#FEMSmicroBlog: How marine picocyanobacteria assimilate nitrogen

15-11-2022

The oceans are some of the biggest oxygen reservoirs on this planet thanks to the multitude of microbes doing photosynthesis. The two marine picocyanobacteria Prochlorococcus and Synechococcus flourish in oceanic areas even though nutrient concentrations are extremely challenging. The review “Regulatory and metabolic adaptations in the nitrogen assimilation of marine picocyanobacteria” in FEMS Microbiology Reviews discusses the abilities of these two bacteria to adapt to the nitrogen-limiting conditions in oceans. Jesús Diez Dapena explains for the #FEMSmicroBlog how these microbial all-rounders became the most abundant photosynthetic organisms on Earth. #FascinatingMicrobes

 

How life on Earth comes about

When thinking about the essential aspects of life, climate, the oxygen we breathe, carbon dioxide or global heating come to mind. One might quickly move towards the huge Amazon forests, the Taiga or the forests in Central Africa and deem them also necessary for life on this planet.

Among other benefits, these forests contribute to food production and oxygen generation. They also show great capacities in capturing carbon dioxide, thus helping in the fight against climate change.

However, many forget the mass that covers the majority of the planet’s surface: the oceans. Particularly in oceanic deserts – areas in which nutrients are scarce – tiny microorganisms thrive that are key agents for life on Earth.

Picocyanobacteria thrive in oceanic deserts – areas where nutrients are scarce – and are key agents for life on Earth.

In fact, marine picocyanobacteria, specifically Prochlorococcus and Synechococcus, are the most abundant photosynthetic organisms on Earth. These small sized organisms are important primary producers as they generate large amounts of photosynthetic oxygen. Also, these bacteria can reduce overall carbon dioxide concentrations by incorporating and converting this gas.

 

Nitrogen limitation in the ocean selects for Prochlorococcus and Synechococcus

The review Regulatory and metabolic adaptations in the nitrogen assimilation of marine picocyanobacteria” in FEMS Microbiology Reviews outlines how picocyanobacteria adapted to the oceanic environments. This work focuses on strategies that Prochlorococcus and Synechococcus use to assimilate nitrogen since this element is often limited in oligotrophic desert areas.

The review analyses how Prochlorococcus and Synechococcus use different nitrogen sources. For instance, ammonium is more abundant at about 10-40 m water depth while nitrate is more abundant at a deeper depth. Thanks to their genomic background, different picocyanobacterial strains are able to adapt to the particular area in response to the predominant nitrogen form.

Next, the study discusses the characteristics of different nitrogen transporters, the main enzymes of the nitrate assimilation pathway and their relationships with carbon metabolisms. In all these subjects, marine picocyanobacteria differ immensely from freshwater strains.

These differences seem to be due to nitrogen being limited in oligotrophic areas of the oceans constantly. In contrast, in freshwater, nitrogen concentrations are rather fluctuating, so freshwater picocyanobacteria need to adapt to these conditions more regularly. This characteristic appears to be an essential selective agent driving niche differentiation of strains.

Picocyanobacteria adapting their nitrogen metabolism to the nitrogen-limiting conditions of the oceans.
Picocyanobacteria adapting their nitrogen metabolism. From Díez et al. (2022).

 

Marine picocyanobacteria and their remarkable plasticity

Finally, ecophysiological factors may explain the contrasting oceanic distribution of these marine picocyanobacteria. During the course of evolution, picocyanobacteria have made a lot of changes in their genomes and metabolisms to save nitrogen.

Picocyanobacteria adapted their genomes and metabolisms to save nitrogen and become the most abundant photosynthetic organisms.

For example, by shortening the genome and lowering its GC content, they manage to save nitrogen. Also, proteins became shorter while still performing the same tasks and simplified regulatory systems adapted to help these microbes be frugal with nitrogen.

On the other hand, assimilating oxidized forms of nitrogen is energetically more expensive than assimilating reduced nitrogen. However, most nitrogen in oceans is of oxidized nature so picocyanobacteria had to develop and optimize their machineries to utilize the little available amount.

Taking all these mechanisms into account, Prochlorococcus and marine Synechococcus have demonstrated remarkable plasticity. They are able to colonize and thrive in all kinds of oceanic niches, from cold waters of the poles to warm intertropical oceans and from nutrient-rich coastal regions to extremely oligotrophic areas. This remarkable adaptability likely helped them become the most abundant photosynthetic organisms on this planet.

 

About the author of this blog

Jesús Diez Dapena is a professor of Biochemistry and Molecular Biology at the University of Córdoba, Spain. He has been always interested in photosynthetic microorganisms and his main research interest has been focused, for more than 45 years, on nitrogen metabolism from green algae and cyanobacteria. During the last 25 years, his research has been centered on marine picocyanobacteria, including the study of carbon metabolism and the relationships between this and nitrogen metabolism. He enjoys reading and mountain hiking.

About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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