Fig. 1: Schematic of kleptoplasty and pyrenoid transformation in R. viridis.

Chloroplasts/kleptoplasts are shown in green, and pyrenoids are shown in yellow. a R. viridis ingests whole cells of a specific Tetraselmis strain by phagocytosis (ingestion event), typically consuming them within several hours at a 1:3 cell ratio. b In the kleptoplast transformation stage (15–20 h after ingestion), R. viridis expels the algal cytoplasm and nucleus while retaining the chloroplasts as three-membrane-bound kleptoplasts. The large, starch-sheathed algal pyrenoid then transforms into multiple Rapaza pyrenoids associated with thylakoid membranes³⁵. The kleptoplasts are further subdivided, possibly by constricting the outermost phagosomal membrane. c During the phototrophic stage, kleptoplasts containing one or more pyrenoids are inherited by daughter cells during exponential growth. R. viridis remains highly photosynthetic for approximately 2 weeks, remaining virtually autotrophic through a 1-week growth phase and a 1-week stationary phase. This autotrophy is supported by suspended growth without external inorganic nitrogen³⁴, culture decline in the absence of light⁹, and cytosolic polysaccharide accumulation in stationary phase cells^9,34. Kleptoplasts do not grow or replicate during this period. d In the absence of new kleptoplast acquisition, vacuoles begin to form after 3 weeks and gradually expand to occupy most of the cell. The cells ultimately die after 4–5 weeks (declining stage). See Karnkowska et al.⁹ for further details. e A Venn diagram showing the distribution of genes encoding proteins involved in key photosynthetic processes in the chloroplast genome, as well as in the nuclear genomes of R. viridis and Tetraselmis sp.