Unprecedented decline in marine viruses in western Mediterranean linked to climate change

A team led by the Institut de Ciències del Mar (ICM-CSIC) has described a sustained and unprecedented decline in the abundance of marine viruses in the northwest Mediterranean over the past two decades. The discovery, published in the journal ISME communicationis based on the longest time series data on marine viruses known to date, from the Bay of Blanes Microbial Observatory (BBMO) in Girona.

The results show that since 2011, the abundance of viruses in seawater has steadily decreased, coinciding with a gradual increase in water temperature and transparency, as well as a significant reduction in nutrients and phytoplankton biomass. These changes reflect a process of oligotrophication, that is to say a depletion of nutrients, which transforms the marine ecosystem into a more pristine and less altered environment.

“What we observe is a clear signal that global change, and in particular climate change, is reconfiguring even the tiniest microbial communities, invisible to the human eye but essential to the functioning of the oceans,” says Xabier López-Alforja, lead author of the study.

“Marine viruses fulfill essential roles in ecosystems: they regulate microbial populations, recycle nutrients and can promote the transport of carbon to the ocean floor, a key process in regulating global climate.”

A unique observatory

The work draws on data sets generated by the Blanes Bay Microbial Observatory, where monthly surface water samples have been collected since 2001. This ongoing effort spanning two decades makes BBMO a unique observatory in the world for studying long-term microbial dynamics.

The research used advanced statistical tools, such as generalized additive mixed models (GAMM), which allowed the team to model and understand the marked seasonal variability of microorganisms and different environmental variables in this part of the Mediterranean. Additionally, the scientific team applied artificial intelligence techniques, particularly neural network models, to unravel the complex interactions between viruses and their environment.

“Through the combination of advanced statistics and machine learning, we were able to see beyond immediate variability and recognize hidden patterns that indicate how viruses respond to warming and nutrient loss,” details López-Alforja.

Ecological and social implications

The reduction in the abundance of marine viruses could have significant consequences on ecosystems and society. By altering the control of microbial populations and the efficiency of nutrient recycling, these changes can accelerate the decline in marine productivity and affect food web stability.

“If viruses decrease, the way nutrients circulate in the ocean is also changed,” adds Dolors Vaqué, researcher at the ICM. “This can affect not only the balance of coastal ecosystems, but also human activities such as fishing, on which entire Mediterranean societies depend.”

Until now, most studies on marine viruses were carried out along spatial gradients or over very short time series, generally less than two years. The lack of long-term data has made it difficult to clearly assess the effects of global change on these organisms.

“Our research raises questions about how this effect on viral communities translates to that of other microorganisms and to biogeochemical cycles,” comments Felipe Coutinho.

“Only with long time series can we distinguish natural fluctuations from trends induced by climate change. And in this case, the signal is unequivocal: viruses are decreasing in parallel with the oligotrophication of the Mediterranean.”

Towards new avenues of research

The study is the result of a collective effort between various ICM-CSIC researchers who keep the Blanes Bay Observatory operational. The research team is now working to sequence and analyze the genetic material of viruses collected over these years to determine whether the loss of abundance is also reflected in changes in the genetic diversity of virus communities.

They also claim that consistency with analyzes of other studies based on shorter time series in the region confirms a generalized trend across the entire Mediterranean basin.