Bioprospecção de bactérias halotolerantes de ambientes litorâneos do estado de Sergipe para biocontrole de fitopatógenos e promoção de crescimento vegetal

Abstract

Halotolerant microorganisms possess adaptive mechanisms that make them promising sources of bioactive compounds with biotechnological potential. In the agricultural context, these microorganisms stand out due to their role in promoting plant growth and controlling phytopathogens. This thesis aimed to prospect, characterize, and functionally evaluate halotolerant microorganisms from coastal environments in the state of Sergipe, Brazil, with a focus on sustainable agricultural applications. In the first chapter, 89 bacteria were isolated from dunes, restingas, and apicuns, of which eight exhibited significant antifungal activity. The selected isolates demonstrated halotolerant profiles, with viable growth in up to 20% NaCl, as well as tolerance to multiple abiotic stresses such as pH variations, high temperatures (up to 55 °C), polyethylene glycol exposure, and heavy metals. Isolates RT242, RT241, and AP245 stood out for their superior performance under saline stress, maintaining growth, cell viability, and biofilm formation. Functionally, all isolates presented at least one plant growth-promoting (PGP) activity, including indole-3-acetic acid production, phosphate solubilization, nitrogen fixation, and extracellular enzyme synthesis. Under saline conditions, isolates AP245 and RT241 significantly enhanced early growth of lettuce seedlings, particularly root development. Greenhouse experiments demonstrated that isolates RT242, RT241, and AP245 significantly stimulated biomass and photosynthetic pigment production. Multivariate analysis confirmed positive associations between these isolates and plant morphophysiological parameters. Phylogenomic identification revealed high similarity with species of the genus Bacillus, including B. velezensis, B. stercoris, and B. safensis. Integrated Bonitur analysis highlighted isolate RT242 as the most promising. In the second chapter, a new halotolerant Streptomyces strain (DN246), isolated from dunes, was characterized. This strain exhibited multiple traits associated with plant growth promotion and phytopathogen biocontrol, including production of indole-3-acetic acid, phosphate solubilization, nitrogen fixation, and expression of extracellular enzymes such as cellulase, chitinase, and lipase. In vitro assays showed that DN246 significantly inhibited the growth of phytopathogenic fungi, notably Alternaria alternata (74% inhibition), and reduced spore germination and growth of phytopathogenic bacteria, especially Xanthomonas arboricola pv. pruni. The cell-free supernatant maintained antifungal activity for up to 96 hours and showed a dose-dependent effect, with minimum inhibitory concentrations (MIC) of 100 mg/mL for A. alternata and 15 mg/mL for X. arboricola, also significantly reducing biofilm formation and cell viability. Bioassays with Brassica oleracea demonstrated that inoculation with DN246 resulted in significant increases in fresh weight and root growth of seedlings.The results obtained in this thesis demonstrate that coastal environments harbor microorganisms with potential for the development of agricultural bioinputs.