Effects of light and temperature on Chara vulgaris (Charophyceae) gyrogonite productivity and polymorphism – palaeoenvironmental implications JOSEP SANJUAN 1,2 *, ALBA VINCENTE 2 ,N ´ URIA FLOR-ARNAU 3 ,TONI MONLEO ´ N 4 ,JAUME CAMBRA 3 AND CARLES MARTI ´ N-CLOSAS 2 1 Departament of Geology, American University of Beirut (AUB), 11-0236, Post Hall 201C, Beirut, Lebanon 2 Departament de Din ` amica de la Terra i de l’Oce ` a, Facultat de Ci ` encies de la Terra, Universitat de Barcelona (UB), Mart´ ı i Franqu ` es s/n, 08028, Barcelona, Catalonia, Spain 3 Departament de Biologia Evolutiva, Ecologia i Ci ` encies Ambientals, Facultat de Biologia, Universitat de Barcelona (UB), Avinguda Diagonal 643, 08028, Barcelona, Catalonia, Spain 4 Departament de Gen ` etica, Microbiologia i Estad´ ıstica, Facultat de Biologia, Universitat de Barcelona (UB), Avinguda Diagonal 645, 08028, Barcelona, Catalonia, Spain ABSTRACT: To test the influence of light and temperature on intraspecific gyrogonite variation, two common European charophyte species, Chara vulgaris and Chara globularis, were studied indoors under controlled environmental conditions. A total of 60 plants of each species were incubated under different combinations of four irradiances (L1 ¼ 30 lmol PAR m 2 s 1 ; L2 ¼ 150 lmol PAR m 2 s 1 ; L3 ¼ 500 lmol PAR m 2 s 1 and L4 ¼ 1000 lmol PAR m 2 s 1 ) and three water temperatures (T1 ¼ 158C; T2 ¼ 258C and T3 ¼ 358C). These parameters simulate temporary ponds from the Mediterranean region in the summer and help in understanding charophyte reproduction in shallow temporary ponds of past geological periods. After 102 days, gyrogonite productivity was evaluated, and a morphometric analysis of the gyrogonites produced was performed. Our results indicated that both irradiance and water temperature play a key role in C. vulgaris intraspecific gyrogonite variation. Gyrogonites tended to increase their size as irradiance and temperature increased. However, in extreme conditions (L4 and T3), an opposite trend was observed, with gyrogonites becoming significantly smaller. Our results agree with the distribution patterns of fossil gyrogonite assemblages. Using the present data, we hypothesise that charophytes in historic temporary floodplain water bodies produced smaller gyrogonites in response to the stressful environments induced by high irradiances and temperatures. KEY WORDS: Charophytes, Fossil, Gyrogonite, Mediterranean, Polymorphism, Palaeoecology, Palaeoenvironment INTRODUCTION Charophytes are a group of submerged freshwater or brackish-water macrophytes that are among relatives of bryophytes and vascular plants. Fossil records of charo- phytes are mainly based on their calcified fructifications, gyrogonites and utricles, dating from the Silurian period to the present (Grambast 1974; Feist et al. 2005). Gyrogonites are important biostratigraphic markers used worldwide to date continental rock formations and reconstruct historic non-marine aquatic environments. However, gyrogonites attract little attention in studies on extant species, since the taxonomic classification of living charophytes is mainly based on vegetative characters (Corillion 1972). Delimitation of extant charophyte species is sometimes controversial due to the morphological variability of their vegetative and reproductive organs in response to environmental variables (Bonis et al. 1993; Blindow et al. 2003). Irradiance, salinity and their combination can determine the growth pattern and morphology of some species of Chara (Flor-Arnau et al. 2006). Furthermore, branch orientation during growth of Chara was modified by irradiance to protect plants from excessive light (Schneider et al. 2006). Asaeda et al. (2007) reported that morphological (shoot structure) and repro- ductive acclimations of charophytes change according to water depth. Water depth highly influences the irradiance and also regulates the effects of water turbulence. Ellawala et al. (2011) showed in indoor experiments that high water turbulence clearly reduced shoot elongation and chlorophyll concentration in Chara fibrosa C.Agardh ex Bruzelius. In such cases, the oospore wall, i.e. the organic pellicle coating the fructification inside the gyrogonite, provides additional taxonomic characters, such as the oospore general morphology and its external ornamentation, thus enabling differentiation between genera and even species (Souli ´ e- M¨ arsche 1989; John et al. 1990). Indeed, the oospore ornamentation pattern and size have helped distinguish particular species of Chara (Ray et al. 2001; Holzhausen et al. 2015). The charophyte fossil record is mainly based on the calcified gyrogonites, the spiral cells surrounding the oospore that result from post-fertilisation development (Souli ´ e- M¨ arsche 1989; Souli ´ e-M¨ arsche & Garc´ ıa 2015 and references therein). These calcified propagules are long-lasting organs that are biologically viable after long periods of burial (Rodrigo et al. 2010). To date, gyrogonites have been little studied in extant species. However, the first taxonomic analyses did discuss polymorphic gyrogonite features and hypothesised the environmental conditions affecting gyro- gonite formation (Souli ´ e-M¨ arsche 1971; Souli ´ e-M¨ arsche et al. 1991). Moreover, several studies have been dedicated to establishing taxonomic relationships between gyrogonites * Corresponding author (js76@aub.edu.lb). DOI: 10.2216/15-140.1 Ó 2017 International Phycological Society Phycologia Volume 56 (2), 204–212 Published 27 December 2016 204