Haematococcus pluvialis is currently cultivated at large scale for its ability to produce high amounts of the high value keto-carotenoid astaxanthin when encysted. Mass cultivation of this species is threatened by the destructive blastocladialean fungus, Paraphysoderma sedebokerense Boussiba, Zarka and James, responsible for the fast collapse of Haematococcus populations. Given the difficulty of maintaining pathogen-free production systems and the lack of treatment options, the selection and development of resistant Haematococcus strains could potentially present an effective and efficient method to control infection.
In the present work, we examined the host specificity of P. sedebokerense (strain PS1) through quantitative phenotyping of 44 Haematococcus strains in a laboratory-controlled infectivity assay. We determined the growth and photosynthetic activity of strains in the presence and absence of PS1 over time (using Chl a in vivo fluorescence) and quantified the degree of infection through the intensity of fluorescence after staining with Wheat Germ Agglutinin (WGA)-Fluorescein, which labels PS1 without interfering with Haematococcus. The measurements were converted into three infectivity proxies, allowing comparisons amongst strains. Eventually, microscopy was performed to check the life stage of Haematococcus upon infection.
Strains of Haematococcus clearly exhibited different levels of susceptibility against PS1 as determined by the three proxies. These were not related to phylogenetic background, nor the sampling origin of the strains. Amongst ten strains with low susceptibility, five occurred as flagellated state cultures, while others were palmelloid and/or aplanospore dominated. In addition, in a long-term selection experiment, we showed that susceptibility to PS1 of a highly sensitive H. pluvialis strain decreased through the dominance of flagellated phenotypes over several generations of infection.
While providing considerable expansion of the relation between PS1 and Haematococcus our study opens the possibility for selection and development of resistant strains.