Dynamic nutrient competition (de-)stabilizes photosymbiosis.

Nutrient competition within photosymbioses can cause a bistable shift from a productive symbiotic state to a dysbiotic one under environmental disturbances.

• Why it matters: Photosymbioses are essential for productivity in aquatic ecosystems but are highly sensitive to anthropogenic changes, yet the mechanisms behind their regulation remain unclear.
• What they did: A mechanistic population model was developed to explore how competition for a limiting nutrient within the holobiont influences ecological dynamics, revealing a bistable system affected by environmental factors and symbiont traits.
• The result: Nutrient competition alone can explain the establishment and breakdown of photosymbiosis, indicating that traits promoting productivity under stable conditions also increase vulnerability to environmental stress, which may threaten ecological stability.