Photosynthetic organisms produce tetrapyrroles, which are coloured pigments that act as chromophores, including chlorophylls and bilins. These pigments play important roles in various physiological processes, including respiration, photosynthesis and photoreception, and have received much research interest in recent years not least because of their applications in artificial photosynthesis and optogenetics. This special issue - guest edited by Tatsuru Masuda, Rei Narikawa, Yoshitaka Saga, Haruki Yamamoto, Matthew Terry, and Yuichi Fujita - presents several review papers and original research articles related to tetrapyrrole synthesis, degradation and function in relation to their photoreception roles in plants and photosynthetic microorganisms (see editorial by Masuda et al. 2025 <https://doi.org/10.1093/pcp/pcaf014>）).
The cover photo exhibits a variety of different marine Synechococcus cultures, showing the astonishingly varied color phenotypes resulting from the differences in pigments found in these cells' phycobilisomes (photosynthetic light harvesting antennae), including a strain capable of Type 4 Chromatic Acclimation, the topic of the review by Kehoe et al., 2025 (<https://doi.org/10.1093/pcp/pcae115>). The photo was taken in front of the Roscoff marine biological station.