What was done
The authors studied the effects of elevated atmospheric CO2 levels on four species of arbuscular mycorrhizal fungi that are symbionts of sagebrush.  Sagebrush seeds were planted in an inert medium inoculated with one of four different species of fungi (Glomus intraradices, Glomus etunicatum, Acaulospora denticulata and Scutellospora calospora) and grown for 16 weeks in controlled-environment growth chambers receiving 350 or 700 ppm atmospheric CO2.  At the end of this period, the plants were harvested, their shoot and root biomass determined, and a number of fungal properties assessed.

What was learned
Elevated atmospheric CO2 did not have any effect on sagebrush shoot biomass over the short period of the study.  However, sagebrush root biomass increased by an average of 35% in the CO2-enriched air in the plants inoculated with the two Glomus fungi.  These two fungal species also increased the structures that facilitate internal nutrient transfer (arbuscules by 70%, internal hyphae by 38%) and reproductive propagation (spores by 70%), while the other two fungal species increased the production of external nutrient transfer structures (external hyphae by 40%) in response to atmospheric CO2 enrichment.

What it means
The results of this work suggest that responses of arbuscular mycorrhizal fungi to increasing levels of atmospheric CO2 are species-specific and may represent complementary strategies for taking advantage of a carbon-rich environment.  With more CO2 in the air to which their hosts are exposed, these fungal species appear to be stimulated in different ways to promote nutrient sharing and transfer to their hosts.  These phenomena, in turn, will likely promote root growth and development (as they did in this study), which should ultimately stimulate aboveground biomass development as well.