Postdoctoral Fellowship: OCE-PRF: Do diatoms use proton-pumping rhodopsins as an alternative energy source under high light

We are well aware that algae living in the global oceans play a critical role in the cycling of carbon and climate. Notably, algae called diatoms are responsible for a large proportion of carbon uptake and sequestration in the world’s oceans. Due to this, diatoms are considered “first responders” to climate change. More specifically they offset increasing carbon dioxide in the Earth’s atmosphere by transferring it to the deep ocean. However, climate change is altering the oceans and the ability of diatoms to survive. Specifically, climate change is increasing the intensity of light in the oceans via a process called “shoaling” and decreasing the amount of iron. Both conditions negatively affect diatoms by hindering photosynthesis. Recently it was discovered some diatoms possess proton pumping rhodopsins, which may serve as an alternative means to generate energy when photosynthesis is not feasible (for example during low iron and high light conditions). Yet while previous work found diatoms increase the use of proton pumping rhodopsins when they are iron-limited, it did not investigate the other condition which hinders photosynthesis, namely high light. The purpose of this project is to investigate how diatom proton pumping rhodopsins function under low iron and high light stress. Broadly, this work will extend to other algae and bacteria which use proton pumping rhodopsins to harvest light and generate energy. In turn, this work will provide insight on how diatoms will respond to the future oceans and how carbon cycling may be altered. This project will broaden diversity and engagement within the field of oceans science through various classroom and community outreach activities in addition to mentoring opportunities. Cumulatively, this work will enhance the ability to model and predict future climate scenarios across the global oceans. Diatoms are phototrophic protists responsible for ~40% of global marine primary production and organic carbon export. Recently, it was discovered some diatoms possess proton-pumping rhodopsins (PPRs), light-driven proton pumps that may contribute as much to cellular energy generation as photosynthesis. Prior data suggests PPR contributions to energy generation increase under iron-limitation, implying diatoms elicit a “phototrophic trade-off” by increasing PPR phototrophy when conditions for photosynthesis are unfavorable. While photosynthesis may become light-saturated at low light levels (e.g., 60-80mol photons m-2 s-1 in polar diatoms), based on photocyclic turnover rates, PPRs become light saturated at ~2000 mol photons m-2 s-1, suggesting PPRs are favored under high light. Yet, diatom PPR phototrophy has not been studied under high irradiance. Further, photosynthesis in iron-limited cells tends to become inhibited at lower light compared to iron-replete cells. This is of pertinence as ocean stratification serves to increase irradiance, and ~30% of marine primary production occurs under iron limitation. This proposal will investigate how high light levels + iron limitation alter the two major phototrophic strategies (and competitive fitness) of a diverse group of marine diatoms via a combination of in vitro laboratory analyses and biochemical assays. Photosynthetic phototrophy will be assessed via 14C isotope tracing, FIRe fluorometry, and photopigment extractions (HPLC). PPR phototrophy will be assessed via intracellular pH and calculations using retinal quantifications (LC-MS/MS). Ecological insights on how the presence vs. absence of PPRs alters diatom competitive fitness under high light + iron limitation will be assessed, with competitive fitness determined via growth dynamics and transcriptomics. Cumulatively, this work will characterize the present role of PPRs in diatom phototrophy and provide insight on the role of PPRs in a more stratified (and variable light) future. Classroom and community STEM engagement will be facilitated with local schools and the Morehead Planetarium NSF Award ID: 2620617 | Program: 01002324DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Brittany Zepernick | Institution: University of North Carolina at Wilmington, WILMINGTON, NC | Award Amount: $104,898 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2620617 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2620617.html