Different strategies to achieve Pb-tolerance by the two Trebouxia algae coexisting in the lichen Ramalina farinacea

Abstract

Elsevier Álvarez, R.; del Hoyo, A.; García Breijo, FJ.; Reig Armiñana, J.; Del Campo, E.; Guera, A.; Casano, L. (2012). Different strategies to achieve Pb-tolerance by the two Trebouxia algae coexisting in the lichen Ramalina farinacea. Lichen thalli are permeable to airborne substances, including heavy metals, which are harmful to cell metabolism. Ramalina farinacea shows a moderate tolerance to Pb. This lichen comprises two Trebouxia phycobionts, provisionally referred to as TR1 and TR9, with distinct physiological responses to acute oxidative stress. Thus, there is a more severe decay in photosynthesis and photosynthetic pigments in TR1 than in TR9. Similarly, under oxidative stress, antioxidant enzymes and HSP70 protein decrease in TR1 but increase in TR9. Since Pb toxicity is associated with increased ROS formation, we hypothesized greater Pb tolerance in this phycobiont. Accordingly, the aim of the present study was to characterize the physiological differences in the responses of TR1 and TR9 to Pb exposure. Liquid cultures of isolated phycobionts were incubated for 7 days in the presence of Pb(NO 3 ) 2 . Thereafter, extracellular and intracellular Pb accumulation, photosynthetic pigments, and photosynthesis (as modulated chlorophyll fluorescence) were analyzed along with the antioxidant enzymes glutathione reductase (GR), superoxide dismutase (SOD), ascorbate peroxidase (APx), and catalase (CAT), and the stress-related protein HSP70. Pb uptake increased with the amount of supplied Pb in both algae. However, while significantly more metal was immobilized extracellularly by TR9, the amount of intracellular Pb accumulation was three times higher in TR1. In neither of the phycobionts were significant effects on photosynthetic pigments or photosynthetic electron transport observed. While under control conditions GR, SOD, and APx levels were significantly higher in TR1 than in TR9, only in the latter were these enzymes induced by Pb. This resulted in quantitatively similar antioxidant activities in the two algae when exposed to Pb. In conclusion, the phycobionts of R. farinacea make use of two different strategies against stress, in which the integration of distinct anatomical and physiological features affords similar levels of Pb tolerance.

Lichen Rehydration in Heavy Metal-Polluted Environments: Pb Modulates the Oxidative Response of Both Ramalina farinacea Thalli and Its Isolated Microalgae

“…As considered in more detail in a previous study [1], there is a vast variability in HM-tolerance among lichens. However, the physiological basis of such tolerance and especially the relative contribution of the mycobiont and photobionts to the HM-tolerance of the holobiont is still a matter of debate [1]. Anyway, the intracellular accumulation of HMs is a potential metabolic risk for lichens.…”

“…In addition, when these phycobionts were incubated in a liquid medium containing Pb, the TR9 algae were able to limit the entry of the toxic metal into the cell, by holding it in the form of aggregates on the outer layer of the cell wall. Under the same conditions, TR1 had a lower level of wall Pb retention capability [1]. Conversely, the intracellular Pb uptake was approximately three times higher in TR1 than in TR9.…”

“…They are among the most valuable biomonitors of environmental pollution both for inorganic and organic compounds, being able to accumulate mineral elements far above their needs [2,3,32,41]. As considered in more detail in a previous study [1], there is a vast variability in HM-tolerance among lichens. However, the physiological basis of such tolerance and especially the relative contribution of the mycobiont and photobionts to the HM-tolerance of the holobiont is still a matter of debate [1].…”

“…In addition, liquid cultures of TR1 and TR9 were similarly tolerant to lead exposure, but employed different morpho-physiological strategies against the heavy metal [1]. In the present study, we characterized the kinetics of intracellular oxygen free-radical formation and two related oxidative parameters (TBARS and chlorophyll autofluorescence) during rehydration, initially with water and then in the presence of two concentrations of PbNO 3 , in isolated and previously desiccated TR1 and TR9 phycobionts.…”

“…However, Pb induced the three enzymes in TR9 while it had no effect on TR1, so that antioxidant activities were quantitatively similar in both phycobionts under Pb treatments. It seemed that each algal species may have developed a specific strategy for the acquisition of similar levels of Pb tolerance [1].…”