Landscape variation in the abundance of pathogen-suppressive Streptomyces in secondary tropical dry forests of Costa Rica

Background/Question/Methods

Actinobacteria in the genus Streptomyces can confer protection against soilborne plant pathogens and reduce disease in plants. The biotic and abiotic factors that select for disease-suppressive phenotypes are poorly understood, especially in forests. The objective of this study was to quantify soilborne Streptomyces densities and pathogen inhibitory activities in regenerating tropical dry forests. Soil was collected from 18 long-term plots in Northwestern Costa Rica that vary in stand age, soil fertility, and forest structure. We used a functional assay to measure suppressive activity of antibiotic-producing Streptomyces against three target pathogens (Streptomyces scabies, Fusarium oxysporum, and Rhizoctonia solani). This method estimates both the frequency and intensity of pathogen suppression. Soil samples were also analyzed for nutrients (N, P, K), carbon (C), extracellular enzyme activity, and total Streptomyces densities. Linear regression and Spearman rank correlations were used to test for bivariate relationships among microbial community variables and soil characteristics.

Results/Conclusions

Streptomyces density ranged from 2.85 x 10⁵ to 4.04 x 10⁷ CFU/g soil. The proportion of the community that was antagonistic to one or more pathogens ranged from less than 4% to ~70%. Pathogen inhibition among Streptomyces varied with edaphic characteristics and was a function of total Streptomyces density. Soil P and K were negatively correlated with the frequency of antagonists but positively correlated with the intensity of inhibition, suggesting that soil fertility influences Streptomyces community phenotypes. Pathogen inhibition was not related to stand age. However, antagonist density increased with the stable isotopic composition of carbon (δ¹³C), which reflects the contribution of grass-derived C to soil organic matter. This suggests that soil legacy effects associated with the transition in dominant vegetation from C4 pasture grasses to C3 trees may be more significant to microbial community composition and function than time since land-use conversion. Our results are consistent with the prediction that antibiotic-production is a successful competitive strategy in high density populations or resource-poor environments. Antibiotic-producing bacteria are an underexplored microbial functional group in tropical forests with potential consequences for community and ecosystem processes.