A UMH Study Sheds Light on How the Most Abundant Marine Bacteria Survive Climate Change

Researchers from the Miguel Hernández University of Elche (UMH) have unravelled the evolutionary mechanisms underlying the ecological success of the most abundant marine bacteria, known as SAR11. The study reveals that these bacteria combine a common 'genetic core' with small regions of 'flexible genes' that allow them to adapt successfully to environmental fluctuations at the population level. "This advancement will help understand how these microbial populations, crucial for global ecological balance, diversify and survive climate change," explain sources from UMH.

The marine microbiome plays an essential role in maintaining ecosystems, driving global biogeochemical cycles and accounting for up to 98% of marine primary productivity. The group of bacteria known as the SAR11 clade are free-living and numerically dominate the ocean's surface waters, representing 20-40% of all prokaryotic cells.

"Despite the abundance and cosmopolitan distribution of these microbes, limitations in recovering the full genetic richness of their natural populations have hindered understanding the relationship between microbial evolution and ecology from a genomic perspective," points out UMH researcher and study leader Mario López Pérez.

The UMH Microbial Genomics and Evolution Group has combined, for the first time, single-cell genomics and long-read metagenomics techniques to reconstruct with high precision the genetic diversity of SAR11 in environmental samples from the Mediterranean. This combination has allowed them to decipher how their genome is organised and how strains that coexist in the same population diversify.

The study shows that these bacteria share a nearly identical genetic core, representing 81% of their genome. The rest, known as the flexible genome, is concentrated in small regions—most with a single gene—and is found in equivalent positions within all strains. "These small variations are always in the same place in the genome and contain genes with equivalent functions, albeit in different versions," explains UMH researcher Carmen Molina Pardines, the study's first author. This genomic pattern favours the coexistence of multiple strains and reduces competition among them.

The study reveals that these bacteria form polyclonal populations, meaning they are composed of multiple genetic variants coexisting in the same environment. This process not only ensures the conservation of essential genes during selective sweeps but also maintains functional redundancy, safeguarding a broad environmental genetic reservoir.

This reservoir provides the population with the ability to respond quickly and adaptively to environmental fluctuations. "These results provide insights into the strategies that explain the ecological success of SAR11 in nutrient-poor marine environments, such as the Mediterranean," clarifies UMH researcher José M. Haro Moreno, also the first author.

Beyond the evolutionary discovery, the study demonstrates that third-generation metagenomics allows overcoming the technical limitations that prevented studying these microorganisms, which are extremely difficult to obtain in pure culture.