Assessment of native cadmium-resistant bacteria in cacao (Theobroma cacao L.) - cultivated soils

Traces of cadmium (Cd) have been reported in some chocolate products due to soils with Cd and the high ability of cacao plants to extract, transport, and accumulate it in their tissues. An agronomic strategy to minimize the uptake of Cd by plants is the use of cadmium-resistant bacteria (Cd-RB). However, knowledge about Cd-RB associated with cacao soils is scarce. This study was aimed to isolate and characterize Cd-RB associated with cacao-cultivated soils in Colombia that may be used in the bioremediation of Cd-polluted soils. Diversity of culturable Cd-RB, qualitative functional analysis related to nitrogen, phosphorous, carbon, and Cd were performed. Thirty different Cd-RB morphotypes were isolated from soils with medium (NC, Y1, Y2) and high (Y3) Cd concentrations using culture media with 6 mg Kg-1 Cd. Cd-RB were identified based on morphological and molecular analyses. The most abundant morphotypes (90%) were gram-negative belong to Phylum Proteobacteria and almost half of them showed the capacity to fix nitrogen, solubilize phosphates and degrade cellulose. Unique morphotypes were isolated from Y3 soils where Burkholderia and Pseudomonas were the dominant genera indicating their capacity to resist high Cd concentrations. P. putida GB78, P. aeruginosa NB2, and Burkholderia sp. NB10 were the only morphotypes that grew on 18 up to 90 (GB78) and 140 mg Kg-1 Cd (NB2-NB10); however, GB78 showed the highest Cd bioaccumulation (5.92 mg g-1). This study provides novel information about culturable Cd-RB soil diversity with the potential to develop biotechnology-based strategies.