INOCULATION WITH AZOSPIRILLUM STRAINS OVERPRODUCING AUXIN IMPROVES BIOMASS AND LIPIDS PRODUCTIVITY OF A MICROALGA UNDER SALT STRESS

PAGNUSSAT LUCIANA; GONORAZKY GABRIELA; DO NASCIMENTO MAURO; MARONICHE, GUILLERMO; CREUS CECILIA; CURATTI LEONARDO

Congreso; SAIB - SAMIGE Joint meeting 2021 on line; 2021

There is currently an increasing interest in the use of microalgae for wastewater treatment and the use of its biomass and lipids as a feedstock for biofuels. Both of these applications are often performed more efficiently by microalgal-bacteria consortia. However, the mechanisms that account for the stability and robustness of this kind of interactions are poorly understood While salinity stress is a mild trigger of lipids accumulation in some microalgae, an increased lipids productivity is mostly offset by the slower growth. Nitrogen starvation is a much stronger trigger of lipids accumulation, but decreases biomass and lipids productivity to a more pronounced level than salt. Previously we showed that the plant growth-promoting bacterium Azospirillum baldaniorum Sp245 also promoted growth of the microalga Scenedesmus obliquus C1S. This alga-bacterium interaction depended on bacterial production of indole-3 acetic acid (IAA), which resulted in a decrease in reactive oxygen species of the algal cells and higher cell densities. S. obliquus showed a clear dose-dependent slow-down of growth and biomass productivity according to the NaCl concentration in the growth medium, in the range 0-225 mM, larger cell sizes are observed and the number of isolated microalgae cells is increased. Additionally, a reduction in the yield of microalgal biomass is observed. For further analysis, NaCl was used al 150 mM as a concentration which reduced algal growth in axenic cultures by 4-fold, but could be partially reverted in preliminary experiments of inoculation with some Azospirillum strains. Because we showed before that nitrogen starvation ameliorates the tolerance response to NaCl of some microalgae, we kept nitrogen sufficiency at 10 mM NaNO3 throughout the study. At this concentration of N, S. obliquus does not normally accumulate lipids. Under these experimental conditions, inoculation with IAA-overproducing bacterial strains, produce higher yields of biomass and neutral lipids than treatments with the wild type A. baldaniorum parental strain. Notably, preliminary results show that inoculation with these strains increased lipids accumulation at 30,79% (w/w). Conversely, inoculation with a bacterial strain impaired in the production of IAA failed to enhance algal biomass and lipids productivity under salt stress conditions. In this case lipids accumulation remained lower at 25,6 % (w/w).The fact that the four A. baldaniorum strains used in this study showed an almost identical survival rate under the stated co-culture conditions, led us to propose a prominent role of IAA in mediating the increase in algal biomass and lipids productivity exerted by A. baldaniorum under salt-stress conditions. This study extends the current knowledge of the mechanisms underlying bacteria-microalgae consortia to improve their technological applications and to better understand ecological relationships in the environment.