YOU CAN'T GET ME - DEVELOPING HAEMATOCOCCUS STRAINS RESISTANT TO PARAPHYSODERMA INFECTION

Industrial cultivation of the unicellular green alga Haematococcus pluvialis for the natural production of the high value compound astaxanthin is currently affected by the parasite Paraphysoderma sedebokerense. Paraphysoderma infection causes economic losses, as it reduces the quality of algal biomass and can eventually causes culture crashes. Contaminant-free cultivation is difficult to achieve in large
scale and so far, a publicly available treatment for Paraphysoderma infection is lacking. Paraphysoderma disseminates by motile spores that can encyst on the host surface. Cysts develop into sporangia feeding on the algal protoplast and release a new generation of spores upon maturation, so spreading the disease. In large scale, Haematococcus is grown in its green phase which is characterised by a heteromorphic life cycle in which cells transition between haematocysts, palmelloids and macrozooids. The motile macrozooid stage is of particular interest, as it is resistant to Paraphysoderma infection. While its abundance during the green phase greatly depends on the Haematococcus strain, culture- and environmental conditions, this motile phenotype is normally lacking in the stressful culture conditions that are necessary for astaxanthin production. In this study, we present the results of a long-term selection experiment in which a highly sensitive H. pluvialis strain gained resistance to Paraphysoderma infection over several generations of infection by developing a culture phenotype dominated by flagellated cells, not only in its green phase, but also throughout the astaxanthin-accumulating red phase. This acquired resistance thus allows cultivation of Haematococcus throught its whole cycle in presence of Paraphysoderma. Monoclonal isolates obtained from the Haematococcus/Paraphysoderma co-culture remained flagellated for several months, indicating that the phenotype is stable in absence of the parasite. This study provides not only a model to study Paraphysoderma resistance, it also opens up the possibility to develop Paraphysoderma resistant Haematococcus strains for cultivation on an industrial scale.