NSS Convention 2026

Directed phytokarst comprises light-oriented pinnacles on limestone surfaces in tropical cave entrances. Although these features have long been linked to phototrophic biofilms, the mineralogical and geomicrobiological processes responsible for focused dissolution of limestone remain poorly understood. Given its exclusive identification under low-light conditions, we wondered whether far-red/near-infrared (FR/NIR)-driven photosynthesis could trigger biochemical pathways that create this dissolution.

We examined directed phytokarst in Clearwater Cave (Gunung Mulu National Park, Malaysia). Field observations indicate that phytokarst was restricted to the twilight zone, with the relative illumination (lux) decreasing from 619.1 lux at the entrance to 6.4 lux (~1% of entrance illumination) in zones with well-developed phytokarst. Petrographic thin-section analyses of the phytokarst revealed flat, laminated calcite layers preserved along a shared original rock surface, supporting a dissolutional origin rather than secondary mineral deposition, while biofilms were densest at the pinnacle base. Excitation emission matrix fluorescence spectroscopy demonstrated red-shifted features in phytokarst-associated biofilms, consistent with the presence of pigments that absorb and fluoresce at longer wavelengths under FR-enriched illumination. Scanning electron microscopy revealed porous, etching-like textures and microborings on carbonate surfaces, suggesting localized bioerosion at the biofilm–mineral interface. Illumina sequencing identified potential NIR-capable cyanobacteria (including Acaryochloris spp.) and green algal taxa.

Collectively, these observations link cave-entrance light gradients to localized biofilm-driven dissolution, explaining directed phytokarst confinement to twilight zones.