RESEARCH ARTICLE A newly isolated Chlorella sp. from desert sand crusts exhibits a unique resistance to excess light intensity 1 Haim Treves 1 , Hagai Raanan 1 , Omri Finkel 1 , Simon M. Berkowicz 1 , Nir Keren 1 , Yoram Shotland 2 & Aaron Kaplan 1 1 Department of Plant and Environmental Sciences, Edmond J. Safra Campus - Givat Ram, The Hebrew University of Jerusalem, Jerusalem, Israel; and 2 Chemical Engineering, Shamoon College of Engineering, Beer Sheva, Israel Correspondence: Aaron Kaplan, Department of Plant and Environmental Sciences, Edmond J. Safra Campus, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. Tel.: ???????????? 2 ; fax: 972 2 6584463; e-mail: aaron.kaplan@mail.huji.ac.il Received 24 April 2013; revised 5 June 2013; accepted 11 June 2013. DOI: 10.1111/1574-6941.12162 Editor: Riks Laanbroek Keywords CO 2 concentrating mechanism; light intensity; photosynthesis; productivity. Abstract We recently isolated a small green alga from a biological sand crust (BSC) in the NW Negev, Israel. Based on its 18S rRNA and rbcL genes, it is a close rela- tive of Chlorella sorokiniana and of certain strains of C. vulgaris and C. variabi- lis, but differs substantially in many aspects from C. sorokiniana. Because the classification of Chlorellales is still not resolved, we designated this species as C. ohadii (Trebouxiophyceae) in honor of Professor Itzhak Ohad. Under con- trolled laboratory conditions, C. ohadii showed marked structural and photo- synthetic performance changes, depending on the carbon source used during growth, as well as remarkable resistance to photoinhibition. CO 2 -dependent O 2 evolution was not affected even when exposed to a light intensity of 3500 lmole photons m À2 s À1 , over 1.5 times the maximal intensity reached at the BSC surface, whereas the variable fluorescence declined sharply. We briefly discuss the use of fluorescence to assess photosynthetic rate and the implica- tions of this finding for the assessment of global BSCs activity. Introduction Biological sand crusts (BSCs) play an important role in stabilizing sandy desert areas by reducing wind erosion (Belnap & Gillette, 1998) and by their impact on biotic compositions (Eldridge & Greene, 1994; Prasse & Born- kamm, 2000; Eldridge & Leys, 2003; Belnap et al., 2004). Destruction of these crusts is considered an important promoter of desertification in arid and semi-arid regions. The crusts are formed by adhesion of the soil particles to extracellular polysaccharides excreted mainly by filamen- tous cyanobacteria, the main primary producers in desert crusts. Other microorganisms, including fungi, microal- gae, lichens, and bacteria, are also abundant, particularly in humid areas that are often covered by a thick crust [(B€ udel, 2002; B€ udel & Veste, 2008; Bates et al., 2012) and references therein]. biological sand crusts represent one of the harshest environments in nature. Organisms inhabiting this ecosys- tem face frequent hydration/dehydration cycles, extreme light intensities, temperature amplitude from subfreezing during winter nights but up to 60 °C in mid-summer days, and vast osmotic potential changes from close to pure rainwater to salt crystals on the crust’s upper layer (B€ udel & Veste, 2008). To cope with such conditions, the organisms inhabiting the BSCs likely possess survival mechanisms, the nature of which remains largely unknown (Potts, 2001; Harel et al., 2004; Ohad et al., 2005; Wright et al., 2005). Crucial for survival is the ability to sense diurnal changes in environmental conditions and rapidly activate metabolism and growth in the short periods when water and sufficient light intensity are available, but to turn metabolism off during desiccation (Potts, 2001). Desert crusts are liable to experience rapid pH rise from around 8 and up to 10.5 (Garcia Pichel & Belnap, 1996) consequent on the photosynthetic activity, thereby reducing the dissolved CO 2 level within the crusts. To cope with the limiting CO 2 level, as compared with the high K 1/2 (CO 2 ) of the carboxylating enzyme ribulose-1,5 bisphosphate carboxylase/oxygenase (RubisCO), a CO 2 concentrating mechanism (CCM) (documented in many photosynthetic microorganisms) is likely activated to enable efficient CO 2 fixation (Palmqvist et al., 1994; Price et al., 1998; Kaplan & Reinhold, 1999; Lange et al., 1999; 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 FEMS Microbiol Ecol && (2013) 1–8 ª 2013 Federation of European Microbiological Societies Published by John Wiley & Sons Ltd. All rights reserved F E M S E C 1 2 1 6 2 B Dispatch: 27.6.13 Journal: FEMSEC CE: Priya Lekshmi S.G. Journal Name Manuscript No. Author Received: No. of pages: 8 PE: Karthick Viswanath