Far-red light regulates phototactic behavior of benthic pennate epipelic diatoms under low irradiance

Diatom-dominated microphytobenthic communities are exposed to steep and dynamic light gradients in intertidal sediments. The vertical migration of epipelic pennate diatoms is a key adaptive trait enabling optimal light acquisition. However, it remains unclear whether these organisms can detect and respond to long-wavelength light, especially because deeper photic layers are enriched in far-red due to attenuation of shorter wavelengths. Here, we investigated the phototactic responses of a natural microphytobenthic biofilm, primarily composed of epipelic pennate diatoms, to long-wavelength light at two low irradiance levels (5 and 20 µmol photons m⁻² s⁻¹), using a custom-built multispectral LED illuminator. Red light (660 and 680 nm) induced a strong upward vertical migration and a high effective quantum yield of photosynthesis. Far-red light (720 and 740 nm) also triggered a significant upward migratory response, although more moderate than red light. In contrast, near-infrared wavelengths (770 and 810 nm) elicited no significant migratory activity, indistinguishable from the dark controls. Phototactic migration was observed even at 5 µmol photons m⁻² s⁻¹, suggesting a high sensitivity to light at intensities potentially below the photosynthetic compensation point. Our results provide evidence that benthic pennate diatoms can behaviorally respond to long-wavelength, low-intensity light. This response, likely mediated by phytochrome-like photoreceptors, suggests the existence of a low-light, long-wavelength sensing mechanism that enables diatoms to detect fine-scale spectral gradients as cues for surface detection and vertical positioning within the sediment matrix.