Water velocity and irradiance effects on internal transport and metabolism of methane in submerged Isoetes alpinus and Potamogeton crispus

Submerged aquatic macrophytes are important methane (CH4) transport pathways, but little is known about how water velocity and illumination affect plant CH4 transport. We studied CH4 transport in vitro in fully-submerged plants of two aquatic macrophytes, Isoetes alpinus and Potamogeton crispus, comparing transport between two water velocities (0.5 and 90 mm s-1) and light versus dark treatments at the shoots. In I. alpinus, CH4 taken up by the roots accumulated in the lacunar system of the leaves at concentrations >40 μmol l-1 at both flow rates, but there was no release from the shoots to the water at flow = 0.5 mm s-1, whereas at 90 mm s-1 release from the shoots to the water was always >0.15 μmol (CH4 g-1 dry wt. h-1). Uptake of CH4 by the roots was 1.5 times greater at the higher than the lower water velocity. In P. crispus, a gradient in lacunar [CH4] developed from the base to the apex of the shoot and was steeper at the higher than the lower water velocity. CH4 release rates from the shoots were <0.1 μmol (CH4 g-1 dry wt. h-1) at both water velocities in this species, due to CH 4 consumption by epiphytic methanotrophic bacteria. Irradiance and its associated root aeration had no effect on CH4 transport in I. alpinus, but decreased root to shoot CH4 transport and release more than 10-fold in P. crispus, apparently by alleviating effects of hypoxia on the root-associated methanotrophic bacteria. Our data support the view that plant-associated methanotrophs are important CH4 sinks in freshwater habitats, and demonstrate the importance of flow velocity in the CH4 dynamics of submerged macrophytes. © 2004 Elsevier B.V. All rights reserved.