Abstract
The microbial loop has been suggested as an alternative route for better utilization of phytate, a poorly available P source to plants. We hypothesized that bacterial grazer activity might dramatically enhance bacterial migration and proliferation, increasing the probability of phytate hydrolysis by bacterial phytases and, thus, phytate mineralization and release of free phosphate. We tested this hypothesis in a two-compartment system with a solid medium containing phytate or free phosphate as the source of P. Two bacterial species, B. subtilis 168 or Bradyrhizobium sp., with or without bacterial grazing nematodes belonging to Acrobeloides sp. previously fed on each of the bacterial species, were inoculated at a single point in the medium. Whatever the P source, nematode migration within both zones allowed the proliferation of bacteria. However, B. subtilis 168 was more efficient in using phytate than Bradyrhizobium sp. since the highest bacterial cell density and free phosphate concentrations were reached by Acrobeloides sp. fed on B. subtilis 168. The grazer activity seemed to be crucial to enhance phytate mineralization, despite Acrobeloides sp. showing a higher preference to feed on Bradyrhizobium sp. This study provides new insights into the effects of bacterial grazer activity on phytate mineralization.
Highlights
The microbial loop has been suggested as an alternative route for better utilization of phytate, a poorly available P source to plants
Spatial expansion of nematodes according to P source and bacterial species When phytate was supplied as the sole P source, the size of Acrobeloides sp. populations did not differ according to the bacterial strain but were roughly twice as large within zone 1 than within zone 2 (Fig. 2A)
Nematode populations fed on bacteria grown with Pi were significantly higher on Bradyrhizobium sp. (+40%) than on B. subtilis in zone 1
Summary
The microbial loop has been suggested as an alternative route for better utilization of phytate, a poorly available P source to plants. The Bradyrhizobium genus displayed the highest relative abundance among all other bacterial genera analyzed in soil samples collected under non-leguminous plant species such as beech or spruce trees (Nacke et al, 2016) Besides their capacity to be associated with legumes to ensure the biological nitrogen fixation in nodules, Bradyrhizobium bacteria could play a large role in Po mineralization. No phytase activity has been reported for this bacterial genus, though the genome of B. japonicum (strain USDA6T) contains a gene encoding a protein with an esterase-like activity of the phytase family (Kaneko et al, 2011), raising the question of the possible phytase capacity of this bacterium Given their abilities to mobilize efficiently recalcitrant P, inoculants containing selected microbes have been widely proposed (Menezes-Blackburn et al, 2018), with some of them commercially available (Owen et al, 2015). These results suggest that P mobilization by bacteria is not efficient enough because they immobilize the released Pi into their biomass (Pistocchi et al, 2018; Spohn et al, 2018), making it non-available to plants, as previously observed by Khan and Joergensen (2009)
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