TRADE‐OFFS,SPATIAL HETEROGENEITY,AND THE MAINTENANCE OF MICROBIAL DIVERSITY

Specialization and concomitant trade‐offs are assumed to underlie the non‐neutral coexistence of lineages. Trade‐offs across heterogeneous environments can promote diversity by preventing competitive exclusion. However, the importance of trade‐offs in maintaining diversity in natural microbial assemblages is unclear, as trade‐offs are frequently not detected in artificial evolution experiments. Stressful conditions associated with patches of heavy‐metal enriched serpentine soils provide excellent opportunities for examining how heterogeneity may foster genetic diversity. Using a spatially replicated design, we demonstrate that rhizobium bacteria symbiotic with legumes inhabiting contrasting serpentine and nonserpentine soils exhibit a trade‐off between a genotype's nickel tolerance and its ability to replicate rapidly. Furthermore, we detected adaptive divergence in rhizobial assemblages across soil type heterogeneity at multiple sites, suggesting that this trade‐off may promote the coexistence of phenotypically distinct bacterial lineages. Trade‐offs and adaptive divergence may be important factors maintaining the tremendous diversity within natural assemblages of bacteria.