PRIMA

Priming in an aquatic ecosystem - Stream biofilms as hotspots for carbon cycling

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 Obiettivo del progetto (Objective)

'Recent findings concerning the role of inland waters in global carbon cycling is currently having a major impact of the view of the global carbon cycle. These findings highlight inland waters - such as streams, rivers and lakes - as major sites of carbon cycling, implying that they must be considered in the context of climate change. Microbial degradation of organic carbon is a process that is central to carbon cycling in all ecosystems. In soils, microbial degradation of recalcitrant carbon is often controlled by the availability of labile carbon sources. This is linked to the priming effect (PE). Mounting evidence suggests that PE is also important in aquatic ecosystems but it has yet to be explicitly addressed. Biofilms are vital components of aquatic ecosystems. In stream biofilms, heterotrophic bacteria and algae coexist in close proximity, exposing the bacteria to both recalcitrant organic carbon of terrestrial origin and labile organic carbon from the algae. This could make stream biofilms hotspots for PE. In PRIMA, I propose an innovative effort cutting across aquatic and terrestrial ecosystems, spanning single-cell to ecosystem scales, and combining methods from biogeochemistry and molecular microbiology to study PE in stream biofilms. Carbon flux in stream biofilm microcosms and in ecosystem scale stream mesocosms will be measured to quantify PE and its implications for carbon cycling in streams. The mechanisms of PE will be addressed on single-cell and community scales using cutting edge methods, such as NanoSIMS and 454-sequencing. I am an experienced researcher trained in Norway and Sweden. In PRIMA, I seek to combine my existing skills with the unique expertise and facilities of Prof. Tom J. Battin at the University of Vienna. The many conceptual and methodological training objectives of PRIMA, as well as its outstanding scientific quality, will strengthen my scientific skills and will enable me to reach my goals as an independent researcher.'

Introduzione (Teaser)

Scientists understand that streams play a key role through their contribution to the global carbon cycle; however, unanswered questions remain. These include how organic carbon, which is bound to an organic compound and protected in the soil over long periods, can be made available to organisms.

Descrizione progetto (Article)

The project 'Priming in an aquatic ecosystem - Stream biofilms as hotspots for carbon cycling' (PRIMA) investigated the stream conditions that lead to microbial biofilms. Biofilms are created when microbial cells stick to one another on a surface, such as a stream bed.

PRIMA hypothesised that terrestrial organic carbon is subject to priming in stream microbial biofilms, which play an important role in aquatic ecosystems. Priming occurs when the addition of carbon or nitrogen affects the rate of decomposition occurring in biofilms. Material labelled with the stable carbon isotope C13 was used to test for priming by tracking and quantifying carbon flux.

Results indicated that priming did not appear to occur in the hyporheic zone of the stream, where shallow groundwater mixes with surface water. However, priming does occur in biofilms, which coexist in close proximity with algae on the bottom of the stream. Material exuded from the algae can undergo change and may serve as the primer. In addition, the biofilm metabolism and changes to dissolved organic carbon may contribute to the generation of 'recalcitrant' organic matter.

Two major experiments were conducted under the auspices of PRIMA. The first mimicked hyporheic biofilms in 25 bioreactors that contained different potential primers, including exuded material from algae. The second experiment mimicked light-dependent bottom-dwelling biofilms grown under different light conditions. The aim was to provide different algae and, therefore, different potential primers.

Both experiments used willow (Salix) grown in a carbon dioxide (CO2)-enriched atmosphere, where the gas was labelled with 13C. The result was fully labelled plant tissue that provided a complex mix of 13C-labelled organic matter, which was decomposed to remove most of the functional groups.

The remaining material was described as recalcitrant and used in both experiments. This methodology enabled researchers to measure the fraction of respiratory CO2 from the degradation of recalcitrant dissolved organic matter.

Work conducted by the PRIMA project provides further insights into the role of inland waters in global carbon cycling. The results confirm their importance as major sites of carbon cycling, which must be taken into consideration when investigating the mechanisms behind climate change.