continue to break heat records in the coming years and we therefore have a pressing need to identify how key marine organism groups such as diatoms will cope. This knowledge will be important to assess their resilience and therefore if and how the ecosystem services they provide will change. To help with this 'resilience assessment', this project will determine how xanthorhodopsins underpin a key ecosystem service: oceanic primary production. To meet this aim, our work will capitalise on three recent ground-breaking publications. In brief, we will characterise genetically modified xanthorhodopsin diatom cell lines to identify the physiological mechanism underpinning diatom growth and biomass formation under unfavourable growth conditions. There is significant diversity in genes encoding xanthorhodopsins between diatom species and even within a single species, which suggests these proteins are a multi-purpose tool ("Swiss Army Knife") helping diatoms to sustain oceanic primary production under adverse environmental conditions. Thus, to reveal if this genetic diversity translates into functional diversity representing different solutions to the same problem, our laboratory experiments will be accompanied by a combination of sequenced-based and 3D structural protein analyses exploring the "100 Diatom Genomes Project" and multi-omics surface ocean datasets. The latter will provide the first insights into the environmental conditions likely responsible for the xanthorhodopsin-based alternative energy system to sustain oceanic primary production under unfavourable growth conditions imposed by a continued warming of the surface oceans.