Annals of Botany

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Moss in detox mystery

Mosses are, in general, very sensitive to sulfur dioxide (SO₂), as discussed by Bharali and Bates (Jorhat, Assam, India and Imperial College, London, UK, pp. 257–263). However, some mosses are able to recover, or partly recover, from the effects of SO₂ even in the continued presence of the pollutant and it is possible that this recovery is at least partly based on detoxification of the SO₂. The authors have investigated this possibility in two species: the calcifuge Pleurozium schreberi and the calcicole Rhytidiadelphus triquetrus. Both were able to reduce markedly the concentration of dissolved SO₂ (supplied as sodium bisulfite) in medium-term (up to 10 h) and longer-term (up to 5 d) incubations, with R.triquetrus being the more effective. In both species, detoxification was much more extensive in the light than in the dark, suggesting the involvement of photosynthesis. This suggestion was supported by the observation that detoxification was inhibited by DCMU, which blocks the chloroplast electron transport chain and the photosynthetic generation of O₂. It is the latter that the authors consider important because the main route for detoxification is likely to be oxidation to sulfate rather than reduction to sulfide. This oxidation, which took place extracellularly, was aided by Fe³⁺ ions taken up into the cell walls in the calcifuge, but not in R. triquetrus. However, external detoxification may not be the whole story because in both species there was a clear reduction in detoxification in the presence of diethyldithiocarbamate (DETC), an inhibitor of superoxide dismutase. Nevertheless, the authors consider it unlikely that reactive oxygen species (ROS) are involved in the removal of bisulfite: the irradiance was too low to induce photoinhibition and the generation of ROS, and in any case, the inhibitory effect of DETC occurred both in the dark and the light. As the authors themselves point out, this requires further investigation.