Acid waters held by tank bromeliads: Causes and potential consequences

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Gretchen North, Erin Brinton, Adriana Fung and Joseph H. Shiina, Biology, Occidental College, Los Angeles, CA, Marvin Browne, Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA, Kyle D. Fukui and Franklin D. R. Maharaj, Biochemistry, Occidental College, Los Angeles, CA, Tiffany Kuo, University of San Francisco, San Francisco, CA, Franklin D. R. Maharaj, Pioneer Astronautics, CO, Stuart Schwab Jr., Botany and Plant Sciences, University of California - Riverside, Riverside, CA

Background/Question/Methods
In the canopy of tropical forests, the water held in the leaf-formed reservoirs of tank bromeliads is often acidic, with a pH usually between 4.0 – 6.5, occasionally < 4.0. The causes and consequences of low pH in tank water with respect to plant resource uptake were investigated in the field at La Selva Biological Station in Costa Rica and in a glasshouse in Los Angeles, CA. Eight species of tank bromeliad were sampled in the field to determine native pH. One species, Guzmania monostachia, was selected to determine the effects of light environment via a shading experiment in the field, as well as by recording diurnal changes in pH for glasshouse plants. The role of diurnal changes in CO2 levels in the tank water was measured using microelectrodes. To separate the contribution of tank contents (such as microbes) from that of the bromeliad, pH was measured in tanks of G. monostachia and in artificial tanks containing the same solution. Possible acidifying activity by the plants was tested by treating tanks with a proton pump blocker. Because low pH can affect aquaporins (membrane water channels) and thus water uptake, aquaporin expression and pH were determined for G. monostachia plants in the field. Water uptake was measured by repeated weighing of plants with acidified vs. control tank contents.
Results/Conclusions
For 156 plants of 8 species in Costa Rica, the average pH of the tank water sampled over time was 4.7 ± 0.06 (SE). Of these species, G. monostachia had a similar pH, which was affected by light exposure. Plants covered with shade cloth had significantly lower pH than uncovered plants; similarly, the lowest pH for glasshouse plants occurred from midday to midafternoon, when light levels were highest. In contrast, lowest pH co-occurred with the lowest levels of CO2 in the tanks. For a solution of tank water divided and placed in plants and artificial tanks, pH decreased in plant tanks but not artificial tanks, implicating plant rather than microbial activity. Proton blocker experiments similarly implicated plant activity in lowering pH. Aquaporin expression was higher for plants with low pH, but water uptake was lower, suggesting that bromeliads maintain acid tank water to assist in securing other resources, perhaps mineral nutrients. Future work will investigate links between phosphorous uptake and acidity, based on preliminary evidence that lower pH may help increase available phosphorous in the tank.