secondary endosymbiosis

Secondary endosymbiosis is a process where a eukaryotic cell (an organism with a nucleus) engulfs another eukaryotic cell that already contains a chloroplast (the organelle responsible for photosynthesis). Instead of digesting it, the host retains the engulfed cell, which becomes an endosymbiont. Over time, this gives rise to complex organisms like certain algae and protists with multiple membranes around their chloroplasts. This process explains how some algae, such as brown and red algae, acquired their photosynthetic ability, expanding the diversity of photosynthetic life beyond plants.