Abstract
Algae are photosynthetic eukaryotic (micro-)organisms, lacking roots, leaves, and other organs that are typical for land plants. They live in freshwater, marine, or terrestrial habitats. Together with the cyanobacteria they contribute to about half of global carbon fixation. As primary producers, they are at the basis of many food webs and they are involved in biogeochemical processes. Algae are evolutionarily distinct and are derived either by primary (e.g., green and red algae) or secondary endosymbiosis (e.g., diatoms, dinoflagellates, and brown algae). Light is a key abiotic factor needed to maintain the fitness of algae as it delivers energy for photosynthesis, regulates algal cell- and life cycles, and entrains their biological clocks. However, excess light can also be harmful, especially in the ultraviolet range. Among the variety of receptors perceiving light information, the cryptochromes originally evolved as UV-A and blue-light receptors and have been found in all studied algal genomes so far. Yet, the classification, biophysical properties, wavelength range of absorbance, and biological functions of cryptochromes are remarkably diverse among algal species, especially when compared to cryptochromes from land plants or animals.
Highlights
What Are Algae?Algae are photosynthetic eukaryotes defined primarily by their lack of roots, leaves, and the stem that are typical for higher plants (Parker et al, 2008)
We focus solely on algal groups with cryptochromes that have been so far phylogenetically described based on existing genomes or have been characterized by their photoreceptors
Phase response curves (PRCs) with blue-light pulses taken from wild type and a Chlamydomonas reinhardtii (Cr) plant cryptochromes (pCRYs) knockdown line revealed the influence of Cr pCRY on PRC
Summary
Algae are photosynthetic eukaryotes defined primarily by their lack of roots, leaves, and the stem that are typical for higher plants (Parker et al, 2008). In the lineages with secondary endosymbionts, plastids are encircled by either four (e.g., diatoms, brown algae, haptophytes, and cryptophytes) or three membranes (euglenophytes, dinoflagellates; Gentil et al, 2017) Some of these lineages still contain the nucleus of the engulfed green or red alga, but in a strongly reduced form, called nucleomorph (e.g., in cryptophytes). Further types of (possibly) novel light sensors were recently discovered in phytoplankton in the open ocean (Coesel et al, 2021) The genomes of these marine microalgae encode lightsensing proteins with new combinations of known domain structures or even fusions of different types of photoreceptors. In the green alga Cr as well as in animal spermatozoa, the BLUF domain is directly associated with dynein and involved in ciliary motility (Kutomi et al, 2021)
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