Eukaryotic Microbiome of Lake Sturgeon Eggs, and Identification of Chemical Thresholds for Infection Control

Eukaryotic microorganisms are an important, but understudied, component of freshwater aquatic ecosystems, and are significant sources of mortality in early life stages of fishes in natural and aquaculture systems. The eukaryotic microbiome colonizing egg surfaces of the lake sturgeon (Acipenser fulvescens) was characterized from eggs collected in natural stream habitats and a streamside hatchery in the Cheboygan River watershed in MI, USA. The taxonomic diversity of members of the Kingdoms Fungi and Stramenopile associated with infections of lake sturgeon eggs during spawning is contributing to lake sturgeon mortality in the hatchery. Characterization of the microbial communities from deposited eggs demonstrated heavy influence of spawning location on the diversity of Pythium, an Oomycete predominating in the microbiome. The Ascomycota also had a strong and distinguishing presence, with members of the Dothidiales found only on eggs from the streamside hatchery. Aureobasidium pullulans, a ubiquitous pigmented yeast, was present in the greatest numbers of egg samples, and Helotiales were found only on samples from the Black River. Independent isolates were collected from egg surfaces and tested for chemical sensitivity to the oomicides ethaboxam and mefenoxam, which are used for control of Oomycete agricultural pathogens. Ethaboxam inhibited mycelial growth almost completely for all Saprolegnia strains tested, while mefenoxam, at 20 × strength, was largely ineffective. Water prevents the natural inactivation of mefenoxam by light, thus is not advisable in aquatic systems, where it could accumulate. Alternatively, ethaboxam may be a nonpersistent, welcome control option for these fish pathogens.