Bioremediation Potential of Native Bacillus sp. Strains as a Sustainable Strategy for Cadmium Accumulation of Theobroma cacao in Amazonas Region

Cacao plant cadmium accumulation has become a major concern, especially for small Amazonian producers. A sustainable alternative to address its toxicity is the use of cadmium removal bacteria. In this regard, 138 rhizosphere isolates from cacao were examined. Supported by their phenotypic characterization and in vitro cadmium tolerance, three hypertolerant bacteria were selected and identified as members of the Bacillus (S1C2, R1C2) and Pseudomonas (V3C3) genera. They were able to grow normally and reduce the cadmium content under in vitro conditions. However, only S1C2 and R1C2 evidenced to employ intracellular Cd2+ accumulation, suggesting the variability of bacterial detoxification mechanisms. Their bioremediation capacity for Theobroma cacao CCN51 was also analyzed. Surprisingly, we found high detectable levels of Cd2+ in the non-cadmium supplemented control, suggesting an extra source of cadmium in the pot. Moreover, despite their cadmium reduction performance under in vitro conditions, they exerted highly variable outcomes on stem cadmium accumulation. While S1C2 and R1C2 showed a considerable reduction of Cd content in cacao stems, the strain V3C3 did not show any effect on Cd content. This highlights the complexity of the plant–bacteria interactions and the importance of the in vivo test for the selection of promising PGPR bacteria. Overall, our results suggest the cadmium alleviation potential and promising prospects of native Bacillus strains associated with Amazonian cacao.