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Biofilm development of the hyperhalophilic archaeon Halobacterium sp. Structural and cellular analysis. Di Meglio LG1, Busalmen JP2 and Nercessian D1 1 Instituto de Investigaciones Biológicas, UNMdP-CONICET, Mar del Plata, Argentina 2 Div Electroquímica y Corrosión INTEMA-CONICET, Mar del Plata, Argentina dnercess@mdp.edu.ar Modern technological and industrial applications frequently use microorganisms as biocatalysts for the optimization of processes. This strategy is more efficient when microorganisms are grown in biofilms, which are the most frequent microbial way of life in nature. These multi-species communities grow at virtually every interface and are the focus of intense research all over the world. Notably, the present knowledge on biofilms composed by hyperhalophilic microorganisms is still limited. In these work a strain of Halobacterium sp. an hyperhalophilic archaeon isolated from La Colorada Grande saltern (La Pampa province, Argentina) was grown forming biofilms onto glass surfaces aiming at evaluating their structure and kinetics of growth. For these aims biofilm was performed in flow-through cells that allowed the direct observation at the phase contrast microscope and the structural study comprising optic sectioning and digital image analysis. Thereby biofilm thickness and cell coverage at different focal planes were determined over the time of growth. Indeed, these parameters were measured on biofilms developed under standard salinity (25% total salts) and under ionic stress generated by low salt concentration (18% total salts), as a way of representing natural environmental pressure on biofilm persistence. Initial observations were related to the unusual elongation of individual cells during early stages of biofilm development. Upon colonizing the surface they elongated to around 6 µm, reaching three times the average length observed at the same growth stage in planktonic cultures. Notably, as biofilms grew the length of cells returned to the typical value, suggesting that elongation may be a colonization related strategy. Under low salinity conditions this behavior was also observed, but in a lower degree. Along biofilm development at standard salinity cells distributed in clusters covering up to 20-30% of the glass surface and reaching a stabilization thickness of about 80 µm. Under ionic stress (18% salts), cluster structure remained the same but the stabilization thickness was about half that typically reached under 25% of salts, evidencing some physiological limitation to growth. Nevertheless, this effect was not related to the viability of cells, as reveled by confocal microscopy using Live-Death probes. Longer cells were again observed in mature biofilms, intertwined in upper biofilm layers well exposed to liquid streams, suggesting that controlling cell size may be a strategy for Halobacterium sp. in overcoming environmental challenges. Supported by CONICET, ANPCyT and UNMdP.