Dynamic Bacterial Community Shifts in Bulk Soil upon Introduction of Root Exudate Cocktail — 6a — Muhammad Yasir Afzal¹ and Volker S. Brözel^1,2

¹Departments of Biology and Microbiology, South Dakota State University, Brookings, SD 57006, USA

²Department of Biochemistry, Genetics and Microbiology, University of Pretoria, Pretoria, 0004, South Africa

Soil bacteria are diverse both physiologically and phylogenetically. Bulk soil contains fewer nutrients than rhizosphere. Plant roots secrete various metabolites called root exudates, and bacteria use them and perform biological processes. Growing roots attract motile bacteria and induce shifts in soil bacterial communities and their functional activities. Long term survival must take precedence over cell division to maintain populations in bulk soil. Bacteria survive in nutrient limited environments using survival structures such as endospores and biofilms. Thus, root exudate compounds play a vital role by exposing bacteria in bulk soil to growth supporting nutrients. We asked how the bulk soil microbial community responds when encountering root exudates and hypothesized that bacteria able to grow rapidly would become predominant upon introduction of root exudates. We determined the bacterial community composition in bulk soil before and after introduction of synthetic root exudate cocktail (REC). We determined the soil bacterial community on d0, d1, d2, d3, d4, d6, d8, d10, d12 and d14 using culturable counts on R2A and MYP media and 16S rRNA gene analysis with QIIME2. Alpha diversity decreased upon introduction of REC and did not return to its original levels, indicating decrease in evenness.  The Chao1 index did not decrease, indicating no loss of species richness. Beta diversity shifted substantially over time. Fast-growing genera increased initially, including Paenarthrobacter. Conversely, slower growing genera initially decreased like Bradyrhizobium but persisted over time. Some bacteria like Mycobacterium did not respond to REC and remained constant. A few genera like Kribella first decreased but rebounded after termination of REC addition. Genera like Skermanella were predominant in the soil and sustained over time. Interestingly, Bacillus responded by germinating but did not increase in population density, rather forming spores again. Collectively, our results show that upon exposure to REC, the bacterial community in soil shifted substantially.

Keywords: Soil bacterial community, 16S rRNA gene, Root exudates, QIIME2, Alpha & Beta diversity

South Dakota State University
Volker Brozel