What the beaks of Galapagos finches have to do with marine bacteria

Closely related, abundant clades of marine Polaribacter avoid competition during spring algae blooms via distinct sugar diets.
Published in Microbiology
What the beaks of Galapagos finches have to do with marine bacteria

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The variety of finch species on the remote Galapagos islands is the most prominent example for Charles R. Darwin’s and Alfred R. Wallace’s theory of evolution through natural selection. Galapagos finch species have developed distinct beak sizes and shapes and thereby have adapted to different food sources. This exemplifies, how even closely related species can effectively make use of available resources, avoid competition and thus co-occur in the same habitat.

The food connection between microalgae and bacteria. The satellite image of a spring bloom in the German Bight (© NASA) and schematic depiction of microalgae and bacteria (©M. Schlösser) are shown. The island Helgoland is encircled in yellow.

This principle is not limited to macrofauna, but also applies in the realm of marine microbes. Satellite photos taken from coastal areas during warm seasons often show that the sea is green rather than blue. This color originates from blooming microscopic marine algae. Such blooms are transient as they are eventually terminated by nutrient-depletion as well as predation by protists and viruses. This in turn leads to the release of large quantities of organic matter into the seawater, including algal polysaccharides. These algal sugars are among the main food sources for heterotrophic marine bacteria.

Scientists at the Department of Molecular Ecology at the Max Planck Institute for Marine Microbiology (Bremen, Germany) have investigated the bacterial response to spring algal blooms off Helgoland Island in the German Bight (southern North Sea) for more than a decade. A close-knit microbial community is recurrently abundant during spring blooms in most years. One of the most prominent community members is Polaribacter, a genus of the Flavobacteriia class. We investigated Polaribacter abundances from 2009 to 2012 during spring blooms and identified co-occurring closely related but distinct clades. Taxonomic and metagenomic analyses revealed that these Polaribacter clades have rather distinct niches with respect to algal polysaccharides. Even though sampling years were few, there were indications of a certain periodicity, as clades tended to occur at different bloom stages. One clade of presumed first responders is characterized by small genomes with a pronounced protein but limited sugar degradation capacity. In contrast, another clade of presumed late responders has larger genomes and the capacity to utilize more complex polysaccharides. One of the clades might be tied to presence of a specific algae (genus Chattonella). It is characterized by large genomes and has the most diverse sugar menu of all investigated Polaribacter clades.

Like Galapagos finches, these results exemplify how also closely-related clades of marine bacteria (here Polaribacter) can forgo direct competition by partitioning available resources (here polysaccharides). We hope that studies as the one presented advance our understanding of the principles that govern microbial community composition in nutrient-rich environments.

Check out the full version of our paper: https://www.nature.com/articles/s41396-020-0601-y

Thanks to Ismail Tsavous for the cover image. 

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