The interaction between the parasitic fungus Paraphysoderma sedebokerense and the commercially high value alga Haematococcus pluvialis

The green microalga Haematococcus pluvialis is produced commercially due to its naturally high productivity of the powerful antioxidant carotenoid astaxanthin, which is mainly used as nutritional additive. Frequent contaminations reduce astaxanthin productivity, and in some cases cause collapse of the culture. We were the first to describe the fungus parasite causing the culture collapse, while developing the biotechnology process for astaxanthin production from H. pluvialis. Understanding host - parasite interactions, as well as inhibiting the parasite’s infection capability are of a great interest scientifically and commercially and were the major aims of our research.

We have isolated the parasite from infected algae cultures, and characterized its life cycle. We have identified the amoeboid swarmers, which recognize and encyst on the host, as the initiators of the disease. Based on both, the parasite life cycle and the 18S ribosomal DNA sequences, the pathogen appeared to represent a novel chytrid fungus closely related to the vascular plant pathogen Physoderma (Blastocladiomycota); it was named Paraphysoderma sedebokerense. While screening for infection with different algae, P.sedebokerense was found to be highly specific to H. pluvialis. We provide evidence that, the host-parasite recognition stage is mediated via host surface carbohydrates residues of Gal-N-acetyl-D-glucosamine and the parasitic Ricinus communis agglutinin 120-like lectin. We developed a reproducible method for the stimulation and isolation of P. sedebokerense propagules, and further used them to study the initial stages of infection, as well as their development under different conditions. The current comprehensive view on this host parasite interaction will be presented.