RESEARCH ARTICLE Response of secondary metabolites to Cu in the Cu-hyperaccumulator lichen Stereocaulon japonicum Hiromitsu Nakajima 1,2 & Naoki Fujimoto 1 & Yoshikazu Yamamoto 3,4 & Takashi Amemiya 1 & Kiminori Itoh 1 Received: 30 June 2018 /Accepted: 29 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Lichen secondary metabolites are known to be associated with heavy metal uptake and tolerance in lichens. Understanding the relationship between their secondary metabolites and heavy metals in them is important for clarifying the mechanisms of their heavy metal accumulation and tolerance. To determine the relationships between the concentrations of secondary metabolites and Cu in the Cu-hyperaccumulator lichen Stereocaulon japonicum and to clarify its response to Cu, we collected Cu-contaminated and uncontaminated samples of the lichen and determined relative concentrations of secondary metabolites and concentrations of Cu, K, glucose, and sugar alcohols in them. We found significant negative correlations between the relative concentrations of secondary metabolites—atranorin and stictic acid—and the concentration of Cu. These negative correlations can be interpreted in one of two ways: (a) S. japonicum itself reduced the relative concentrations of secondary metabolites in response to the increase of Cu concentration or (b) its carbon and energy metabolism was damaged by Cu stress, resulting in the reduction of the relative concentrations of secondary metabolites. The analysis of K, glucose, and sugar alcohols showed no effect of Cu on these concentrations, which means that the carbon and energy metabolism was not damaged by Cu stress. Therefore, the negative correlations can be interpreted that S. japonicum itself reduced the relative concentrations of secondary metabolites with the increase of Cu concentration. These findings provide a deeper understanding of the response of secondary metabolites to Cu in the lichen. Keywords Stereocaulon japonicum . Copper hyperaccumulator . Lichen secondary metabolite . Atranorin . Stictic acid . Glucose . Sugar alcohol . Carbon and energy metabolism Introduction Lichen secondary metabolites are important factors in metal homeostasis and pollution tolerance in lichens; moreover, some of the metabolites are associated with heavy metal up- take and tolerance (Purvis et al. 1987; Molnár and Farkas 2010). Hence, by examining these abiotic roles of the metab- olites, the mechanisms of heavy metal accumulation and tol- erance in lichens must be understood, which will provide use- ful information to use lichens for monitoring of heavy metal pollution. These abiotic roles, however, have not been entirely explored (Molnár and Farkas 2010). Results of relationships between concentrations of second- ary metabolites and heavy metals in lichens remain controver- sial (Pawlik-Skowrońska and Bačkor 2011; Hauck et al. 2013; Nakajima et al. 2015; Kalinowska et al. 2015; Gauslaa et al. 2016); in particular, concentrations of lichen secondary me- tabolites have been positively (Pawlik-Skowrońska and Bačkor 2011; Hauck et al. 2013; Gauslaa et al. 2016) or neg- atively (Gauslaa et al. 2016) correlated with those of heavy metals in lichens. This contradiction indicates that the relation- ships between concentrations of secondary metabolites and heavy metals in lichens are not fully understood. To clarify Responsible editor: Elena Maestri * Hiromitsu Nakajima h-nakaji@uec.ac.jp 1 Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan 2 Division of General Education, Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan 3 Graduate School of Bioresource Sciences, Akita Prefectural University, Shimoshinjo-nakano, Akita 010-0195, Japan 4 Osaka Museum of Natural History, 1-23 Nagai Park, Higashi-Sumiyoshi-ku, Osaka 546-0034, Japan Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-3624-4