Climate change can cause stress in herring larvae

When herring larvae are exposed to multiple stressors simultaneously, their ability to react to these changes at the molecular level is reduced: A combination of two factors is enough to prevent a protective response. This is the result of an experiment conducted by a team led by Dr Andrea Franke from the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg (HIFMB) in Germany. The researchers exposed herring larvae from the Baltic Sea – a marginal sea of the Atlantic Ocean east of Denmark – to elevated temperatures, bacteria and a combination of both. They published their results in the journal “Science of the Total Environment”. The study also involved researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel and Kiel University, also in Germany.

In their experiment, the researchers looked at three types of responses to find out how the herring larvae reacted to different combinations of environmental changes. First, they examined the activity of all larval genes, known as gene expression. Second, the team examined so-called microRNA. These are short molecules that can have an inhibitory or activating effect in the complex process of protein synthesis. Through gene expression and microRNA, an organism has the ability to regulate protein production and thus react to environmental changes. Thirdly, the researchers looked at the microbiome of the herring larvae: they used genetic analyses to determine which microbes live on and in the animals.

The team saw changes in all three analyses. In terms of gene expression, the larvae showed a particularly impressive reaction: when the animals were exposed only to a heatwave or only to bacteria, the researchers observed that a large proportion of the genes were downregulated. The scientists interpret this as a cellular stress response that serves to protect proteins and the genetic material DNA and helps to prevent irreversible cell damage.

“We did not expect that the larvae would no longer show a response at the gene expression level when several stressors occur simultaneously.”

What surprised the researchers, however, was that this stress response was completely absent when the larvae were exposed to a heat wave and the bacteria at the same time. “We did not expect that the larvae would no longer show a response at the gene expression level when several stressors occur simultaneously. This can lead to protein damage and cell damage in the larvae, which may impair their growth and survival in the long term,” says first author Franke. This would be bad news for the herring stock in the western Baltic Sea, which is already severely depleted.

So far, comparable experiments have mostly investigated the effect of individual stressors on fish larvae. However, this does not necessarily reflect reality, Franke emphasizes: “Climate change often means that marine organisms and ecosystems are exposed to several stressors at the same time.” According to Franke, future long-term studies using multiple stressors could investigate the impact of climate change on the growth and survival of herring larvae.

The HIFMB was founded in 2017 and is an institutional cooperation between the Alfred Wegener Institute in Bremerhaven Helmholtz Centre for Polar and Marine Research (AWI), and the Carl von Ossietzky University in Oldenburg. It researches marine biodiversity and its importance for the function of marine ecosystems. In doing so, it develops the scientific basis for marine nature conservation and ecosystem management.