Gall-forming protistan parasites infect southern bull kelp across the Southern Ocean, with prevalence increasing to the south

Abstract

Protistan pathogens can have devastating effects on marine plants, yet the processes that affect their distributions and infection intensities are poorly understood. Species within the brown algal genus Durvillaea are major ecosystem engineers throughout the sub-Antarctic and cold-temperate Southern Hemisphere, and a newly described genus of protistan parasite, Maullinia, was recently found infecting D. antarctica in Chile. We set out to address 3 key questions. (1) Is there evidence for trans-oceanic dispersal of Maullinia? (2) Does Maullinia infect other Durvillaea species? (3) Does infection prevalence vary throughout the hosts' ranges? We sampled Maullinia on Durvillaea populations along coasts in Chile (D. antarctica, from 32 to 42 S: 8 sites), Australia (D. potatorum and D. amatheiae, from 36 to 38 S: 5 sites) and sub-Antarctic Marion Island (46 53' 47'' S, 3743' 32'' E). We used a genetic marker (18S rRNA) to verify the presence of Maullinia on Durvillaea at all sites and visual surveys of Maullinia galls to assess infection prevalence in Chile and Australia. We confirm that Maullinia infects Australian Durvillaea species, but our results indicate that each host species is parasitised by a different Maullinia lineage. Maullinia infection prevalence increased with latitude. Long-and short-distance dispersal events are inferred to have occurred based on genetic patterns. We conclude that Maullinia protists are broadly distributed and affect multiple host species, including at least 3 Durvillaea species (2 in Australia, and 1 in both Chile and Marion Island), and that environmental factors influence host susceptibility to infection.