Elevated temperatures have a signiicant effect on microalgae, causing photoinhibition before other cell functions are impaired (Harrison & Platt 1986, Davison 1991). Extreme temperatures limit electron transport and carbon ixation by reducing the ability of the algae to use light. This results in excess light energy and causes photoinhibition by damaging the Photosystem II (PSII) apparatus (Levasseur et al. 1990; Anning et al. 2001). PSII is the most thermo-sensitive component of photosynthesis (Falkowski & Raven 1997). Temperature inluences algae photosynthesis by changing the photosynthetic rate, or by inducing phenotypic or genotypic changes among algae species (Davison 1991). In a review by Eppley (1972), the author concluded that phytoplankton cultures grown at low temperature showed low P max and low saturating intensity for photosynthesis. The changes in P max with temperature were caused by effects on the enzymatic complex of inorganic carbon ixation (Davison 1991). Algae show different photosynthetic responses when exposed to temperatures above and below their optimal temperature. High temperatures may cause instability in the structure of the thylakoid membrane, primarily by affecting the composition of membrane lipids (Jensen & Knutsen 1993; Kirk 1994; Falkowski & Raven 1997). Davison (1991) suggested that elevated temperatures were able to modulate the cellular concentrations of RUBISCO and Calvin cycle enzymes that directly decreased the effective quantum yield (∆F/F m' ). Theoretically ∆F/F m' is the operating quantum eficiency of PSII; as such, it is a measure of the proportion of light absorbed by PSII that is used for photochemistry rather than being quenched (Maxwell & Johnson 2000). Thus a decrease in the ∆F/F m' may suggest damage to PSII or increased quenching activity (NPQ) (Ralph et al. 2005; Campbell et al. 2006). For example, El-Sabaawi & Harrison (2006) observed a decline in ∆F/F m' of sub-artic diatoms species when exposed to temperatures from 4ºC to 20ºC. Microalgae exposed to extreme low temperatures lose lexibility in their membranes which then become crystalline or freeze (Falkowski & Raven 1997). The response of photosynthesis to temperature is also dependent on the available light, with the response at sub-saturating light levels being very different from that at saturating light levels (Davison 1991). The benthic microalgal communities near Casey Station, eastern Antarctica, experience extreme variations in photoperiod and range of irradiance, ranging from low irradiances under ice cover in winter to high irradiance after the ice melts away or breaks up in spring. Irradiance on the sea loor has been reported to vary from 2 µmol photons m –2 s –1 under ice cover to 308 µmol photons m –2 s –1 in the absence of ice at depths of approximately 10 m (McMinn et al. 2004). Despite experiencing such extreme changes, the microalgal communities were able to adapt to both low and high irradiances. They were able to regulate their irradiance absorption eficiencies in response to the ASM Sci. J., 4(1), 81–88 81 Effects of Temperature on the Photosynthetic Parameters of Antarctic Benthic Microalgal Community S. Salleh 1,2 * , A. McMinn 1 , M. Mohammad 2 , Z. Yasin 3 and S.H.A. Tan 3 Elevated temperature affects marine benthic algae by reducing growth and limits the transport of electron or carbon ixation which may reduce the ability of the cell to use light. This resulting excess light energy may cause photoinhibition. In this study, the photosystem II of the benthic microalgal communities from Casey, eastern Antarctic were relatively unaffected by signiicant changes in temperatures up to 8ºC, along with high PAR level (450 µmol photons m –2 s –1 ). Similarly, the community was able to photosynthesize as the temperature was reduced to –5ºC. Recovery from saturating and photoinhibiting irradiances was not signiicantly inluenced by temperatures at both –5ºC and 8ºC. These responses were consistent with those recorded by past experiments on Antarctic benthic diatoms and temperate diatoms which showed that climate change did not have a signiicant impact on the ability of benthic microalgae to recover from photoinhibitory temperature stress. Keywords: photoinhibition; recovery; temperature; light; Antarctic; benthic microalgae; photosystem II; Casey; 1 Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Private Bag 77, Hobart 7001, Tasmania, Australia 2 Centre for Marine and Coastal Studies, Universiti Sains Malaysia, 11800, Penang, Malaysia 3 School of Biological Sciences, Universiti Sains Malaysia, 11800, Penang, Malysia * Corresponding author (e-mail: sazlinam@utas.edu.au) paper 10.indd 81 7/31/2010 9:42:46 PM