Genotype specific and microbiome effects of hypoxia in the model organism Daphnia magna

The fitness of the host is highly influenced by the interplay between the host and its associated microbiota. The flexible nature of these microbiota enables them to respond swiftly to shifts in the environment, which plays a key role in the host's capacity to withstand environmental stresses. To understand the role of the microbiome in host tolerance to hypoxia, one of the most significant chemical changes occurring in water ecosystems due to climate change, we performed a reciprocal gut transplant experiment with the freshwater crustacean Daphnia magna. In a microbiome transplant experiment, two genotypes of germ-free recipients were inoculated with gut microbiota from Daphnia donors of their own genotype or from the other genotype, that had been either pre-exposed to normoxic or hypoxic conditions. We found that D. magna individuals had a higher survival probability in hypoxia if their microbiome had been pre-exposed to hypoxia. The bacterial communities of the recipients changed over time with a reduction in alpha diversity, which was stronger when donors were pre-exposed to a hypoxic environment. While donor genotype had no influence on the long-term survival probability in hypoxia, donor genotypes was the most influential factor of the microbial community 3 days after the transplantation. Our results indicate that microbiome influencing factors mediate host fitness in a hypoxic environment in a time depending way.

The interplay between prior selection, mild intermittent exposure, and acute severe exposure in phenotypic and transcriptional response to hypoxia

Hypoxia has profound and diverse effects on aerobic organisms, disrupting oxidative phosphorylation and activating several protective pathways. Predictions have been made that exposure to mild intermittent hypoxia may be protective against more severe exposure and may extend lifespan. Here we report the lifespan effects of chronic, mild, intermittent hypoxia, and short‐term survival in acute severe hypoxia in four clones of Daphnia magna originating from either permanent or intermittent habitats. We test the hypothesis that acclimation to chronic mild intermittent hypoxia can extend lifespan through activation of antioxidant and stress‐tolerance pathways and increase survival in acute severe hypoxia through activation of oxygen transport and storage proteins and adjustment to carbohydrate metabolism. Unexpectedly, we show that chronic hypoxia extended the lifespan in the two clones originating from intermittent habitats but had the opposite effect in the two clones from permanent habitats, which also showed lower tolerance to acute hypoxia. Exposure to chronic hypoxia did not protect against acute hypoxia; to the contrary, Daphnia from the chronic hypoxia treatment had lower acute hypoxia tolerance than normoxic controls. Few transcripts changed their abundance in response to the chronic hypoxia treatment in any of the clones. After 12 h of acute hypoxia treatment, the transcriptional response was more pronounced, with numerous protein‐coding genes with functionality in oxygen transport, mitochondrial and respiratory metabolism, and gluconeogenesis, showing upregulation. While clones from intermittent habitats showed somewhat stronger differential expression in response to acute hypoxia than those from permanent habitats, contrary to predictions, there were no significant hypoxia‐by‐habitat of origin or chronic‐by‐acute treatment interactions. GO enrichment analysis revealed a possible hypoxia tolerance role by accelerating the molting cycle and regulating neuron survival through upregulation of cuticular proteins and neurotrophins, respectively.