How does rising atmospheric CO2 affect marine organisms?

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

What was done
Open-top chambers were constructed on serpentine and sandstone grasslands near Stanford, California, USA, to determine the long-term effects of elevated CO2 on plant growth, grassland community structure, and soil biota dynamics.  In this particular study, soil samples were removed from the grasslands and analyzed after six years of continuous exposure to ambient and elevated (ambient plus 350 ppm) atmospheric CO2 concentrations.

What was learned
Colonization of plant roots by arbuscular mycorrhizal fungi was significantly enhanced in both grasslands due to atmospheric CO2 enrichment.  The proportion of roots containing arbuscules, which are short-lived symbiotic organs that facilitate carbon and nutrient exchange between mycorrhizal fungi and their host plants, were approximately ten- and three-fold greater in CO2-enriched sandstone and serpentine grasslands, respectively, than they were in comparable grasslands grown at ambient CO2.

After six years of CO2 enrichment, elevated CO2 had not significantly impacted soil bacterial populations.  However, atmospheric CO2 enrichment significantly enhanced the number of fungal colony-forming units within the soil, and increased microarthropod numbers by 108 and 39% in sandstone and serpentine grasslands, respectively.  It is important to note that many microarthropods survive by feeding upon fungal colonies; thus, this research demonstrates how the increasing CO2 content of the air can stimulate the fungal food chain.

What it means
As the atmospheric CO2 concentration continues to rise, plants growing in serpentine and sandstone grasslands should exhibit increased rates of photosynthesis and growth.  Some of the additional carbohydrates produced under higher levels of atmospheric CO2 will likely find their way belowground where they can enhance fungal growth and root colonization.  Such robust symbiotic relationships between plants and fungi should positively feedback to stimulate even greater increases in plant growth.  In addition, greater fungal populations in the rhizosphere will likely provide a greater superstructure for supporting organisms that feed upon them; as was the case for soil microarthropods in this study.  Such enhancements in important food chains can result in greater productivity within various communities and ecosystems across the globe.

Reviewed 1 November 1999