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Abstract Many organisms have formed symbiotic relationships with nitrogen (N)-fixing bacteria to overcome N limitation. Diatoms in the family Rhopalodiaceae host unicellular, N-fixing cyanobacterial endosymbionts called spheroid bodies (SBs). Although this relationship is relatively young, SBs share many key features with older endosymbionts, including coordinated cell division and genome reduction. Unlike free-living relatives that fix N exclusively at night, SBs fix N largely during the day; however, how SB metabolism is regulated and coordinated with the host is not yet understood. We compared four SB genomes, including those from two new host species (Rhopalodia gibba and Epithemia adnata), to build a genome-wide phylogeny which provides a better understanding of SB evolutionary origins. Contrary to models of endosymbiotic genome reduction, the SB chromosome is unusually stable for an endosymbiont genome, likely due to the early loss of all mobile elements. Transcriptomic data for the R. gibba SB and host organelles addressed whether and how the allocation of transcriptional resources depends on light and nitrogen availability. Although allocation to the SB was high under all conditions, relative expression of chloroplast photosynthesis genes increased in the absence of nitrate, but this pattern was suppressed by nitrate addition. SB expression of catabolism genes was generally greater during daytime rather than at night, although the magnitude of diurnal changes in expression was modest compared to free-living Cyanobacteria. We conclude that SB daytime catabolism likely supports N-fixation by linking the process to host photosynthetic carbon fixation. 

Denticula costata Hustedt was originally described from fossil material from Sumatra and later assigned to the genus Tetralunata. Although Tetralunata was thought to be endemic to Indonesia, D. costata has been reported from wetwalls from South Africa. This disparity in locality prompted us to investigate the D. costata in South Africa further. D. costata (now T. costata) specimens and the species reported from South Africa were different in size, shape, and structure of the raphe system. These differences, as well as comparisons to other Denticula species, allowed us to determine that the South African specimens have not been described previously. Valve ultrastructure, including the canal raphe, areolae with volate occlusions, and presence/structure of the septa suggest this new species belongs to the genus Tetralunata. This is the first report of Tetralunata from outside of Indonesia. Herein, we describe Tetraluanata schoemanii sp. nov. and its systematic placement close to, but separate from, Epithemia. This first report of Tetralunata from outside of Indonesia, increases our understanding of the genus range and displays a unique biogeographical pattern that warrants further investigation in the future.