Bromeliads compete with microorganisms for nutrients in their phytotelm

Tank bromeliads impound water between their leaves, and have been successfully used as a model system to develop ecological theory. One fundamental interaction remains, however, unexplored in this system: the potential for bromeliad plants to compete with aquatic microorganisms for nutrients in their water-filled tanks. Here we conduct the first experimental test of this interaction by directly blocking bromeliads from accessing phytotelm water and quantifying change in the concentrations of dissolved inorganic nitrogen, phosphate, autotrophic algae and heterotrophic bacteria. To understand any context-dependency of responses, we crossed this manipulation with different combinations of light exposure and allochthonous resource type. We found that bromeliads reduced the concentrations of dissolved inorganic nitrogen, especially when the resource was nutrient-rich. Bromeliads also reduced the concentration of heterotrophic bacteria within their tank regardless of environmental context and algal concentrations particularly under high light, nutrient-rich resource conditions. Our results demonstrate that bromeliads can strongly impact their phytotelm by reducing the concentration of key nutrients, concomitantly reducing densities of algae under optimal growth conditions. This strong competition with algae for nutrients may explain why high abundances of algae within wild tank bromeliads are only found under specific field conditions. Bacteria may have a more commensal relationship with bromeliads, as they tend to increase bromeliad nutrition through decomposition. In conclusion, our study underlines that tank bromeliads do not only provide habitats for other species, but also interact strongly with the aquatic ecosystem that they create.