Effects of CO2 in fungi

Abstract

Carbon dioxide supplies carbon for photosynthetic species and is a major product of respiration for all life forms. Inside the human body where CO₂ is a by-product of the tricarboxylic acid cycle, its level reaches 5% or higher. In the ambient atmosphere, ∼.04% of the air is CO₂. Different organisms can tolerate different CO₂ levels to various degrees, and experiencing higher CO₂ is toxic and can lead to death. The fungal kingdom shows great variations in response to CO₂ that has been documented by different researchers at different time periods. This literature review aims to connect these studies, highlight mechanisms underlying tolerance to high levels of CO₂, and emphasize the effects of CO₂ on fungal metabolism and morphogenesis.

Figure 1.

Direct effects of CO₂ on fungal metabolism. CO₂ is a precursor (red) and a product (blue) of numerous metabolic reactions in the cell. 1) CO₂ is converted to bicarbonate spontaneously or by carbonic anhydrase, causing intracellular acidification through generating hydrogen ions. 2) CO₂ and pyruvate are synthesized into oxaloacetate. 3) CO₂ is used to synthesize AIR in the adenine biosynthesis pathway, and 4) is a precursor for carbamoyl phosphate.

Figure 2.

CO₂ sensing in C. albicans. 1) CO₂ is converted to bicarbonate by carbonic anhydrase leading to activation of adenylyl cyclase (AC) and cAMP signaling, and 2) causing intracellular acidification through generating hydrogen ions and activating Rim101. 3) The homeostasis of bicarbonate and/or unknown products of bicarbonate regulates Pkh1/2 and TOR activity. 4) In high CO₂, Sch9 is activated by Pkh1/2 and TOR through phosphorylation and inhibits the transcription factor Rca1.5) In low CO₂ levels, Rca1 activates expression of carbonic anhydrase. 6) CO₂ induces Flo8 expression. 7) Ras1 acts antagonistically to CO₂ signaling. 8) CO₂ interacts with Ptc2 to form aggregates in the cell. 9) Ptc2 activates Wor1 through dephosphorylation.

Figure 3.

CO₂ tolerance in C. neoformans. 1) CO₂ disrupts membrane fluidity and/or integrity. 2) CO₂ is converted to bicarbonate by carbonic anhydrase. 3) Bicarbonate activates adenylyl cyclase leading to cAMP production. 4) HCO₃⁻ is utilized to produce malonyl-CoA, critical for fatty acid biosynthesis. 5) CO₂ is utilized to synthesize carbamoyl phosphate. 6) CO₂ activates the TOR pathway. 7) The RAM and Calcineurin pathways may post-transcriptionally regulate genes important for CO₂ tolerance. 8) Increased CO₂ causes degradation of Rim101, preventing its activity as a transcription factor.