Acid water in the tanks of bromeliads: Causes and potential consequences

Background/Question/Methods

The small bodies of water contained by the overlapping, encircling leaf bases or tanks of bromeliads in the neotropical rainforest provide sustenance for the plants themselves as well as for rich communities of microbes, invertebrates, and even small vertebrates. The composition of the tank community and the function of the bromeliad plant are interrelated with the pH of the tank water, as has been shown in numerous studies. Most notably, the pH can be quite low for an external body of water, with values below 4.0 measured in the field and in glasshouse studies. The consequences of such acidity for the bromeliad could involve effects on water uptake via aquaporins, pores that admit water yet can be closed at low pH, as well as effects on nutrient supply via microbial activity and debris breakdown in the tank. Our preliminary observations and work by other researchers led to the hypothesis that pH is lower for plants growing in high light than in low light, and lower during the daytime than at night. A related hypothesis is that the plant is the primary regulator of pH, either through the action of proton pumps or by respiratory production of carbon dioxide.

Results/Conclusions

Plants of the tank bromeliad Guzmania monostachia at La Selva Biological Station in Costa Rica growing in a forest gap were either exposed to ambient light or covered with shade cloth that reduced light by 80%. After four weeks, the pH of the tank water of ambient plants was 4.45 ± 0.04 and that of shaded plants was 4.62 ± 0.05 (P = 0.007). After shade cloth was removed, the pH difference between the two groups disappeared (4.62 ± 0.03 vs. 4.66 ± 0.06; P =0.485). For plants of G. monostachia in the glasshouse, the minimum pH of the tank water, 3.81 ± 0.15, occurred at midday when light was highest, and the maximum, 4.00 ± 0.14, occurred at night. Experiments with briefly sterilized chamber-grown plants and purified water confirmed that pH of tank water of plants in high light was lower than in low light, suggesting that microbial activity did not cause the difference. When the proton pump stimulator fusicoccin was added to pH 6.0 buffer in the tank, the pH decreased by a mean of 47% vs. a decrease of 26% for control plants, implying active pH regulation by the plant that may in turn regulate water uptake through aquaporins.