A marine fungus kills the alga behind toxic blooms

Ostreopsis cf. ovata is a dinoflagellate that forms blooms in the Mediterranean and produces ovatoxin. Contact with it causes a runny nose, coughing and skin irritation in people, and a bloom can close a beach for days. In 2025 the journal Mycologia carried the description of an organism that kills it.

Algophthora mediterranea is a new genus and species of chytrid fungus, described by the team of Núria Pou-Solà and Kensuke Seto. The fungus parasitises Ostreopsis cells and kills them within a few days. Time-lapse imaging captured every ten minutes over four days allowed the whole cycle to be followed. The first traces of its presence were noticed in Spanish waters as early as 2021, but a formal description required culture and phylogenetic analysis.

Its lack of fussiness is the most interesting part. Unlike many chytrids, which are tied closely to a single host, Algophthora mediterranea infects more than one algal species and can also feed on pollen. A generalist among algal parasites is unusual in this group — and it is precisely that trait that determines how such an organism behaves in an ecosystem.

The temptation is obvious: if the fungus kills a toxic dinoflagellate, perhaps it could be used against blooms. That deserves to be cooled down explicitly. Describing a species is not a biocontrol method, and releasing a generalist into the sea means risk to species nobody intended to control. The authors raise the question of what role these fungi play in regulating blooms, not of how to deploy them.

The wider context is this: parasitic chytrids are a factor in aquatic ecosystems that went unaccounted for a long time. They break down algal biomass and pass it on through the food web, and their significance only began to be described once environmental sequencing showed how many of them are there.

The phenomenon even has a name. The "mycoloop", described in limnology, is a circuit in which a chytrid parasitising a large alga — too large for zooplankton to eat — converts its biomass into its own zoospores, and those are of a size small crustaceans can take. The fungus acts as an adapter between two levels of the food web that would otherwise not connect.

The genus Batrachochytrium, known for killing amphibians, belongs to the same phylum. That is a useful reminder that chytrids are not a uniform group of "water fungi": they include both algal parasites that regulate blooms and vertebrate pathogens responsible for extinctions. What they share is their structure and a single-flagellate zoospore, not a way of life.

The naming is worth taking literally. The genus name Algophthora means something close to "destroyer of algae", and the epithet mediterranea marks the sea the species was described from. Scientific names are sometimes a record of what the authors considered most important about the organism.

Date and application limits — 29 August 2026

Algophthora mediterranea was described online on 15 December 2025; the Mycologia issue is dated 2026. Morphology and rDNA phylogeny support the new genus and species. In cross-infection experiments the chytrid infected not only the toxic dinoflagellate Ostreopsis cf. ovata, but also two Alexandrium species, three diatoms and pine pollen; infection was very rare in some diatoms. This confirms a broad host range in culture, but weakens a simple “cure for blooms” narrative. There has been no field trial or assessment of effects on non-target organisms. Population ecology, specificity and safety tests must come first.