Carbon and Energy Sources of Mycorrhizal Fungi: Obligate Symbionts or Latent Saprotrophs?

It has been debated whether ectomycorrhizal fungi have retained saprotrophic capabilities and whether that allows them to obtain carbon, as implied by the alternate carbon source hypothesis, or to obtain nitrogen and phosphorus, as implied by the nutrient mining through priming hypothesis. This chapter reviews the evidence for both hypotheses. It concludes that there is very little support for the alternate carbon source hypothesis but substantial support for the nutrient mining through priming hypothesis. Next to enzymatic oxidation of recalcitrant organic matter, ectomycorrhizal fungi can acquire organic nutrients through destabilization of mineral-protected carbon by low-molecular-weight organic anions. Acquisition of organic nutrients from organic carbon by ectomycorrhizal fungi is similar to priming of organic matter degradation by saprotrophic microorganisms, in which the same principles of enzymatic priming of recalcitrant carbon and chemical priming of mineral-protected carbon occur. Although arbuscular mycorrhizal fungi lack the ability to degrade organic matter, they may equally be involved in priming, enhancing, or retarding carbon fluxes.