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

The results show that since 2011, the abundance of viruses in seawater has continuously decreased, coinciding with a progressive increase in water temperature and transparency, along with a significant reduction in nutrients and phytoplankton biomass. These changes reflect an oligotrophication process—meaning an impoverishment in 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 especially climate change, is reconfiguring even the most minute microbial communities—those invisible to the human eye but essential for the functioning of the oceans," explains Xabier López-Alforja, lead author of the study.

"Marine viruses fulfill critical roles in ecosystems: they regulate microbial populations, recycle nutrients, and can favor the transport of carbon to the ocean floor, a key process in global climate regulation."

A unique observatory

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

The research utilized advanced statistical tools, such as Generalized Additive Mixed Models (GAMMs), which allowed the team to model and understand the marked seasonal variability of both microorganisms and different environmental variables in this part of the Mediterranean. Additionally, the scientific team applied artificial intelligence techniques—specifically, neural network models—to unravel the complex interactions between viruses and their environment.

"Thanks to the combination of advanced statistics and machine learning, we've been 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 decline in the abundance of marine viruses could have significant consequences for ecosystems and for society. By altering the control of microbial populations and the efficiency of nutrient recycling, these changes can accelerate the decrease in marine productivity and affect the stability of the food web.

"If viruses decrease, the way nutrients circulate in the ocean is also modified," adds Dolors Vaqué, an ICM researcher. "That can affect not only the balance of coastal ecosystems but also human activities like fishing, upon which entire societies in the Mediterranean depend."

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

"Our research raises questions about how this effect on viral communities translates to those 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 research avenues

The study is the result of a collective effort among various ICM-CSIC researchers who keep the Blanes Bay Observatory operational. The research team is now working on sequencing and analyzing the genetic material of the viruses collected over these years to determine if the loss of abundance is also reflected in changes in the genetic diversity of viral communities.

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