Cooperation between species is important in many natural and agricultural ecosystems, from coral reefs to soybean plants that depend on bacteria (rhizobia) for nitrogen. Given the benefits of cooperation, why does it sometimes break down, as in the case of bleaching when corals expel symbiotic algae, or rhizobia that either parasitize their plant hosts or ignore them altogether? Previous NSF-funded research showed that plants reduce resource supply to rhizobia that provide them with less nitrogen, so why are less-beneficial rhizobia still common in some soils? Maybe they escape host plant sanctions by hiding inside root nodules with more beneficial rhizobia, or maybe they somehow manipulate their hosts into providing resources they haven't earned. Both hypotheses will be tested experimentally. Additional experiments will determine whether a documented reduction in oxygen supply to less-beneficial rhizobia is the primary way that plants reduce the reproduction and spread of these rhizobia, or whether other plant responses are also important. The earliest practical benefits of this work are likely to be agricultural, but understanding cooperation between species is also relevant to a wide variety of ecological problems, from the decline of coral reefs to the spread of invasive plants. The gained understanding in symbiosis applies to understanding human social systems. The studies will provide research opportunities for students at all levels.
