Mythbusters: Unravelling the pollutant uptake processes in mosses for air quality biomonitoring

Abstract

Terrestrial mosses have been used for more than 50 years to monitor air pollution. We argue, however, that their value as biomonitors is based on two widespread but partially erroneous assumptions concerning their morphological structure (the structural myth) and physicochemical characteristics (the physicochemical myth). The structural myth consists of the oversimplification of the moss morphological structure. The physicochemical myth recognizes their high cation exchange capacity (CEC) as the only pathway for pollutant uptake. Here, we gather and discuss the evidence demonstrating that these assumptions are misleading and focus the discussion on the aspects that make mosses good biomonitors of air pollution. First, we show that these plants (i) do have a cuticle, whose structure and composition differs among species, (ii) can have epidermal cells, that differ in shape and thickness from other cell types, (iii) have a vascular system, whose degree of development is lineage- and species-specific, and (iv) have rhizoids, that can absorb water, nutrients and pollutants. The effect of these traits in the pollutant uptake processes has been understudied in biomonitoring studies. Second, we show that mosses (i) do not concentrate as much pollutants as they could according to their high CEC, (ii) can retain large amounts of particles containing atmospheric pollutants in their surfaces, and (iii) in many contexts, the spatial structure of the concentrations of pollutants in mosses depicts the pattern expected for atmospherically deposited particles. Thus, the quality of these organisms as biomonitors of air pollution also lies in their high capacity to retain particles. We do call for more research on how moss structural and physiological traits affect pollutant uptake dynamics and recommend researchers using mosses as air quality biomonitors to face critically to inaccurate or insufficiently demonstrated assumptions in this context.