Diatoms Reveal Surprising Survival Strategies When Light and Heat Strike Together

When the tide recedes on a sunny afternoon, the microscopic diatoms coating estuarine mudflats face one of the harshest combinations that photosynthetic life can endure: intense solar irradiance beating down on sediments that are simultaneously heating toward 35 degrees Celsius. A team of researchers from the University of Aveiro in Portugal, the University of Brest in France, and the University of Rostock in Germany has now dissected, species by species, how these tiny algae survive such repeated assaults. Their findings, published in Frontiers in Photobiology, reveal that the capacity to withstand consecutive days of combined light and heat stress depends less on the number of stress events and more on which species is under attack and how its photosynthetic machinery was tuned before the stress arrived.

The study focused on three pennate diatom species isolated from the intertidal mudflats of the Ria de Aveiro: two benthic forms, Navicula phyllepta and Entomoneis paludosa, which live permanently within the upper sediment layers, and one tychoplanktonic species, Cylindrotheca closterium, which alternates between the sediment and the water column. These organisms collectively form the microphytobenthos, the biofilm of microalgae that drives primary production on intertidal flats and anchors food webs supporting meiofauna, benthic invertebrates, and migratory shorebirds. During daytime low tides, when diurnal emersion coincides with midday sun, these communities endure high irradiance, extreme temperatures, salinity swings, and localized carbon limitation, conditions that can strike on consecutive days and potentially accumulate damage if not fully repaired between events.

Photoinhibition, the light-induced decline in photosynthetic efficiency, arises from an imbalance between the direct photoinactivation of photosystem II (PSII) and the counteracting repair processes that rebuild damaged units. PSII is the protein complex that captures light energy to generate ATP and NADPH and ultimately fix carbon, and its D1 protein is uniquely vulnerable to photodamage. Diatoms mitigate this threat through two principal mechanisms: non-photochemical quenching, which dissipates excess absorbed energy as heat before it reaches PSII, and the xanthophyll cycle (XC), which converts the pigment diadinoxanthin (DD) into diatoxanthin (DT) to enable that protective dissipation. Concurrently, cells activate photorepair through de novo synthesis of D1, a process that demands substantial energy. The researchers quantified the rate constants of PSII photoinactivation (kPI) and photorepair (kREC) using pulse-amplitude-modulated chlorophyll fluorescence, and measured the de-epoxidation state (DES) of XC pigments by high-performance liquid chromatography to gauge photoprotective capacity.

To simulate successive diurnal low tides, the team exposed unialgal cultures of each species to three consecutive daily periods of combined high light and heat: 650 micromoles of photons per square meter per second at 35 degrees Celsius for 90 minutes, conditions within the range documented during diurnal emersion at the collection sites. Cultures were grown under contrasting low-light (10 micromoles photons per square meter per second) and high-light (120 micromoles photons per square meter per second) regimes to induce distinct photoacclimation states, and samples were treated with the antibiotic lincomycin to block D1 synthesis, allowing the researchers to separate the contribution of photoprotection from that of photorepair. The maximum quantum yield of PSII, Fv/Fm, was measured before and after each stress event and during recovery, providing a sensitive readout of functional PSII loss and regeneration.

The results exposed a striking divide between the benthic and the tychoplanktonic species. In both N. phyllepta and E. paludosa, PSII photoinactivation was largely counterbalanced by efficient photorepair, leading to near-complete recovery after each stress event, with untreated samples reaching between 89.6 and 99.5 percent of control Fv/Fm values across low- and high-light-acclimated cultures. By contrast, C. closterium showed markedly higher photoinactivation rates and limited photorepair capacity, with Fv/Fm remaining below 27.2 percent of controls in nearly every case. The rate constants told the same story: in the benthic species, kPI and kREC were roughly equivalent, indicating that repair could offset damage, whereas in C. closterium kPI exceeded kREC by more than 2.7-fold in high-light-acclimated cultures and more than 4.0-fold in low-light-acclimated ones, consistent with its overall poor recovery.

The two benthic species, however, achieved their resilience through different balances of the two protective strategies. N. phyllepta relied predominantly on highly efficient photorepair, showing low Fv/Fm in lincomycin-treated samples that betrayed a high susceptibility to photoinactivation but a strong capacity to rebuild damaged PSII once the inhibitor was absent. E. paludosa, in contrast, combined moderate photoprotection with good repair, maintaining higher Fv/Fm even when D1 synthesis was blocked, suggesting that its xanthophyll cycle and photosynthetic adjustments prevented a larger fraction of photoinactivation in the first place. The researchers note that E. paludosa can raise its maximum electron transport rate and light-saturation parameter more than N. phyllepta under high irradiance, which may reduce excitation pressure delivered to PSII and thereby lessen D1 photoinactivation.

The xanthophyll cycle itself behaved in unexpected ways depending on photoacclimation state. In all low-light-acclimated cultures, exposure to combined light and heat triggered a substantial increase in DES, confirming activation of DD-to-DT conversion, with C. closterium reaching the highest values at 63.4 percent on average. Yet in the high-light-acclimated benthic species, DES showed no significant increase under stress, a result the authors attribute to several converging factors: these cells already possessed large constitutive pools of DD and DT, including substantial pre-existing DT that may have sufficed to cope with the applied irradiance; the 650 micromoles per square meter per second stress level may not have been extreme enough for cultures already tuned to high light; a portion of the DD pool may be structurally inaccessible to the de-epoxidase enzyme in high-light-acclimated cells; and elevated temperatures can suppress the proton gradient across the thylakoid membrane that activates DD de-epoxidation, while high-light acclimation shifts thylakoid lipid composition toward sulfoquinovosyldiacylglycerol, a lipid that inhibits the enzyme.

Crucially, the limited xanthophyll cycle activity in the high-light-acclimated benthic species provided a natural experiment on the photoprotective role of DT. Across both N. phyllepta and E. paludosa, samples unable to produce additional DT under stress suffered the highest photoinactivation rates, and the inverse relationship between the stress-induced increase in DES and kPI was highly significant for both species. This supports the conclusion that the xanthophyll cycle effectively shields PSII from abiotic stress. In C. closterium, however, both DES and kPI reached high values simultaneously, indicating an intrinsic susceptibility to photoinactivation against which pigment-based photoprotection alone was insufficient. The authors suggest that in this species, heat stress may act primarily by impairing the repair of photoinactivated PSII rather than by directly accelerating photoinactivation, rendering efficient DD de-epoxidation ineffective at preventing net photoinhibition.

Despite the pronounced short-term effects of each stress exposure, no significant accumulation of irreversible PSII damage occurred over the three days, likely because the extended recovery intervals between events, roughly 22.5 hours including 12 hours of darkness at growth temperature, favored effective photorepair. Low to moderate light during these intervals supports the ATP production required for D1 protein turnover while keeping new photoinactivation below the counteracting repair rate, and centric diatoms have been shown to continue PSII protein recycling even in darkness, making it plausible that benthic species operate a similar mechanism overnight. The most consistent cumulative response across species was a gradual decline in stress-induced DD de-epoxidation over successive days, suggesting either cumulative negative impacts on xanthophyll cycle function or ongoing photo- and thermoacclimation to the intermittent stress.

The findings carry implications that extend well beyond the laboratory. Under scenarios of increasing heatwave frequency, species with higher photorepair capacity, such as the benthic forms studied here, are likely to be favored, potentially driving shifts in the composition and productivity of intertidal microphytobenthos communities. A decline in tychoplanktonic forms like C. closterium, disadvantaged by limited photorepair and poor recovery from cumulative stress, could alter biofilm structure, nutrient cycling, and benthic-pelagic coupling, since these species are periodically resuspended and contribute to pelagic primary production. Such changes would cascade into higher trophic levels, given the central role of microphytobenthos as a food source for meiofauna, benthic invertebrates, and shorebirds. The authors acknowledge that intertidal communities can face more extreme conditions than those applied, including higher irradiances, temperatures exceeding 35 degrees Celsius, and ultraviolet-B radiation, and that motile diatoms in sediment can use vertical migration to regulate their light exposure. Nevertheless, the suspension-based approach used here characterized the inherent photophysiological capacities of each species without the confounding influence of migratory behavior, revealing significant intrinsic differences between co-occurring species. Overall, the study demonstrates that species-specific physiological traits and photoacclimation state play a dominant role in determining the capacity of diatoms to cope with the extreme conditions found in estuarine intertidal environments.

Subject of Research: Photobiology

Article Title: Interspecific diversity in PSII photoinhibition and photoprotection of benthic diatoms during consecutive light and heat stress

Article References: Frankenbach, S., Ferreira, M. J., Esteves, A. C., Bastos, A., Morelle, J., Lepetit, B., Lavaud, J., & Serôdio, J. (2026). Interspecific diversity in PSII photoinhibition and photoprotection of benthic diatoms during consecutive light and heat stress. Frontiers in Photobiology, 4, Article 1822079. https://doi.org/10.3389/fphbi.2026.1822079

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1822079

Keywords: diatoms, photoinhibition, photosystem II, xanthophyll cycle, microphytobenthos, heat stress, intertidal, photoacclimation, Interspecific, diversity, PSII, photoprotection