Phylogeny, lipid composition, pigment signature, and growth of the fish-killer Heterosigma akashiwo from Chilean Patagonia

The raphidophyte Heterosigma akashiwo is widely distributed in coastal ecosystems where
it has caused massive events of fish mortality and shellfish damage. In April 2021, high cell
abundances of H. akashiwo (> 70,000 cells mL-1) killed > 6,000 tons of salmon in the Chilean
fjords. This study investigated the molecular phylogeny, pigment composition, fatty acid
profile, and cell growth of the H. akashiwo (strain CREAN_HA03) from Chilean waters. A
phylogenetic reconstruction based on the large sub-unit ribosomal nucleotide showed that the
Chilean strain belongs to the H. akashiwo clade. The genetic distance was 0.00 relative to strains
from Japan, Australia, U.S.A., China, and New Zealand. The pigment signature showed that the
CREAN_HA03 strain is mainly dominated by fucoxanthin (48.4%), chlorophyll c2 (18.3%),
chlorophyll a (14.7%), and violaxanthin (9.8%). A factorial T-S growth experiment showed a
μmax of 0.48 d−1 at 17 °C and salinity of 35. The maximum cell abundance of ~50,000 cells mL-1
was reached at 12°C and a salinity of 25. The fatty acid profile exhibited a high abundance of
polyunsaturated fatty acids: 20.94% palmitic acid (16:0), 13.04% EPA (20:5ω3), and 10.44%
stearidonic acid (18:4ω3), which have all been described as highly cytotoxic against fish gill
tissue. Further characterization of more Chilean H. akashiwo isolates is needed to understand
the precise ichthyotoxic mechanisms and environmental drivers that trigger massive bloom
events in Chilean Patagonia.