Soil microorganisms can overcome respiration inhibition by coupling intra- and extracellular metabolism: 13C metabolic tracing reveals the mechanisms

CO2 release from soil is commonly used to estimate toxicity of various substances on microorganisms. However, the mechanisms underlying persistent CO2 release from soil exposed to toxicants inhibiting microbial respiration, for example, sodium azide (NaN3) or heavy metals (Cd, Hg, Cu), remain unclear. To unravel these mechanisms, NaN3-amended soil was incubated with position-specifically C-13-labeled glucose and C-13 was quantified in CO2, bulk soil, microbial biomass and phospholipid fatty acids (PLFAs). High C-13 recovery from C-1 in CO2 indicates that glucose was predominantly metabolized via the pentose phosphate pathway irrespective of inhibition. Although NaN3 prevented C-13 incorporation into PLFA and decreased total CO2 release, C-13 in CO2 increased by 12% compared with control soils due to an increased use of glucose for energy production. The allocation of glucose-derived carbon towards extracellular compounds, demonstrated by a fivefold higher C-13 recovery in bulk soil than in microbial biomass, suggests the synthesis of redox active substances for extracellular disposal of electrons to bypass inhibited electron transport chains within the cells. PLFA content doubled within 10 days of inhibition, demonstrating recovery of the microbial community. This growth was largely based on recycling of cost-intensive biomass compounds, for example, alkyl chains, from microbial necromass. The bypass of intracellular toxicity by extracellular electron transport permits the fast recovery of the microbial community. Such efficient strategies to overcome exposure to respiration-inhibiting toxicants may be exclusive to habitats containing redox-sensitive substances. Therefore, the toxic effects of respiration inhibitors on microorganisms are much less intensive in soils than in pure cultures.