Photosystems are ganging up to form megacomplexes in the dinoflagellate Prorocentrum cordatum

Phytoplankton, although small in biomass, have been estimated to account for around 45% of the global primary production (Field et al. 1998).Among phytoplankton, dinoflagellates are an ecologically successful group of unicellular eukaryotes that display high morphological diversity and varying lifestyles ranging from free-living cells to endosymbionts and parasites (Taylor et al., 2008).Photosynthetic dinoflagellates are evolutionarily distant from land plants and green algae and have structurally variable chloroplasts that originate from secondary and tertiary endosymbiosis (Taylor et al., 2008; Kalvelage et al., 2024).Prorocentrum cordatum is a bloom-forming marine dinoflagellate found from tropical to temperate waters (Heil et al., 2005).It is highly adaptable to changing environments and tolerant to wide range of salinity and temperature (Dougan et al., 2023).The large P. cordatum haploid genome (4.15 GB) is composed of 62 chromosomes with over 85 000 gene models (Dougan et al., 2023).The genome is characterized by a high number of dispersed duplicate genes and gene families (Dougan et al., 2023).The availability of genomic resources enables the more detailed molecular characterization of the highly adaptable P. cordatum.In this issue of Plant Physiology, Kalvelage et al. ( 2024) reveal the intracellular structure of heterotrophic, free-living P. cordatum, characterize the organization of the photosynthetic complexes at the thylakoid membranes and identify the proteins involved in photosynthesis.The authors use focused ion beam scanning electron microscopy (FIB/SEM) to reconstruct a 3D model of P. cordatum cellular architecture and show that P. cordatum cell encloses one large barrel-shaped chloroplast that lines the cell envelope occupying roughly 40% of the cell volume (Figure 1).The thylakoid membranes within do not form distinct grana structures, as in land plants, but are arranged as loose stroma-like membranes with more compact regions, as previously seen in other aquatic organisms (Grouneva et al., 2013).In P. cordatum, the large chloroplast encases a single mitochondrion of roughly 5% of the cell volume that forms distinct reticulate structures with close contacts to the chloroplast suggesting a tight functional connection between the two organelles.