Photosynthetic carbon allocation of an Antarctic sea ice diatom (Fragilariopsis cylindrus) Sarah C. Ugalde a,b, ⁎, Klaus M. Meiners a,c , Andrew T. Davidson a,c , Karen J. Westwood a,c , Andrew McMinn b a Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart 7001, Tasmania, Australia b Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart 7001, Tasmania, Australia c Australian Antarctic Division, Dep. of Sustainability, Environment, Water, Population and Communities, 203 Channel Highway, Kingston, Tasmania 7050, Australia abstract article info Article history: Received 9 January 2013 Received in revised form 21 May 2013 Accepted 24 May 2013 Available online 20 June 2013 Keywords: Antarctica Carbon fractionation Extracellular polymeric substances Microalgae pH Primary production Antarctic sea ice provides an ephemeral but important habitat for algal productivity and is characterised by extreme physicochemical variations. In this study, we assess the ability of a sea ice diatom (Fragilariopsis cylindrus) to cope with physicochemical changes through examination of physiological status and allocation of 14 C-incorporated organic carbon into particulate and extracellular fractions, using closed-bottle incubations over 49 d. Carbon allocation was found to vary with growth stage and shifts in the physicochemical environment, in particular the carbonate system. Total extracellular organic carbon was comprised of at least 85% low molec- ular weight 14 C-colloidal-organic carbon. The relative contribution of 14 C-extracellular polymeric substances and 14 C-total extracellular organic carbon to 14 C-total primary production varied from lag to senescent growth phases, increasing from 0 to 5.7% and 32.9% to 69.5%, respectively. Carbon allocation into 14 C-extracellular polymeric substances was correlated with a decline in CO 2 availability and increased pH. Overall, the results demonstrate that carbon exudation may play an important role in adaptive algal physiology by buffering cells against biogeochemical shifts within brine channels, induced through photosynthetic activity. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The variable extent of Antarctic sea ice is a significant seasonal event, advancing to ~18–19 × 10 6 km 2 at its maximum in September– October and retreating to ~3–4 × 10 6 km 2 each summer (Comiso, 2010). This process has critical effects on ocean–atmosphere interac- tions (Thomas and Dieckmann, 2010), and is integrally linked to productivity and ecosystem dynamics of the Southern Ocean (Bluhm et al., 2010; Frazer et al., 1997; Loeb et al., 1997). Antarctic sea ice is structurally complex, comprised of a network of brine channels which provide an extensive habitat for microbial communities (Horner et al., 1992; Thomas and Dieckmann, 2010). Pennate diatoms typically dominate the sea ice flora, comprising > 90% of standing stocks during austral spring and often exceed 300 mg Chl a m 2 (Arrigo et al., 2010; Palmisano and Sullivan, 1983; Trenerry et al., 2002). An abundant Antarctic diatom, Fragilariopsis cylindrus (Grunow) Krieger (Bacillariophyceae), occurs commonly in both sea ice and open water column assemblages (Kang and Fryxel, 1992). It is therefore an ideal representative organism for physiolog- ical studies related to sea ice research (Mock and Valentine, 2004). Prolonged photosynthetic activity within the confines of brine chan- nel systems can result in the alteration of biogeochemical properties, such as depletion of carbon dioxide, increased pH, reduced availability of nitrate and silicate, high ammonia concentrations, and high concen- trations of dissolved organic matter (e.g., Gleitz et al., 1995; Meiners et al., 2009; Papadimitriou et al., 2007; Thomas and Dieckmann, 2010). The mechanisms employed by sea ice algae to tolerate these bio- geochemical extremes are poorly understood, however high cell abun- dances within brine channels infer significant adaptation. Arctic and Antarctic sea ice characteristically contain high concen- trations of mucilage and dissolved organic carbon, which is thought to be comprised of extracellular polymeric substances (EPS) (Krembs et al., 2002; Meiners et al., 2003; Underwood et al., 2010). EPS are pro- duced by a range of micro-organisms, including bacteria and algae (Aslam et al., 2012a,b; Collins et al., 2010; Krembs and Deming, 2008), and are defined as extracellular organic compounds that precipitate in a polar solvent, usually 70% ethanol (Decho, 1990; Underwood and Paterson, 2003). The compounds are large and complex macromole- cules, encompassing a wide range of polysaccharides, uronic acids, and sulphated sugars (Underwood and Paterson, 2003). Numerous studies have reported significant variability with respect to the abun- dance and composition of ice-associated EPS (Aslam et al., 2012a; Krembs et al., 2002; Meiners et al., 2003; Underwood et al., 2010) and this is likely to reflect the spatial and temporal variability that Journal of Experimental Marine Biology and Ecology 446 (2013) 228–235 Abbreviations: chl a, chlorophyll a; colloidal-OC, colloidal organic carbon; DIC, dissolved inorganic carbon; EPS, extracellular polymeric substances; POC, particulate organic carbon; TA, total alkalinity; TEOC, total extracellular organic carbon; TPP, total primary production. ⁎ Corresponding author at: Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart 7001, Tasmania, Australia. Tel.: +61 6232 3186. E-mail address: sarah.ugalde@utas.edu.au (S.C. Ugalde). 0022-0981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jembe.2013.05.022 Contents lists available at SciVerse ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe