Like tiny escape artists, microscopic algae in tidal mudflats dive deeper into the sediment to dodge harmful sunlight. These diatoms, a type of single-celled algae, form slimy biofilms on muddy shores and must cope with intense light that can damage their cellular machinery through the buildup of reactive oxygen species — unstable molecules produced during photosynthesis.
Diatoms have long been known to shift vertically in the sediment as a protective tactic, but the exact signal that prompts this downward movement has remained unclear. The findings, published February 5 in BioRxiv, reveal that reactive oxygen species themselves act as the alarm, prompting the diatoms to retreat deeper into the mud. The study examined communities dominated by the diatom species Pleurosigma strigosum.
“Results showed a consistent downward migration response under all oxidative stresses,” the researchers note, “highlighting the key role of reactive oxygen species, especially hydrogen peroxide, in triggering this microphytobenthic behavior.”
Alexandre Desparmet
The team, led by Alexandre Desparmet, collected muddy sediment samples from a coastal pond where the biofilms develop under regular tidal cycles. They prepared two types of experimental setups: one with sediment layers allowing the diatoms to move vertically, and another without sediment to restrict movement. These communities were then exposed to three sources of oxidative stress, intense light mimicking midday sun, a cold atmospheric plasma generating a mix of reactive molecules, and a direct dose of hydrogen peroxide, all while monitoring changes in pigments and photosynthetic efficiency through chemical analyses and fluorescence measurements.
Regardless of the stress source, the diatoms in sediment rapidly migrated downward, with the strongest response under hydrogen peroxide, which slashed surface biomass by over 75 percent in minutes. The movement was reversible; once the stress ended, the algae resurfaced. In setups without sediment, where migration was blocked, the diatoms ramped up an alternative defense by converting certain pigments to dissipate excess energy harmlessly.
The researchers observed that this downward migration works hand-in-hand with pigment-based protections. Under intense light, the two strategies combine to limit damage, with pigment adjustments stepping up more forcefully when movement is limited.
“By establishing that a rapid and efficient migration could be induced by reactive oxygen species and could act in synergy with the xanthophyll cycle in epipelic cells,” the study emphasizes, it “provides key insights into the molecular basis of microphytobenthic responses to cellular and environmental oxidative stresses.”
Citations
Author et al. Reactive oxygen species trigger downward vertical migration in diatom microphytobenthic biofilms as a strategy to cope with oxidative stress. BioRxiv. Published online December 16, 2025. DOI: 10.1101/2025.07.10.664094
