Molecular-phylogenetic, structural and biochemical features of a cold-adapted, marine ichthyosporean near the animal-fungal divergence, described from in vitro cultures

A hitherto undescribed eukaryotic microorganism, provisionally named Sphaeroforma arctica, was isolated from the arctic marine amphipod Gammarus setosus. Phylogenetic inference based on a partial sequence of the small subunit ribosomal RNA (SSU-rRNA) gene, grouped the organism with members of the Ichthyophonida sub-clade of the recently established class Ichthyosporea. Its closest relative by SSU-rDNA analysis was Pseudoperkinsus tapetis, a Perkinsus-like clam pathogen of the Iberian peninsula. S. arctica and P. tapetis were found to share 99.3% sequence similarity. Under the in vitro growth conditions employed, the organism showed a continuous vegetative growth, manifested as steady proliferation of cells 5-7 mum in diameter within a spherical wall derived from its parent cell. After approximately 48 h at 12 degreesC, the wall ruptured with the release of 120-150 individual cells, and the cycle was repeated. No budding, hyphal, amoeboid, sporal or flagellated growth stages were observed. The mitochondrial cristae of S. arctica, were flattened. The cell wall of cells cultured in vitro was composed almost exclusively of carbohydrates, with N-acetyl-galactosamine as the major monomer, constituting close to 70% of the dry weight. Polyunsaturated fatty acids, mostly eicosapentaenoic and docosahexaenoic acid, constituted approximately 70% of total fatty acids, supporting an evolutionary adaptation to a life at permanently low temperatures. We conclude that S. arctica is a new species belonging to a new genus within the Ichthyosporea class of protists.