Mitochondrial density and cell area changes in the ciliate Paramecium bursaria under constant darkness: effects of symbiotic Chlorella variabilis and nutrient availability.

Paramecium bursaria and its symbiotic association with Chlorella variabilis influence host organelles. Previous studies have reported reduced mitochondria and trichocysts in algae-bearing P. bursaria cells, suggesting that the digestion of symbiotic algae may provide nutrients for trichocyst synthesis. However, the response of host mitochondria to symbiont loss under prolonged darkness remains unclear. Here, we examined the mitochondrial dynamics and cell morphology in algae-bearing and alga-free P. bursaria under constant darkness combined with feeding or starvation. Algal reduction was quantified using differential interference contrast image intensity, and host mitochondria were visualized using MitoBright LT Green. Under dark conditions with starvation, symbiotic algae and cell area decreased markedly, whereas mitochondrial fluorescence remained largely unchanged in algae-bearing cells. Gradual loss of algae despite feeding preserved both cell area and mitochondrial density. In alga-free cells, starvation caused early mitochondrial decline, followed by partial recovery, whereas feeding supported maintenance or enhancement. These findings indicate that mitochondrial density does not increase as algae decrease; instead, nutrient availability is critical for sustaining mitochondria in prolonged darkness. Our results provide insights into organelle-level responses to symbiont loss and the mechanisms underlying endosymbiotic resilience under environmental stress, with implications for mutualistic stability in changing ecosystems.