SYMNIF

Evolution of symbiosis between ciliates and nitrogen-fixing prokaryotes

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 Obiettivo del progetto (Objective)

'During the history of the planet Earth, acquisition of symbiont-encoded metabolic pathways has allowed hosts to exploit new ecological niches. A mechanistic understanding of the interactions between host and symbionts as well as knowledge about the role of symbionts in the functional diversification and species formation in hosts is, however, currently lacking. It is well known that aquatic ciliates may harbor phototrophic algae or methanogenic Archaea as symbionts. Moreover, I recently found nitrogen-fixing (diazotrophic) symbionts in ciliates (Protozoa) and recent studies report diazotrophic symbionts in marine dinoflagellates and zooplankton. Accordingly, I will in this project examine symbioses in model systems consisting of diazotrophic prokaryotes and aquatic ciliates to address the importance of symbionts for the diversification of hosts. The study implicates usage of cutting-edge microbiological, molecular and marine ecological methodologies. Various symbioses in marine and freshwater ciliates will be examined when exposed to different levels of nitrogen availability. The project will provide new insights into the formation, maintenance and specificity of symbioses between aquatic ciliates and their prokaryotic symbionts in relation to environmental conditions. These results have implications for our understanding of the importance of symbiosis in the formation of new species. Moreover, the study will provide first estimates of the nitrogen fixation of diazotrophic prokaryotes associated with ciliates. This is of relevance to the global cycling and availability of nitrogen in aquatic environments and thereby intimately linked to marine carbon budgets.'

Introduzione (Teaser)

Researchers in the EU have investigated how symbiotic marine microorganisms work together to harvest gaseous nitrogen into a form they can use.

Descrizione progetto (Article)

This process is known as nitrogen fixation, and serves an important ecological function by channelling environmental nitrogen into the food web. A newly discovered group of marine nitrogen fixers called diazotrophs work in symbiosis with other microorganisms called ciliates to fix nitrogen in the ocean.

The EU-funded 'Evolution of symbiosis between ciliates and nitrogen-fixing prokaryotes' (SYMNIF) project aimed to investigate the symbiosis between diazotrophs and their hosts.

Over 100 different samples of ciliates (a type of zooplankton) were collected from the North Atlantic and tested for the presence of diazotroph symbionts. Almost half of the ciliates tested harboured a nitrogen-fixing symbiont.

Tests on five years' worth of cultivated zooplankton showed that the same diazotroph species was present in many different types of ciliate.

Samples collected in spring and summer showed more symbionts than during winter, likely due to nitrogen depletion in seawater over summertime. However, adding nitrogen to growing zooplankton did not influence the nitrogen fixation rate, showing that the diazotrophs were not free-living species.

The SYMNIF project shed light on an important group of marine microorganisms for the first time. Understanding how these symbiotic microorganisms work together to trap nitrogen will help researchers better understand global nitrogen flows.