Free-living stage of the unicellular algae Coccomyxa sp. parasite of the blue mussel (Mytilus edulis): Low-light adaptation, capacity for growth at a very wide salinity range and tolerance to low pH

•Growth of parasitic Coccomyxa sp. Metis-sur-mer in liquid culture was investigated.•Maximum growth rate was 0.75 day−1 and growth saturated at 100 μmol quanta m−2 s−1.•Growth was maximal over the wide salinity range 15-35.•In natural spring water, the growth rate was still nearly half of maximum.•Unlike a closely related species, this species could not grow in acidic waters.

Coccomyxa parasitica and similar, unidentified Coccomyxa species infect the soft tissues of many bivalve species, giving them a conspicuous green coloration and leading to mantle and shell deformities. Very little information exists regarding the ecophysiology of parasitic Coccomyxa sp. and this limits our ability to understand how it can achieve its unusual life history. In the present study, the growth of Coccomyxa sp. Metis-sur-mer (KJ372210) in liquid culture was investigated. Coccomyxa sp. maximum growth rate was 0.75 day−1 (equivalent to a doubling time of 22 h), growth saturated at ∼100 μmol quanta m−2 s−1 and was still ∼20% of maximum at 13 μmol quanta m−2 s−1, the lowest photosynthetically available radiation (PAR) intensity tested. Coccomyxa sp. showed a very wide tolerance to salinity, with growth rate practically invariable over the salinity range 15-35. Even in natural spring water enriched with f/2 media, the growth rate was nearly half of maximum. Unlike the closely related acid-tolerant Coccomyxa sp. CPCC 508, Coccomyxa sp. Metis-sur-mer could not grow in acidic waters. Considering the ability of Coccomyxa sp. to achieve a high growth rate at low irradiance and its relative insensitivity to the prevailing salinity, it is somewhat surprising it has not yet infected bivalves at a larger scale worldwide.

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