Do Tropical Nickel Hyperaccumulators Mobilize Metals into Epiphytes? A Test Using Bryophytes from New Caledonia

Abstract

Hyperaccumulator plants mobilize large amounts of certain elements from the soil into their tissues. Those elements then may be transferred to other organisms in those communities. Using a humid tropical forest site in New Caledonia, we tested whether epiphytes (mosses and liverworts) growing on Ni hyperaccumulator hosts contained greater levels of Ni (and seven other metals) than those growing on non-hyperaccumulator hosts. We selected two Ni hyperaccumulator species, Psychotria douarrei and Hybanthus austrocaledonicus, pairing individuals of each species with similar-sized non-hyperaccumulators and collecting epiphytes from each for elemental analysis. Samples of epiphytes and host plant leaves were analyzed for concentrations of eight metals (Co, Cr, Fe, Mg, Mn, Ni, Pb, and Zn). Two-way ANOVA was used to assess the influence of host type (hyperaccumulator or non-hyperaccumulator), epiphyte group, and the interaction term. Leaves of both Ni hyperaccumulator species had greater Ni concentrations than the paired non-hyperaccumulator species, but leaf concentrations of other metals (Co, Cr, Fe, Pb, and Zn) were higher as well in one or both cases. The strongest influence on epiphyte elemental composition was found to be the host type factor for Ni. Epiphytes collected from hyperaccumulator hosts had significantly greater Ni concentrations than those collected from non-hyperaccumulator hosts. Epiphyte Ni concentrations often exceeded the threshold used to define Ni hyperaccumulation (1000 µg/g), showing that some epiphytes (in most cases those growing on Ni hyperaccumulators) also hyperaccumulate Ni. Six of the epiphytes we analyzed, four liverworts (Frullania ramuligera, Schistochila sp., Morphotype #1 and Morphotype #13) and two mosses (Calyptothecium sp. and Aerobryopsis wallichii), had at least one specimen containing more than 1000 µg Ni/g and hence qualify as Ni hyperaccumulators. We conclude that Ni could move from Ni hyperaccumulator hosts to their epiphytes, either from leachates/exudates from tissues or from accumulated external dust, thus potentially mobilizing Ni still further into the food webs of these humid tropical forests.