Physiological acclimation to light in Chara intermedia nodes Miriam Gerhardt a , Susanne Schneider b , Wolfgang Bru ¨ ggemann a, * a University of Frankfurt, Institute for Ecology, Evolution and Diversity, Siesmayer Street 70, D-60323 Frankfurt, Germany b Norwegian Institute of Water Research, Gaustadalle ´en 21, NO-0349 Oslo, Norway 1. Introduction Charophytes (Charales, Charophyceae) are submerged green algae with a macroscopic thallus and an equisetum-like growth form, attached to the substratum by rhizoids. Charales can be found both in brackish and in fresh water habitats (Krause, 1997) and colonise depths from close to the water surface down to 30 m (Schwarz et al., 1996) with irradiances of as low as 14.5 mmol m 2 s 1 or less (Steinman et al., 1997). Underwater light climate is highly variable because of sun position, weather conditions, water colour, turbidity, depth and wave form (Schneider et al., 2006; Schubert et al., 2001). Reported acclima- tion strategies of other algae, including marine Charales, to different light climate conditions include both mechanisms for effective light use at deep, low light stands as well as protective mechanisms against excessive light (Sorrell et al., 2001; Schwarz et al., 2002; Ku ¨ ster et al., 2004). Light acclimation also includes chromatic acclimation. In several unicellular plankton algae of the Chlorophyta, it has been shown that blue-light enhances photosynthetic activity at various levels (summarized in Mouget et al., 2004). Chara intermedia is a species of oligotrophic freshwater stands. Especially in deep clear lakes near the Alps C. intermedia is very dominant and forms dense algae beds. Its thallus reaches a length of up to 80 cm (Krause, 1997). It has been shown earlier (Schneider et al., 2006) that C. intermedia (as well as Chara hispida) shows a morphological light acclimation by differential growth responses: under low-light, its lateral branches are spread horizontally outwards and under high-light they grow vertically upwards towards the light. This has been explained in terms of optimal light use under conditions of growth in dense algae beds, because light can penetrate into deeper areas of an algae bed. It is also assumed that the upward orientation of the youngest (top) branches could be a protection against excessive light. In the present contribution, two questions will be addressed: 1: Does C. intermedia react both morphologically and physiologi- cally to different irradiances?and 2: Does light quality influence the acclimation responses (i.e. are blue-light receptors or phytochromes responsible)? Aquatic Botany 91 (2009) 151–156 ARTICLE INFO Article history: Received 1 October 2008 Received in revised form 4 May 2009 Accepted 6 May 2009 Available online 18 May 2009 Keywords: Chara intermedia Light acclimation Photosynthesis Physiology Morphology Pigment ratios ABSTRACT In this study we investigated the ability of Chara intermedia to acclimate to different irradiances (i.e. ‘‘low-light’’ (LL): 20–30 mmol photons m 2 s 1 and ‘‘high-light’’ (HL): 180–200 mmol photons m 2 s 1 ) and light qualities (white, yellow and green), using morphological, photosynthesis, chlorophyll fluorescence and pigment analysis. Relative growth rates increased with increasing irradiance from 0.016  0.003 (LL) to 0.024  0.005 (HL) g g 1 d 1 fresh weight and were independent of light quality. A growth-based branch orientation towards high-light functioning as a mechanism to protect the plant from excessive light was confirmed. It was shown that the receptor responsible for the morphological reaction is sensitive to blue-light. C. intermedia showed higher oxygen evolution (up to 10.5 (HL) vs. 4.5 (LL) nmol O 2 mg Chl 1 s 1 ), photochemical and energy-dependent Chl fluorescence quenching and a lower Fv/Fm after acclimation to HL. With respect to qP, the acclimation of the photosynthetic apparatus depended on light quality and needed the blue part of the spectrum for full development. In addition, pigment composition was influenced by light and the Chl a/Car and Antheraxanthin (A) + Zeaxanthin (Z)/Violaxanthin (V) + A + Z (DES) ratios revealed the expected acclimation behaviour in favour of carotenoid protection under HL (i.e. decrease of Chl a/Car from 3.41  0.48 to 2.30  0.35 and increase of DES from 0.39  0.05 to 0.87  0.03), while the Chl a/Chl b ratios were not significantly affected. Furthermore it was shown that morphological light acclimation mechanisms influence the extent of the physiological modifications. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 69 79824745; fax: +49 69 79824822. E-mail address: w.brueggemann@bio.uni-frankfurt.de (W. Bru ¨ ggemann). Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot 0304-3770/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2009.05.002