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Diatoms are a group of unicellular eukaryotic microalgae of high evolutionary success and of ecological importance. Like all photosynthetic organisms, they need to tightly regulate their photosynthetic machinery to both generate enough energy in low light conditions and avoid photodamage by absorption of excess light energy. Many other aspects of diatom biology, from central metabolism and cell cycle to migration patterns, have been shown to be influenced by light of certain wavelengths. One possible mechanism to determine both light quantity and light wavelength is the useof photoreceptors. The genomes of diatoms encode up to four different classes of photoreceptors: (1) Phytochromes are a class of photoswitchable red/far red-light absorbing photoreceptors, which are evolutionary widespread; (2) Blue-light sensing cryptochromes, which share a high homology to DNA-repairing photolyases, are also evolutionary widespread; (3) Aureochromes, a group of blue-light dependent transcription factors which were identified in 2007 and are restricted to the Stramenopiles. (4) Heliorhodopsins, a novel class of green-yellow light-absorbing photoreceptors identified in 2018 from a metagenome study, have had their domain topology reversed compared to previously known Rhodopsins. In this chapter, we summarize the mode of action of each of these photoreceptors with examples from what is known from other species, such as plants, give an update on their phylogenetic distribution among diatom species and describe what is known about the respective photoreceptors in diatoms both using expression data from microarray and RNA-seq experiments and functional data derived from overexpression, knockdown and knockout strains.

Cell walls are not just passive barriers; they are dynamic and adaptable structures that are actively remodeled in response to both internal and external cues. They are crucial in defining cellular identity, ensuring structural integrity, and mediating interactions with the environment. The concept of cell wall integrity (CWI) encompasses the mechanisms by which cells monitor and maintain their walls, ensuring proper function and response to challenges. While significant knowledge has been accumulated on CWI in certain model organisms, there remains a vast landscape of uncharted territory in others. In this review, we aim to bridge this gap, offering a comparative perspective on CWI across different evolutionary lineages, from the well-studied yeasts to the diverse world of plants. We focus especially on the green lineage –the group of green algae and land plants, hence the green wall–, but also consider some insights from organisms with radically different lifestyles and cell wall arrangements, which serves as a base to some intriguing questions about the role of CWI across evolution and environmental adaptation.