Tracking the early events of photosymbiosis evolution

Cellular interactions and evolutionary origins of endosymbiotic relationships with ciliates

“…The associations between ciliates and photosynthetic microbes are influenced by environmental factors such as light and oxygen availability. Aerobic algae with oxygenic photosynthesis offer ciliates additional oxygen and organic matter, sometimes fulfilling the host’s requirement for vegetative growth [ 12 , 23 ]. In oxygen-deficient environments, ciliates acquire phototrophy from anaerobic primary producers like purple bacteria, which fix inorganic carbon without producing oxygen [ 190 , 191 ].…”

“…However, these relationships are mainly controlled by hosts, which have developed various mechanisms to manage symbiont conditions [ 203 ]. Host ciliates optimize the utilization of symbionts through domestication or modification, retaining only useful organelles [ 12 ]. Symbiont populations are regulated by host control over intake, consumption, and proliferation.…”

“…Alternatively, in scenarios where both symbiotic partners derive benefits from their collaboration, the relationship is categorized as mutualism [ 9–11 ]. Furthermore, symbiotic relationships can be further categorized based on the differing nutritional functions and levels of interdependence between partners into obligate and facultative symbiosis, although the delineation between these categories may blur under specific circumstances [ 12 , 13 ].…”