Soru

Passage:
Karst landscapes, which develop through the dissolution of soluble bedrock such as limestone and dolomite, encompass approximately 15 percent of the global ice-free continental land area. While subterranean karst networks were historically viewed as passive geological conduits that primarily facilitated groundwater transit, recent biogeochemical investigations have revealed their critical function as active subterranean sinks for atmospheric methane ($CH_4$). Atmospheric methane oxidation within these subterranean voids is mediated predominantly by high-affinity aerobic methanotrophic bacteria, which are phylogenetically distinct from the low-affinity methanotrophs that proliferate in methane-rich environments such as wetlands and agricultural soils.

Unlike low-affinity methanotrophs that require ambient methane concentrations exceeding 1,000 parts per million (ppm) to sustain metabolic activity, high-affinity methanotrophs—particularly members of the uncultivated upland soil cluster gamma (USC $\gamma$) lineage—can utilize methane at near-ambient or even sub-ambient concentrations (below 1.9 ppm). In deep cave chambers, where air circulation is constrained and residence times of subterranean air masses range from several days to months, these bacteria utilize membrane-bound particulate methane monooxygenase enzymes to oxidize methane to methanol, which is subsequently converted to formaldehyde and assimilated into cellular biomass via the ribulose monophosphate pathway.

However, the efficiency of this subterranean methane sink is strictly governed by microclimatic venting regimes driven by external-to-internal temperature differentials. During warm summer periods, when cave air is colder and denser than outside air, downward convective airflow creates a unidirectional chimney effect, expelling cave air through lower karst openings while drawing ambient surface air into upper fissures. This continuous influx replenishes atmospheric methane supplies to the biofilm-coated limestone walls, maintaining high oxidation rates. Conversely, during winter inversions, when internal cave temperatures exceed external temperatures, upward convective ventilation reverses the airflow pattern, frequently causing internal relative humidity to decline near cave entrances. This localized desiccation inhibits methanotrophic biofilm viability, causing methane oxidation rates in peripheral cave zones to plummet by up to 80 percent, even though deep-interior bacterial populations remain metabolically stable.

Furthermore, geochemical analysis demonstrates that the structural substrate itself influences methanotrophic activity. While limestone host rock composed of high-purity calcite ($CaCO_3$) provides an ideal alkaline buffering matrix that neutralizes metabolic acids generated during methanotrophy, dolomitic formations with high magnesium content exhibit lower bacterial colonization densities. Researchers attribute this disparity to differences in surface porosity and trace mineral bioavailability rather than variations in ambient relative humidity.

Question:
According to the passage, which of the following is explicitly identified as a direct cause of the substantial decline in methane oxidation rates in peripheral cave zones during winter inversions?