A new flagellated life stage in Paraphysoderma sedebokerense, a pathogen of industrially relevant microalgae

Industrial-scaled microalgal cultivation is a rapid growing market worldwide. Economically important microalgae in this regard are, among others, species of Chlorella, Scenedesmus and Haematococcus. The latter, Haematococcus pluvialis, represents an important source for the red pigment astaxanthin, which is commonly added as colorant in cosmetics, nutraceuticals and food. Pathogen-caused disease outbreaks can significantly hamper the production of valuable biomass representing an economic loss. Although reports on this problem are scarce, pathogens of microalgae are omnipresent. In a pilot project, we have set-up a laboratory controlled pathosystem between the green alga Haematococcus pluvialis and the blastocladialean pathogen Paraphysoderma sedebokerensis. Life cycle analysis via microscopy strongly suggests that Paraphysoderma has a uniflagellated, motile zoospore phase in contrast to the original description of this pathogen as aplanosporic organism. The zoospores then transform into the known amoeboid phase after some time. Therefore, the life cycle might even be more complex than previously assumed. Importantly, its understanding is a major step towards the development of efficient disease control strategies