Mesodinium

Mesodinium is a genus of single-celled ciliates in the phylum Ciliophora. Members are found in marine and freshwater environments around the world, with coastal and estuarine waters hosting many populations. The best studied species is Mesodinium rubrum, known for forming vivid red-water blooms that color affected seas.

Morphology and biology: Like other ciliates, Mesodinium species possess cilia, a macronucleus and micronuclei, and reproduce primarily by asexual binary fission. They are typically elongated and motile, equipped for feeding on smaller algae and protists. Their ecological significance is heightened by their ability to engage in kleptoplasty, a form of temporary plastid acquisition.

Ecology and kleptoplasty: A defining feature of M. rubrum is its mixotrophic lifestyle. It feeds by phagotrophy on smaller algae, especially cryptophyte prey, and sequesters their chloroplasts to drive photosynthesis. The stolen plastids can remain functional for days to weeks, enabling photosynthetic growth even when prey is intermittently scarce. This combination of heterotrophy and kleptoplasty underpins dense red-water blooms in nutrient-rich coastal waters.

Ecological impact and research: Blooms of Mesodinium rubrum can influence local light regimes and food-web dynamics, and they have been reported in waters worldwide. The genus is of particular interest to scientists studying the evolution of mixotrophy and the mechanisms by which ciliates can maintain functional plastids from prey. Further research continues to clarify species diversity, prey relationships, and the duration of kleptoplastid functionality.