The work, in which researchers from the Institut de Ciències del Mar (ICM) have participated, links genetic data to cell images to better study these bacterial and archaeal groups.
While most people these days have their attention firmly fixed on carbon because of the role of CO2 in climate control, we shouldn’t forget about nitrogen. This is because like carbon, nitrogen is one of the major components of life’s cellular building blocks. Also, this element is one of the most important growth-limiting nutrients for both land plants and marine phytoplankton, which is responsible for half of the total photosynthesis on the planet.
This may come as a surprise given that 80 % of the atmosphere is composed of nitrogen gas or dinitrogen (N2). However, N2 is very stable and resistant to chemical reactions, and hence it cannot be assimilated by most organisms. That’s why the small but diverse group of microbes that are able to “fix” atmospheric N2 into more accessible forms for living organisms are so crucial for the Earth ecosystem, acting as natural fertilizers.
Now, a group of researchers from the Institut de Ciències del Mar (ICM) has participated in a study published recently in the prestigious journal Nature Communications that offers an improved global overview of the abundance, diversity and distribution of marine nitrogen-fixers, a few bacterial and archaeal groups microorganisms also known as diazotrophs.
These organisms cover a broad size spectrum -from μm to mm- due to their diverse lifestyles: single cells, cell aggregates, colonies, free-living, and symbiotic associations. This challenges the researchers’ ability to collect and separate them from the co-occurring and more abundant marine microbial populations.
On land, however, where more than 90% of nitrogen fixation is performed by bacteria living in symbiosis with a diverse group of land plants, nitrogen-fixers are far better characterized given their vital importance to agriculture. In addition, the exploration of the diversity, activity, and distribution of marine nitrogen-fixers is hindered by the vastness of the global ocean, so it is not that easy to observe and study them.
Data from the Tara Oceans Expedition
To carry out the study, the authors linked genetic data to cell images in the global ocean. They analyzed data from the Tara Oceans Expedition, that from 2009 to 2013 sailed the main oceanic regions collecting environmental data along with plankton samples.
“This expedition provided a unique framework to unveil the distribution of marine diazotrophic microorganisms across the global ocean, covering different plankton size fractions and combining “omics” and imaging large datasets”, notes the ICM researcher Silvia González Acinas, co-author of the study.
At all stations, DNA samples were taken from surface to a depth of 1,000 metres, while some locations included parallel samples to generate molecular (e.g. DNA sequencing) and microscopy imaging datasets. Then, using machine learning prediction tools for analysing >2 million images in combination with the analysis of 30 terabases of DNA sequencing data, an improved global overview of abundance, diversity and distribution of nitrogen-fixers was obtained.
“The combination of new methods and approaches is uncovering more and more diversity of nitrogen fixers in planktonic compartments where we were not looking at before. Our findings will lead to better estimations of nitrogen fluxes in the ocean as we incorporate these newly discovered nitrogen fixers in predictive biogeochemical models”, comments Francisco Cornejo Castillo, another co-author of the study that will join the ICM in the coming months.
Among the most relevant findings of the study we can find the detection of new abundant “hotspots” in understudied oceanic regions where multiple nitrogen-fixers appear to coexist, and the identification of “ultrasmall” (less than 0.2µm) nitrogen-fixers.
In this regard, the leading author of the study, Juan Pierella Karlusich, from the Institute of Biology of the Ecole Normale Supérieure (IBENS) points out that “these results will advance the research field by bringing attention to these new high density regions and the new types of nitrogen-fixers”.
All in all, the work offers a proof of concept that integrative analyses of molecular and imaging data can lead to more accurate explorations of ocean microbes.