Photoprotection and high-light acclimation in semi-arid grassland lichens – a cooperation between algal and fungal partners

Abstract

In lichens, each symbiotic partner cooperates for the survival of the symbiotic association. The protection of the susceptible photosynthetic apparatus is essential for both participants. The mycobiont and photobiont contribute to the protection against the damaging effect of excess light by various mechanisms. The present study investigated the effect of seasonality and microhabitat exposure on photoprotection and photoacclimation in the photo- and the mycobiont of six lichen species with different thallus morphology in inland dune system in the Kiskunság region (Hungary) with shaded, more humid and exposed, drier dune sides. High-Performance Liquid Chromatography, spectrophotometry, chlorophyll a fluorescence kinetic technique were used, and micrometeorological data were collected. The four years data series revealed that the north-east-facing side was characterized by higher relative humidity and lower light intensities compared to the south-west-facing drier and more exposed sides. The south-west facing side was exposed to direct illumination 3–4 hours longer in winter and 1–2 hours shorter in summer than the north-east facing side of the dune, influencing the metabolism of sun and shade populations of various species. Because rapid desiccation caused short active periods of lichens during bright and drier seasons and on exposed microhabitats, the rapid, non-regulated non-photochemical quenching mechanisms in the photobiont had a significant role in protecting the photosynthetic system in the hydrated state. In dehydrated conditions, thalli were mainly defended by the solar screening metabolites produced by the mycobiont and curling during desiccation (also caused by the mycobiont). Furthermore, the efficacy of light use (higher chlorophyll and carotenoid concentration) increased because of short hydrated periods. Still, a lower level of received irradiation was appropriate for photosynthesis in dry seasons and on sun exposed habitats. In humid seasons and microhabitats, more extended active periods lead to increased photosynthesis and production of solar radiation protectant fungal metabolites, allowing a lower level of photoprotection in the form of regulated non-photochemical quenching by the photobiont. Interspecific differences were more pronounced than the intraspecific ones among seasons and microhabitat types.

The long-term effect of removing the UV-protectant usnic acid from the thalli of the lichen Cladonia foliacea

“…It can be concluded that the algae were more sensitive to the seasonally changing environment than the acetone treatment. The treatment mostly affected the mycobiont partner and the algae did not elevate their photoprotective role (φNO and φNPQ did not differ in the control and treated thalli), as Veres et al ( 2022 ) pointed out in Cladonia furcata . Probably the initial amount of UV-protectant lichen metabolite originated from a high level of constitutive photoprotection (Asplund et al 2009 ), and the system was over-secured.…”

“…Therefore, the thalli of Cladonia foliacea can survive for years after acetone treatment. Veres et al ( 2022 ) pointed out that C. foliacea contained approximately four times more cortical pigment (usnic acid) compared to, for example, C. furcata or C. magyarica containing atranorin in their cortex derived from the same microhabitat collected at the same time. Probably the usnic acid can significantly contribute to the survival of C. foliacea in habitats with extreme irradiation and drought, helping the thalli against photodamage.…”

“…Nybakken and Julkunen-Tiitto 2006 ). Probably the thinner cortical layer of the mountain thalli could cause them to reflect more sensitively the seasonal changes of the environment (Veres et al 2022 ), but this needs further evidence.…”

“…It was assumed that because of the lowered level of usnic acid, the algae would take over the photoprotection role of the fungi and increase the level of non-photochemical quenching mechanisms to avoid photodamage. It was expected that the concentration of usnic acid in the treated and transplanted thalli would return to its original control level and local samples within the experimental period and supposedly within 18–20 months (estimation based on Solhaug et al 2003 and Veres et al 2022 ).…”

Effect of light stress on maximum photochemical efficiency of photosystem II and chloroplast structure in cryptogams Cladonia mitis and Pleurozium schreberi

“…As part of our investigation, we not only measured the photosynthetic activity of the lichens in their nonsoaked state, but also irrigated them for a period of two hours, since the importance of hydrating the samples under investigation before measuring their physiological parameters has already been shown in the previous study (Węgrzyn et al 2021b). Exposure to the sunlight of the dry lichen thallus can cause the accumulation of severe damage in PS II, and water is the endogenous factor for the regeneration of the damaged apparatus (Veres et al 2022a). In the study, the hydrated form of C. mitis showed signi cantly higher levels of photosynthetic activity than the non-soaked thallus, which is a natural process that occurs in lichens (Grimm et al 2021;Wegrzyn et al 2021).…”