Abstract
Tropical epiphytic plants within the family Bromeliaceae are unusual in that they possess foliage capable of retaining water and impounded material. This creates an acidic (pH 3.5–6.5) and anaerobic (<1 ppm O2) environment suspended in the canopy. Results from a Costa Rican rainforest show that most bromeliads (n = 75/86) greater than ~20 cm in plant height or ~4–5 cm tank depth, showed presence of methanogens within the lower anoxic horizon of the tank. Archaea were dominated by methanogens (77–90% of recovered ribotypes) and community structure, although variable, was generally comprised of a single type, closely related to either hydrogenotrophic Methanoregula or Methanocella, a specific clade of aceticlastic Methanosaeta, or Methanosarcina. Juvenile bromeliads, or those species, such as Guzmania, with shallow tanks, generally did not possess methanogens, as assayed by polymerase chain reaction specific for methanogen 16S rRNA genes, nor did artificial catchments (~100 ml volume), in place 6–12 months prior to sample collection. Methanogens were not detected in soil (n = 20), except in one case, in which the dominant ribotype was different from nearby bromeliads. Recovery of methyl coenzyme M reductase genes supported the occurrence of hydrogenotrophic and aceticlastic methanogens within bromeliad tanks, as well as the trend, via QPCR analysis of mcrA, of increased methanogenic capacity with increased plant height. Methane production rates of up to 300 nmol CH4 ml tank water−1 day−1 were measured in microcosm experiments. These results suggest that bromeliad-associated archaeal communities may play an important role in the cycling of carbon in neotropical forests.
Highlights
Because of the significant influence of methane on atmospheric chemistry and it’s prominent role as a greenhouse gas, it is becoming increasingly important to understand its sources and sinks on Earth (Crutzen, 1991; IPCC, 2001)
Methanogenic archaea can use a number of substrates as precursors of methane, with CO2 reduction and acetate fermentation being most important in wetland soils
METHANE PRODUCTION IN BROMELIAD TANK INCUBATIONS Methane production potential was assayed via incubation of bromeliad tank water in gas tight serum vials
Summary
Because of the significant influence of methane on atmospheric chemistry and it’s prominent role as a greenhouse gas, it is becoming increasingly important to understand its sources and sinks on Earth (Crutzen, 1991; IPCC, 2001). Methanogenesis is the dominant terminal mineralization process in wetlands and freshwater sediments that experience prolonged flooding and are limited in more energetically favorable electron acceptors (e.g., sulfate, nitrate, and metal oxides; Whalen, 1993; Grosskopf et al, 1998; Galand et al, 2002; Cadillo-Quiroz et al, 2006) Methane emissions from these environments, both natural and man-made, are substantial (∼200 Tg year−1) and are estimated to contribute up to 40% of total global CH4 emissions annually (Crutzen, 1991; Grosskopf et al, 1998; Watanabe et al, 1999; IPCC, 2001; Galand et al, 2002). Methane production by these groups can be influenced by many factors, including availability of substrates and electron donors (e.g., H2), which are often formed catabolically by bacteria, quantity and quality of organic carbon, the presence of possible substrate competitors (e.g., bacteria), and environmental conditions such as pH and oxygen levels (Capone et al, 1983; Galand et al, 2002; Juottonen et al, 2005)
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