Carbonic anhydrase modulates photosynthetic responses to UV radiation in diatoms across temperature gradients

Diatoms are major contributors to marine primary production and global carbon cycling, while they face increasing physiological stress from climate change-driven shifts in temperature, light regimes, and carbon availability. Carbonic anhydrase (CA) is a key enzyme in diatom carbon-concentrating mechanisms, catalyzing the reversible conversion of CO₂ and HCO₃^- to facilitate carbon fixation. Here, we examined how CA inhibition influences the growth and photosynthetic performance of two morphologically distinct diatoms-Skeletonema costatum and Nitzschia sp. (centric vs. pennate)-under ultraviolet radiation (UVR) across three temperatures (15, 20, and 25 °C). Cultures were exposed to photosynthetically active radiation (PAR) or PAR + UVR (PAB), with or without ethoxyzolamide (EZ), a membrane-permeable CA inhibitor. In S. costatum, EZ completely suppressed growth at all temperatures, indicating a strong dependence on CA-mediated CO₂ supply. Nitzschia sp. maintained growth under EZ at 15 and 20 °C but was more affected at 25 °C, suggesting greater resilience through alternative carbon acquisition pathways. Photophysiological measurements showed that CA inhibition substantially reduced maximum relative electron transport rate (rETR_max) and light saturation point (I_k) in S. costatum, with smaller effects in Nitzschia. Under UVR, effective quantum yield (EQY) declined in both species, but the reduction was amplified by CA inhibition, most severely in S. costatum, where UVR-induced EQY inhibition exceeded 75 % at 25 °C. These results highlight that CA plays a critical role in mitigating UVR stress by sustaining CO₂ availability, and that species-specific traits, including differences in cell geometry, carbon uptake systems, and photoprotective capacity, modulate diatom vulnerability to combined warming and UVR. Such species-specific responses could drive shifts in diatom community composition and alter coastal carbon cycling under future climate scenarios.