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Fungi (In Association with Grasses) -- Summary
Nearly all grassland species form symbiotic relationships with mycorrhizal fungi; and the arbuscular mycorrhizal fungi that commonly colonize roots of grasses typically form symbiotic structures known as arbuscules, which are short-lived organs that facilitate carbon and nutrient exchanges between the fungi and their plant hosts.  These symbiotic relationships often increase grassland vitality and productivity, making it important to understand how they may be altered by rising atmospheric CO2 concentrations.  In this summary, we review published papers that provide insight into the effects of atmospheric CO2 enrichment on these important plant-fungal interactions in grasses.

To build a small background for this review, it is important to understand that even under ambient atmospheric CO2 concentrations, the presence of symbiotic interactions between grasses and arbuscular mycorrhizal fungi often leads to robust increases in growth.  In the study of Wilson and Hartnett (1998), for example, the authors grew 36 grass and 59 forb species common to tallgrass prairie ecosystems with and without arbuscular mycorrhizal fungi inoculation.  Among the grasses, they reported that fungal inoculation increased the average dry mass of perennial C4 species by 85%.  However, fungal inoculation had no significant effect on dry mass production in perennial C3 species or in any annual grass species, regardless of photosynthetic phyisology.  With respect to the forbs, over 80% of the perennial species exhibited significant increases in dry mass with fungal inoculation, while only 15% of the annual species displayed enhanced growth with inoculation.  Thus, a large number of plant-fungal interactions exist at ambient CO2 concentrations and may be modified by exposure to elevated CO2.

In a four-month study, Rillig et al. (1998a) grew monocultures of three grasses and two herbs that co-occur in Mediterranean annual grasslands in pots placed within open-top chambers receiving ambient and twice-ambient concentrations of atmospheric CO2.  They reported that elevated CO2 significantly increased the percent root colonization by arbuscular mycorrhizal fungal hyphae in all five species, which could ultimately lead to greater biomass production in these annual grassland plants.

In another four-month study using the same experimental enclosures, Rillig et al. (1998b) grew the single annual grass Bromus hordeaceus at ambient and elevated atmospheric CO2 concentrations.  In contrast to the above findings, elevated CO2 did not increase the percent root colonized by fungal hyphae.  However, it did significantly increase the percent root colonized by arbuscules, indicating that elevated CO2 can cause enhanced fungal-plant interactions by modifying fungal structures other than hyphae.

Lastly, in two related long-term studies, Rillig et al. (1999a and 1999b) constructed open-top chambers on two adjacent serpentine and sandstone grassland communities in California, USA, and fumigated them with air containing 350 and 700 ppm CO2 for six years.  In corroboration of earlier short-term results, they reported that elevated CO2 did not increase the percent root colonized by fungal hyphae (1999a).  However, and also in agreement with other earlier short-term results, they found that atmospheric CO2 enrichment enhanced the percent root colonized by arbuscules in serpentine and sandstone grasslands by three- and ten-fold, respectively (1999b).  In addition, using these same experimental plots, Hungate et al. (2000) reported that elevated CO2 significantly enhanced the overall biomass of fungal organisms, regardless of grassland type.

In summary, it is important to understand that as the air's CO2 concentration increases, it will likely impact plant-fungal interactions in grasslands by increasing the percent root colonized by either mycorrhizal fungal hyphae or arbuscular structures, both of which aid in carbon and nutrient exchanges between the two interacting symbionts.  Thus, we should expect grasslands to exhibit enhanced productivity and growth due to these CO2-enhanced relationships that can make soil nutrients more readily available for plant uptake and usage, especially since the findings of Rillig et al. (1999b) indicate that short-term CO2-induced changes in mutually beneficial plant-fungal interactions remain robust over the long-term.

References
Hungate, B.A., Jaeger III, C.H., Gamara, G., Chapin III, F.S. and Field, C.B.  2000.  Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2Oecologia 124: 589-598.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999a.  Fungal root colonization responses in natural grasslands after long-term exposure to elevated atmospheric CO2Global Change Biology 5: 577-585.

Rillig, M.C., Field, C.B. and Allen, M.F.  1999b.  Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands.  Oecologia 119: 572-577.

Rillig, M.C., Allen, M.F., Klironomous, J.N., Chiariello, N.R. and Field, C.B.  1998a.  Plant species-specific changes in root-inhabiting fungi in a California annual grassland: responses to elevated CO2 and nutrients.  Oecologia 113: 252-259.

Rillig, M.C., Allen, M.F., Klironomos, J.N. and Field, C.B.  1998b.  Arbuscular mycorrhizal percent root infection and infection intensity of Bromus hordeaceus grown in elevated atmospheric CO2Mycologia 90: 199-205.

Wilson, G.W.T. and Hartnett, D.C.  1998.  Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie.  American Journal of Botany 85: 1732-1738.