Functional relationships between Cyanobacteria and Bacteria: dynamic of the phycosphere and internal N recycling

Informations

Funding country

France

Acronym

PHYCOCYANO

URL

-

Start date

4/30/2012

End date

-

Budget

180,000 EUR

Fundings

Abstract

Cyanobacterial blooms are known to modify the aquatic ecosystem functioning by strongly decreasing the biodiversity of the phytoplankton communities, and enhancing the biomass production. Moreover, they could generate health problems, by their capacities of producing harmful cyanotoxines for humans and animals. Although it is now well established that nutrient loadings (mostly N and P) and climate conditions (water column stability) play a major role in the determinism of the phytoplankton blooms; numerous factors and processes involved in the enhancement of these proliferations, are still poorly understood. For example, positive/negative interactions within the microbial communities could play a major role. Few knowledge of the phycosphere (equivalent to the rhizosphere concept) of the cyanobacteria is available, although the matter and energy fluxes (mucus and exudates, N2 fixation products, organic matter mineralization) are certainly intense within the phycosphere and the cyanobacteria. In the project proposed herein, the bacterial communities will be studied within the phycosphere of two toxic cyanobacteria, Microcystis aeruginosa, and Aphanizomenon flos aquae, well known for their proliferations in many worldwide ecosystems. During the seasonal bloom (May to October), the succession of both species within the phytoplankton community was observed: with a predominance of A. flos aquae in May-June, followed by the codominance in June-July, and the dominance of M. aeruginosa from July to October. More precisely, in this project we will determine the bacterial structural and functional diversities and activities within the phycospheres, with a special attention given to the ones involved in the N recycling. Effectively, M. aeruginosa is not able to fix N2, while A. flos aquae can develop differentiated cells for the N2 fixation (heterocysts) within their filaments. Thus our main hypothesis is that the N2-fixing capability will have strong impact on the diversity and functioning of the bacterial consortia associated to both cyanobacterial types. To test this, we will develop an integrative approach based on metatranscriptome analyses, molecular quantification of genes involved in the N recycling, combined with the 15N pool dilution method and measures of potential activities. These approaches would allow to: (i) compare the structure and composition of the bacterial communities associated to both cyanobacteria, and the free-living bacteria fraction (ii) to characterize the metabolism and activities of the genes involved in the N recycling within the active bacterial fraction, (iii) determine the N fluxes and N20 emissions within the bacteria-cyanobacteria consortia, in comparison to the ones from the free-living bacterial fraction. Altogether, these results will inform us on the relationships existing between the cyanobacteria and the bacteria associated, which could play a significant role in the ecological success of the cyanobacteria in aquatic ecosystems.