Toxicity of the ichthyotoxic marine flagellate Pseudochattonella (Dictyochophyceae, Heterokonta) assessed by six bioassays

Recurrent blooms of the marine flagellate genus Pseudochattonella have caused several fish kills in Northern Europe and Japan during the last two decades. Six different bioassays using fish cells, fish and brine shrimp nauplii were assessed as test systems for investigating ichthyotoxic mechanisms of Pseudochattonella. Metabolic activity in embryo cells from chinook salmon (CHSE-214) was inhibited in 2 strains, increased in another strain but was unaffected in a further 2 strains when exposed to extracts from strains of Pseudochattonella farcimen while it increased after exposure to both strains of Pseudochattonella verruculosa. Primary hepatocyte cells from rainbow trout showed metabolic inhibition and loss of membrane integrity when exposed to DMSO extracts both from Pseudochattonella spp. and from the putatively non-toxic heterokonts Pseudopedinella pyriforme and Skeletonema sp. while the same algae extracted with H₂O showed no adverse effects in the bioassays used. No acute mortality was observed after 24h exposure of cod fry and salmon smolts to Pseudochattonella and Pseudopedinella. However, histological examinations of fish gills showed strong effects on secondary lamellae (hypertrophy, edema, thickened secondary lamellae and necrotic cells). No acute toxicity of brine shrimp larvae could be detected after 24h exposure to any of the Pseudochattonella strains. This study shows that both Pseudochattonella and Pseudopedinella affect cell metabolism and can cause damage to gills. The toxicity of Pseudochattonella varied by species, strain and bioassay system, and was most clearly demonstrated when salmon smolts or cod fry were exposed to bloom concentrations of P. verruculosa from Japan.