Quaeritorhiza haematococci is a new species of parasitic chytrid of the commercially grown alga, Haematococcus pluvialis.

Aquaculture companies grow the green alga Haematococcus pluvialis (Chlorophyta) to extract the carotenoid astaxanthin to sell, which is used as human and animal dietary supplements. We were requested to identify an unknown pathogen of H. pluvialis from an alga growing facility in the southwestern United States. To identify this zoosporic fungus and determine its phylogenetic placement among other chytrids, we isolated it into pure culture, photographed its morphology and zoospore ultrastructure, and sequenced and analyzed portions of nuc rDNA 18S and 28S genes. The organism belongs in the Chytridiomycota, but a comparison of rDNA with available representatives of the phylum did not convincingly place it in any described order. The unique zoospore ultrastructure supports its indeterminate ordinal position, and the morphology, as determined by light microscopy, did not match any described species. Consequently, we have placed this chytrid in the new genus, Quaeritorhiza, and described it as the new species Q. haematococci in the family Quaeritorhizaceae but otherwise incertae sedis in the Chytridiomycetes. This new taxon is important because it increases the known diversity of Chytridiomycota and the organism has the ability to disrupt agricultural production of an algal monoculture.