Prochlorococcus: Capable of more than we thought?

It is well known that Prochlorococcus Cyanobacteria are the most abundant marine photosynthetic microorganisms. Because of this they exist in a range of different light conditions and can be broadly classified into two ecotypes: low-light adapted and high-light adapted. Obviously these two ecotypes will face different limiting factors and therefore must have different adaptations to cope with external stressors. For instance, nitrogen availability often proves to be a limiting factor unless a microorganism is able to use inorganic sources of nitrogen. Currently though over one hundred single-cell-amplified partial genomes have been described for Prochlorococcus but not one was able to use nitrate as a source of nitrogen.

However, in a study by Astorga-Elo et al, it was found that certain uncultivated lineages of deep water Prochlorococcus populations showed nitrate assimilation rates. They were found to have the necessary genes for this process in both the global ocean sampling metagenomics database and in metagenomes of flow-cytometry-sorted populations. These uncultivated lineages thrive in Anoxic Marine Zones (AMZs) where oxygen concentration is below the threshold for detection by modern sensors and light is scarce but inorganic nutrients are abundant. It is therefore likely that the genomic potential for nitrate assimilation acts as an adaptation for survival in these relatively harsh conditions.

The paper reports the results of metagenomic analysis carried out on environmental sequences from samples collected within the AMZ of eastern tropical Pacific. The results showed that the microbial community was enriched in Prochlorococcus, the majority of which (~90%) were found to possess the genes for nitrogen assimilation. In addition to this, de novo assembled contigs found that there was a single contig that encodes the genes related to urea and nitrate uptake and assimilation. These genes were in synteny with those found in Synechococcus WH8102. In other words, the analysis suggests that the genetic potential for nitrate assimilation has not been recently obtained via horizontal gene transfer but is a characteristic retained from a common ancestor.

The results of this study are indicative of what is likely to be an adaptation of Prochlorococcus to the nutrient-rich environment of AMZs. I believe this study to be important as it provides new information on a major marine microorganism, helping us to further understand both its adaptive abilities and evolutionary history. And, by demonstrating that certain lineages of the cyanobacterium have retained this specific adaptive characteristic they show how environmental pressures can alter the process of genetic streamlining.

Astorga-Elo, M., Ramirez-Flandes, S., DeLong, E. F.. (2015). Genomic potential for nitrogen assimilation in uncultivated members of Prochlorococcus from an anoxic marine zone. The ISME Journal. 9, 1264-1267.