Eukaryotic algal phytochromes span the visible spectrum

Abstract

Plant phytochromes are photoswitchable red/far-red photoreceptors that allow competition with neighboring plants for photosynthetically active red light. In aquatic environments, red and far-red light are rapidly attenuated with depth; therefore, photosynthetic species must use shorter wavelengths of light. Nevertheless, phytochrome-related proteins are found in recently sequenced genomes of many eukaryotic algae from aquatic environments. We examined the photosensory properties of seven phytochromes from diverse algae: four prasinophyte (green algal) species, the heterokont (brown algal) Ectocarpus siliculosus, and two glaucophyte species. We demonstrate that algal phytochromes are not limited to red and far-red responses. Instead, different algal phytochromes can sense orange, green, and even blue light. Characterization of these previously undescribed photosensors using CD spectroscopy supports a structurally heterogeneous chromophore in the far-red–absorbing photostate. Our study thus demonstrates that extensive spectral tuning of phytochromes has evolved in phylogenetically distinct lineages of aquatic photosynthetic eukaryotes.

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

“…Recent characterization of phytochromes from taxonomically diverse algae unveiled significant structural diversity in these proteins and wide spectral variations that are not restricted to R and FR light (Duanmu et al, 2014;Rockwell et al, 2014b). Our study in diatoms establishes the presence of this important photoreceptor class in prominent marine species and further enhances our knowledge of phytochrome diversity.…”

“…Three DPHs found in A. coffeaeformis showed an additional Cys residue in the GAF domain, at a similar position to the chromophore binding site in plant and cyanobacterial (Cphs) PHYs as well as in E. siliculosus PHYs (Supplemental Figure 1) (Rockwell et al, 2014b). Moreover, domain prediction analyses with different tools (Supplemental Data Set 1) revealed a possible diversification of the DPH N-terminal part that typically contains a PAS domain in other eukaryotic PHYs.…”

“…This makes the recent discovery of PHYs in a growing number of aquatic organisms a puzzling issue and opens novel perspectives on the possible aquatic origin of PHYs. Found in numerous algal lineages that originated after cyanobacterial endosymbiosis (Streptophyta, Prasinophyta, and Glaucophyta), PHYs have also been identified in some algae derived from secondary endosymbiosis (Cryptophyta and Heterokonts) (Falciatore and Bowler, 2005;Keeling, 2013;Duanmu et al, 2014;Rockwell et al, 2014b;Li et al, 2015). The characterization of representative PHYs from multicellular and unicellular marine phototrophs (phytoplankton) revealed a wide spectral variation among them.…”

“…The characterization of representative PHYs from multicellular and unicellular marine phototrophs (phytoplankton) revealed a wide spectral variation among them. Several of these PHYs exhibit a R-FR photocycle, while others sense orange, green, and even blue light wavelengths (Duanmu et al, 2014;Rockwell et al, 2014b). Similarly to proteorhodopsin-based sensing (Man et al, 2003), this variability has been interpreted to be a result of selective pressure to tune the photoreceptor absorbance to the spectral characteristics of light in marine environments.…”

Algae hold clues to eukaryotic origins of plant phytochromes

“…It is the only phytochrome thus far detected that shares with Viridiplantae phytochromes a double internal PAS (Per-ARNT-Sim) repeat that separates the PCM from the C-terminal histidine-kinase-related module (HKM). Glaucophytes have a single internal PAS (14), whereas prokaryotic phytochromes lack an internal PAS (6 Within Viridiplantae, however, the findings of Duanmu et al fill important gaps in our knowledge of phytochrome evolution and function. Their focus on prasinophyte algae allowed them to establish that diverse members of the chlorophyte algae do possess phytochromes, despite their absence in sequenced genomes of species from Chlamydomonas, Chlorella, Volvox, Bathycoccus, and Ostreococcus.…”