Distinct accumulation behaviour of Marchantia polymorpha L. grown along a soil metal gradient triggers fitness penalties

Abstract Background and aims Liverworts are often used as bioindicators of metals in the environment. Here, we evaluated the growth, metal uptake and ultrastructural responses of a model liverwort Marchantia polymorpha subsp. ruderalis to a concentration gradient of Zn, Cd and Pb derived from native metalliferous soil, in the absence of their aerial deposition. Methods Thalli of M. polymorpha were cultivated from gemmae on substrates containing a Zn, Cd and Pb gradient, established by mixing metalliferous and non-metalliferous soils with comparable physicochemical characteristics. Biochemical, ionomic, elemental localisation and ultrastructural analyses were subsequently performed. Results Increasing soil Zn, Cd, Pb concentrations constrained thallus growth already within the first week of metal exposure and resulted in four- to fivefold reductions in biomass, accompanied by impaired photosynthetic performance, activation of stress-related defence mechanisms, decreased uptake of K, Mn and P, and increased accumulation of Ca, Fe, S, Zn, Cd and Pb. While Zn and Pb concentrations in thalli increased linearly with their substrate availability, a hyperaccumulation-like response was displayed for Cd. Concentration of Cd in thallus, however, remained well below the hyperaccumulation threshold established for vascular plants. Metals were sequestered in ventral thallus tissues, spatially separated from photosynthetically active parts, occurring alongside ultrastructural changes such as thickening of the ventral cuticle, chloroplast modifications, starch and plastoglobuli accumulation, and proliferation of electron-dense cytoplasmic vesicles. Conclusions When grown across a soil metal gradient, M. polymorpha exhibits Zn- and Pb-indicator behaviour and previously undescribed Cd-accumulator behaviour, accompanied by reduced overall fitness. Hence, exposure to increasing substrate metal concentrations triggered a variety of physiological, ionomic and structural changes indicating this species is weakly tolerant to Zn, Cd and/or Pb taken up from the soil.