Abstract
Due to the selective attenuation of solar light and the absorption properties of seawater and seawater constituents, free-floating photosynthetic organisms have to cope with rapid and unpredictable changes in both intensity and spectral quality. We have studied the transcriptional, metabolic and photo-physiological responses to light of different spectral quality in the marine diatom Phaeodactylum tricornutum through time-series studies of cultures exposed to equal doses of photosynthetically usable radiation of blue, green and red light. The experiments showed that short-term differences in gene expression and profiles are mainly light quality-dependent. Transcription of photosynthesis-associated nuclear genes was activated mainly through a light quality-independent mechanism likely to rely on chloroplast-to-nucleus signaling. In contrast, genes encoding proteins important for photoprotection and PSII repair were highly dependent on a blue light receptor-mediated signal. Changes in energy transfer efficiency by light-harvesting pigments were spectrally dependent; furthermore, a declining trend in photosynthetic efficiency was observed in red light. The combined results suggest that diatoms possess a light quality-dependent ability to activate photoprotection and efficient repair of photodamaged PSII. In spite of approximately equal numbers of PSII-absorbed quanta in blue, green and red light, the spectral quality of light is important for diatom responses to ambient light conditions.
Highlights
Sunlight is the primary source of energy and a crucial source of information for all photoautotrophs
P. tricornutum cells cultured in continuous white light (CWL) were kept in darkness for 48 h (D48) before being re-exposed to either the initial white light (WL) or to blue light (BL), green light (GL) or red light (RL) at equal Photosynthetically Usable Radiation (PUR)
Transcriptional profiling of nuclear and plastid transcripts and light quality dependency The differences in molecular acclimation to light of BL, GL and RL compared to WL at corresponding time points were elucidated, focusing on the regulation of genes encoding photosynthesis-related proteins, proteins involved in assembly and repair of photodamaged PSII, ROS scavenging enzymes and photoreceptors
Summary
Sunlight is the primary source of energy and a crucial source of information for all photoautotrophs. Marine diatoms, which are responsible for close to 40% of the primary productivity in the world’s oceans [1, 2], experience high spatial, temporal and spectral variability in growth irradiance regime due to selective attenuation of solar irradiance in aquatic medium [3]. In addition to incident solar radiation and time of day, light quality and quantity is highly affected by the presence of coloured dissolved organic matter (cDOM) and suspended particles [4]. Through a complex system of photoreception and sensory- and metabolic pathways [5, 6, 7], diatoms manage to continuously sense, evaluate and acclimate their photosynthetic apparatus to changes in the intensity of the downwelling irradiance (Ed, Photosynthetically Available Radiation (PAR); 400–700 nm), its spectral quality (El), its orientation and the day length [8]
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