Physical stress and diversity-productivity relationships: The role of positive interactions

If environmental stress provides conditions under which positive relationships between plant species richness and productivity become apparent, then species that seem functionally redundant under constant conditions may add to community functioning under variable conditions. Using naturally co-occurring mosses and liverworts, we constructed bryophyte communities to test relationships between species diversity (1, 2, 4, 8, 16, 24, or 32 species) and productivity under constant conditions and when exposed to experimental drought. We found no relationship between species richness and biomass under constant conditions. However, when communities were exposed to experimental drought, biomass increased with species richness. Responses of individual species demonstrated that facilitative interactions rather than sampling effects or niche complementarity best explained results—survivorship increased for almost all species, and those species least resistant to drought in monoculture had the greatest increase in biomass. Positive interactions may be an important but previously underemphasized mechanism linking high diversity to high productivity under stressful environmental conditions.

Dominance of one bacterial phylotype at a Mid-Atlantic Ridge hydrothermal vent site.

Microbial community structure in natural environments has remained largely unexplored yet is generally considered to be complex. It is shown here that in a Mid-Atlantic Ridge hydrothermal vent habitat, where food webs depend on prokaryotic primary production, the surface microbial community consists largely of only one bacterial phylogenetic type (phylotype) as indicated by the dominance of a single 16S rRNA sequence. The main part of its population occurs as an ectosymbiont on the dominant animals, the shrimp Rimicaris exoculata, where it grows as a monoculture within the carapace and on the extremities. However, the same bacteria are also the major microbial component of the free-living substrate community. Phylogenetically, this type forms a distinct branch within the epsilon-Proteobacteria. This is different from all previously studied chemoautotrophic endo- and ectosymbioses from hydrothermal vents and other sulfidic habitats in which all the bacterial members cluster within the gamma-Proteobacteria.

Molecular mechanisms of defense by rhizobacteria against root disease.

Genetic resistance in plants to root diseases is rare, and agriculture depends instead on practices such as crop rotation and soil fumigation to control these diseases. "Induced suppression" is a natural phenomenon whereby a soil due to microbiological changes converts from conducive to suppressive to a soilborne pathogen during prolonged monoculture of the susceptible host. Our studies have focused on the wheat root disease "take-all," caused by the fungus Gaeumannomyces graminis var. tritici, and the role of bacteria in the wheat rhizosphere (rhizobacteria) in a well-documented induced suppression (take-all decline) that occurs in response to the disease and continued monoculture of wheat. The results summarized herein show that antibiotic production plays a significant role in both plant defense by and ecological competence of rhizobacteria. Production of phenazine and phloroglucinol antibiotics, as examples, account for most of the natural defense provided by fluorescent Pseudomonas strains isolated from among the diversity of rhizobacteria associated with take-all decline. There appear to be at least three levels of regulation of genes for antibiotic biosynthesis: environmental sensing, global regulation that ties antibiotic production to cellular metabolism, and regulatory loci linked to genes for pathway enzymes. Plant defense by rhizobacteria producing antibiotics on roots and as cohabitants with pathogens in infected tissues is analogous to defense by the plant's production of phytoalexins, even to the extent that an enzyme of the same chalcone/stilbene synthase family used to produce phytoalexins is used to produce 2,4-diacetylphloroglucinol. The defense strategy favored by selection pressure imposed on plants by soilborne pathogens may well be the ability of plants to support and respond to rhizosphere microorganisms antagonistic to these pathogens.