6. Final Report Summary - CHEMOARCH (Identity and biogeochemical role of chemoautotrophic prokaryotes in aquatic ecosystems)
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Abstract: The project aimed to study the process of dark CO2 fixation in aquatic systems, by identifying chemoautotrophic prokaryotes at the phylogenetic level and studying their abundance, dynamics and metabolic activity. While photosynthetic CO2 assimilation has been extensively studied, the incorporation of CO2 in the dark had been almost overlooked in environmental studies. Recent studies suggested that this process is more widespread than assumed in aquatic systems and thus, it is critical to understand the magnitude of dark CO2 incorporation and the microorganisms involved under different environmental conditions. We focused on the group Crenarchaeota, since these microorganisms are widespread in aquatic environments and able to fix CO2 while gathering energy from oxidation of ammonia.
The three main objectives of the project were:
1. to identify which prokaryotic groups are the 'key players' in non-photosynthetic fixation of CO2 in different aquatic ecosystems.
2. to determine the main environmental factors which influence the distribution of chemoautotrophic prokaryotes throughout natural environmental gradients.
3. to study the metabolic strategies associated with the growth of chemoautotrophs.
An important part of the project was the participation in two polar cruises to the Amundsen Gulf (Arctic) and the Western Antarctica (Amundsen & Ross Seas). Specific work in polar environments was not originally emphasised, but there are several reasons why these systems are relevant, e.g. Crenarchaeota are highly abundant in polar systems, while their diversity and ecology is mostly unknown. Polar oceans are also highly sensitive to climate change. The work during the cruises aimed to explore the relevance of dark CO2 assimilation and identify microbes involved in the process. Microautoradiography combined with fluorescence in situ hybridisation (MARFISH) was used to assess the single cell activity in CO2 uptake. BrdU-immunocapture was used to identify the replicating populations and metagenomic sequencing was applied to broadly describe the gene pool of Crenarchaea-dominated communities. High throughput 454 sequencing of archaeal 16S rRNA genes was applied along with quantitative PCR targeting functional genes involved in ammonia oxidation.
The main results are:
1. Identification of active marine bacterial groups in Arctic dark CO2 assimilation. Some typically heterotrophic taxa within Gammaproteobacteria were very active in the dark uptake of CO2, implying that dark CO2 assimilation could be very important for the metabolism and survival of polar heterotrophic bacteria (Objective 1).
2. Discovery of an archaeal diversity hotspot in Antarctic circumpolar deep waters. Marine Archaea are abundant in polar environments, but their diversity and biogeography are unknown. We found marked variations in archaeal diversity among different Antarctic water masses, showing that hydrography shapes the biogeography of marine Antarctic Archaea. This is the first study of Antarctic archaeal diversity based on 454 pyrosequencing (Objective 2).
3. QPCR data revealed that the majority of Crenarchaeota in Antarctic water masses contain the gene for ammonia oxidation and thus are likely chemoautotrophs while single-cell activity showed that they were less active in CO2 fixation. The same pattern was found in Arctic waters, indicating that the contribution of Crenarchaeaota to CO2 fixation in polar waters is lower than assumed. An ongoing analysis of a metagenomic sample collected during the Arctic winter will provide in depth information on the metabolic properties of Archaea in polar systems (Objective 3).
These results will improve our current understanding of the biogeochemical role of Archaea in polar systems. Due to the great sensitivity of these environments to global change, understanding the key biogeochemical players and regulating mechanisms are urgent issues for the scientific community, and society in general.