Nutrient competition and coevolutionary antagonistic interactions between Fusarium and Streptomyces in soil

Abstract

Plant impacts on nutrient use profiles among rhizosphere microbes are poorly understood. We examined the effects of plant host and plant species richness on nutrient use profiles of Streptomyces isolated from the rhizosphere of the prairie plants Andropogon gerardii (Ag) and Lespedeza capitata (Lc) growing in communities of 1 (monoculture) or 16 (polyculture) plant species. Growth on 95 carbon sources was assessed over time. Mean niche width and mean growth were significantly greater for isolates from polyculture vs. monoculture plots, and for Streptomyces from Lc vs. Ag. Isolates from high-carbon (polyculture) or high-nitrogen (Lc) soils had larger niche widths than isolates from low-C (monocultures) or low-N (Ag) soils. Isolates from polycultures were significantly more niche (nutrient) differentiated than isolates from monocultures. We also characterized antagonistic phenotypes and nutrient use among sympatric Streptomyces and Fusarium. Streptomyces from monocultures were more antagonistic against sympatric Fusarium populations than isolates from polycultures. In contrast, Fusarium isolates from polycultures were more inhibitory against sympatric Streptomyces than isolates from monocultures. Fusarium isolates from monocultures had greater niche overlap with Streptomyces than those from polycultures, suggesting greater potential for Fusarium to compete with Streptomyces in monoculture plant communities. In contrast, Streptomyces from polycultures had greater niche overlap with sympatric Fusarium than those from monocultures, suggesting that Fusarium experience greater competition from Streptomyces in polyculture than monoculture. These patterns are consistent with selection for Fusarium-antagonistic Streptomyces populations in the presence of strong Fusarium competition in monocultures, and selection for Streptomyces-inhibitory Fusarium populations in the presence of Streptomyces resource competition in polycultures. Finally, we found evidence for local adaptation between Fusarium and Streptomyces populations as evidenced by the presence of significantly greater inhibition among sympatric than allopatric populations in prairie soil. Additionally, for both taxa, there was a significant positive correlation between the strength of inhibition against each taxon and the intensity of resource competition from that taxon, supporting the hypothesis that antibiotics act as weapons in soil communities. Collectively, these results suggest that coevolutionary antagonistic interactions between Fusarium and Streptomyces in soil are driven by resource competition.