Diatom Phytochromes Reveal the Existence of Far-Red-Light-Based Sensing in the Ocean

Abstract

The absorption of visible light in aquatic environments has led to the common assumption that aquatic organisms sense and adapt to penetrative blue/green light wavelengths but show little or no response to the more attenuated red/far-red wavelengths. Here, we show that two marine diatom species, Phaeodactylum tricornutum and Thalassiosira pseudonana, possess a bona fide red/far-red light sensing phytochrome (DPH) that uses biliverdin as a chromophore and displays accentuated red-shifted absorbance peaks compared with other characterized plant and algal phytochromes. Exposure to both red and far-red light causes changes in gene expression in P. tricornutum, and the responses to far-red light disappear in DPH knockout cells, demonstrating that P. tricornutum DPH mediates far-red light signaling. The identification of DPH genes in diverse diatom species widely distributed along the water column further emphasizes the ecological significance of far-red light sensing, raising questions about the sources of far-red light. Our analyses indicate that, although far-red wavelengths from sunlight are only detectable at the ocean surface, chlorophyll fluorescence and Raman scattering can generate red/far-red photons in deeper layers. This study opens up novel perspectives on phytochrome-mediated far-red light signaling in the ocean and on the light sensing and adaptive capabilities of marine phototrophs.

Diurnal fluctuations in chloroplast GSH redox state regulate susceptibility to oxidative stress and cell fate in a bloom‐forming diatom

“…Therefore, the redox potential of the chloroplast, derived from photosynthetic activity, serves as a potential sensor for light–dark changes in photosynthetic cells such as diatoms (Fortunato et al. ). We provide strong evidence for reduction in chloroplast steady‐state prior to the transition to the dark phase, when cells are adapted to light–dark cycle.…”

“…, Fortunato et al. ), the molecular components underlying diatom's mechanisms of sensing and responding to stress and their ecological success are relatively unexplored. The genus Thalassiosira is comprised of centric bloom‐forming diatoms, widely distributed throughout the world's oceans (Malviya et al.…”

Phytochrome diversification in cyanobacteria and eukaryotic algae

“…3). Phytochrome is present in some but not all photosynthetic stramenopiles and has radiated into a small gene family in complex phaeophyte algae (kelps) [9,34,35]. Most dinoflagellates lack phytochromes, but recent transcriptomic studies on the dinotom Durinskia baltica [36] have shown that its diatom symbiont retains a phytochrome gene (Fig.…”

“…The light-dependent accumulation of phytochrome in the nucleus of the prasinophyte alga Micromonas pusilla thus implicates light-dependent nuclear localization as a conserved mechanism of phytochrome function throughout the Viridiplantae [18]. A red/far-red phytochrome regulates gene expression in response to the levels of far-red light in the diatom Phaeodactylum tricornutum [35], and the presence of apparent orthologs in many diatoms implicates similar functions in such organisms as well. Hopefully, these promising studies will lead to further insights in the near future.…”

Phytochromes in Diatoms: Sensing Far-Red Light in the Deep Blue Sea

“…Substantial divergence of signaling output domains has also occurred in the green plant lineage (Li et al, 2015). In a new Breakthrough Report, Fortunato et al (2016) examine phytochromes (DPHs) from the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana. These DPHs showed typical conserved domain architecture with a conserved photosensory control module; however, phylogenetic comparison and biochemical analysis suggest that this domain binds biliverdin IXa, the precursor of plant and green algal phytochrome chromophores.…”