CONICET | Buscador de Institutos y Recursos Humanos

Dwindling fossil fuel reserves, and an increasing global warming, prompts the prospecting of alternative energy sources. Harvesting solar energy as photosynthetic biomass, and its further processing into different biofuels as energy carriers is one of the most promising strategies for the years to come. Last-generation biofuels technology includes culturing of either cyanobacteria or unicellular Chlorophytes. The most remarkable advantages of this feedstock in comparison to traditional crops are superior yield, no requirement of fertile land (avoiding competition with food production and preventing deforestation) and more efficient management of water and nutrients, among others. Although genetic engineering of microalgae to improve different aspects of biofuels technology is in the pipeline in many laboratories worldwide, transference of major metabolic pathways such as nitrogen fixation seems not to be possible in the near future. To overcome that drawback, we aim at assembling synthetic microbial communities made up of model organisms with complementary metabolic capabilities. In this work we show some properties of mixed cultures of the unicellular non-nitrogen fixing cyanobacteria Synechocystis sp. PCC 6803 and the diazotrophic, plant growth-promoting proteobacteria Azotobacter vinelandii in a minimal synthetic medium using air as the carbon and nitrogen sources. Addition of A. vinelandii cells to a cyanobacterial culture produced a modest but reproducible increase in biomass, cells counting and chlorophyll accumulation. A. vinelandii mutants with complete deletion of either the transcription activator NifA or the anti-activator NifL, producing a concerted silencing or constitutive over-expression of nitrogen fixation (Nif) genes, respectively, were obtained. Changing Nif properties of A. vinelandii appeared to alter more significantly biomass accumulation and cell countings than chlorophyll accumulation. Strain PCC 6803, had an increased plating efficiency when co-cultured with A. vinelandii onto solid medium. Moreover, the cyanobacterium show, in addition to the regular small round colonies, some larger and diffuse colonies, which corresponded to cell patches containing both microorganism. Strain PCC 6803 colonies, show unidirectional gliding motility towards A. vinelandii colonies when present at a minimal threshold proximity, suggesting a positive taxism. This study shows: i) a proof-of-concept for the synthetic community approach using two genetically characterized microorganisms; ii) the initial genetic dissection of A. vinelandii cyanobacterial growth-promoting properties; iii) one of the first steps towards the improvent of photosynthetic-biomass production properties of synthetic microbial communities by genetic engineering. Supported by ANPCyT & UNMdP.

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