A warmer world with more extreme weather events is here, and things will almost certainly get worse. Scientists are currently working to understand how animals may respond to these new environmental stressors, and particularly want to understand the effects of combined stressors that will likely co-occur in the real world. 

Two commonly occurring stressors that occur in aquatic environments and are often individually studied are high water temperatures and low water oxygen content (hypoxia). A new study by researchers at Canada's University of Guelph looks at the effects of these two stressors in concert on zebrafish embryos and larvae. Specifically, the researchers wondered whether zebrafish had increased tolerance to hypoxia when exposed to high temperature early in life. This could happen through a phenomenon known as "cross-talk," which occurs at the cellular level.

First, they needed to identify if exposure to just high temperature or just hypoxia would elicit similar responses from zebrafish embryos. They measured the average expression of genes from each stress pathway for each group to determine if there were differences in expression between embryos exposed to high temperatures and those exposed to hypoxic environments. They found that either stressor caused an increase in gene expression in the other stress pathway as well, suggesting that there was cellular cross-talk!

Next, to determine if the gene expression differences caused physiological changes for the fish themselves (not just within their cells), embryos from each treatment group were reared until the larval stage and then exposed to hypoxia. The researchers were looking for differences in larval tolerance to low-oxygen environments between larvae that had experienced the high-temperature environments as embryos and those that had already been exposed to hypoxic environments as embryos. They expected that more tolerant larvae would be able to maintain normal levels of activity (e.g. swimming) and have better survival rates than their counterparts at lower water oxygen levels. But contrary to their expectations, there was no significant increase in either tolerance or survival regardless of what conditions the larvae had been exposed to as embryos.

So although there was cross-talk at the cellular level, this did not translate into higher-level differences in zebrafish stress responses. Still, this research opens interesting avenues for better understanding whether animals will be able to increase their physiological tolerances to the warmer and less oxygenated environments that may develop as the climate changes.