546 Phycologia (2006) Volume 45 (5), 546–556 Published 1 September 2006 Differences in acclimation potential of photosynthesis in seven isolates of the tropical to warm temperate macrophyte Valonia utricularis (Chlorophyta) ANJA EGGERT 1,2 *†, RONALD J.W. VISSER 1 ,PHILIP R. VAN HASSELT 2 AND ANNEKE M. BREEMAN 1 1 Department of Marine Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands 2 Laboratory of Plant Physiology, Department of Plant Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands A. EGGERT, R.J.W. VISSER, P.R. VAN HASSELT AND A.M. BREEMAN. 2006. Differences in acclimation potential of photosyn- thesis in seven isolates of the tropical to warm temperate macrophyte Valonia utricularis (Chlorophyta). Phycologia 45: 546–556. DOI: 10.2216/05-03.1 The potential to acclimate photosynthesis to sub- and supra-optimal temperatures was investigated in seven isolates of Valonia utricularis (Roth) C. Agardh, a green macrophyte with a tropical to warm-temperate distribution. Photosynthesis– light response curves were obtained by measuring chlorophyll a fluorescence characteristics of algae grown at optimal (25°C), sub- and supra-optimal temperatures. Suboptimal temperatures were chosen to support 30% of the maximum relative growth rate in each isolate. Thermal acclimation was investigated by comparing short-term and long-term temperature effects on the initial rate of increase of the relative electron transport rate (rETR) and the maximum rETR under light-saturating conditions. Isolates from the northeast Atlantic and the Mediterranean all showed a strong potential to acclimate maximum rETR to suboptimal growth temperatures, i.e. short-term temperature effects were diminished after acclimation. However, photoinhibition, measured as a decrease of the maximal quantum yield (F v /F m ), was found when plants were grown at 30°C. The isolates reduced light harvesting at 30°C by decreasing total chlorophyll content and by increasing the chlorophyll a/b ratio. Up-regulation of photoprotective processes by the xanthophyll cycle pigments was not observed. In contrast, isolates from the Indo-west Pacific were unable to acclimate photosynthesis to suboptimal growth temperatures and these tempera- tures were strongly photoinhibiting, even though adjustments on the pigment level were observed. All Indo-west Pacific isolates reached comparable maximum rETR values at 30° and 25°C. Thus, the Atlantic/Mediterranean isolates had a stronger potential to acclimate photosynthetic rates at suboptimal growth temperatures compared to the Indo-west Pacific isolates, which was accompanied by losses at 30°C. The results are discussed in a biogeographical context. KEY WORDS: Light-harvesting, Photosynthesis, Thermal acclimation, Valonia utricularis INTRODUCTION The green macrophyte Valonia utricularis (Roth) C. Agardh (Chlorophyta) has a world-wide tropical to warm-temperate distribution. Temperature is a major factor in determining the biogeographical distribution of marine macrophytes (Van den Hoek et al. 1990). Species with a broad distribution may be composed of ecotypes, each of which is adapted to its partic- ular habitat. Previous research on the temperature require- ments for growth, survival and reproduction of V. utricularis isolates has shown that a cold-adapted thermal ecotype de- veloped in the warm-temperate regions of the northeastern Atlantic/Mediterranean, in contrast to a cold sensitive ecotype in the Indo-west Pacific (Eggert et al. 2003a). However, op- timum growth temperatures were similar in all tested isolates (25°C). Plants or ecotypes adapted to thermally contrasting habitats generally exhibit photosynthetic temperature responses that reflect adaptation to the temperature regimes of their habitat. Populations from colder environments reach higher photosyn- thetic rates at low temperatures and optimum photosynthetic rates occur at lower temperatures than in populations native to warmer environments (Berry & Bjo ¨rkman 1980; Larcher * Corresponding author (anja.eggert@uni-rostock.de). † Present address: University of Rostock, Institute of Bioscience, Albert-Einstein-Straße 3, D-18051 Rostock, Germany. 1995). For example, Wiencke et al. (1993) and Eggert & Wiencke (2000) report maximal photosynthesis for Antarctic marine macrophytes at 10–15°C and substantially reduced rates at 25°C, whereas optimum temperatures for photosyn- thesis are much higher (25–35°C) in warm-temperate/tropical species (Terrados & Ros 1992). The impairment of photosyn- thesis at suboptimal temperature appears to be primarily re- lated to impaired synthesis and function of photosynthetic pig- ments (e.g. Robertson et al. 1993; Nie et al. 1995) and re- duced activities of key enzymes in the Calvin cycle (e.g. Kingston-Smith et al. 1997; Olesen & Madsen 2000). In con- trast, the impairment of photosynthesis at high temperatures is thought to be a direct result of thermal disruption of the thylakoid membranes leading to an inactivation of PSII activ- ity (Havaux & Tardy 1996). Thus, low and high temperatures have adverse effects on the quantum yield of electron trans- port and therefore generally increase the risk of oxidative stress to the photosynthetic apparatus, which can lead to chronic damage to PSII as indicated by a decrease in the max- imal quantum yield of PSII (F v /F m ) (Krause 1994). Plants may have the ability to acclimate photosynthesis in response to changes in growth temperature, allowing them to optimise photosynthesis over a range of temperatures. The po- tential for thermal acclimation of photosynthesis varies be- tween species. Plants native to habitats with large annual tem- perature variations generally display a stronger ability to ac-