Chlorophyll fluorescence imaging analysis of the responses of Antarctic bottom-ice algae to light and salinity during melting K.G. Ryan a, ⁎, M.L. Tay a , A. Martin a,b , A. McMinn b , S.K. Davy a a School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand b Institute of Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia abstract article info Article history: Received 7 July 2010 Received in revised form 5 January 2011 Accepted 11 January 2011 Keywords: Algae Antarctic Chlorophyll fluorescence Imaging PAM Salinity Sea ice Bottom-ice algae within Antarctic sea ice were examined using chlorophyll fluorescence imaging. The detailed structure of the bottom-ice algal community growing in the platelet and congelation layers of solid pieces of sea ice was evident for the first time in chlorophyll imaging mode. Strands of fluorescence representing algal cells were clearly visible growing upward into brine channels in a fine network. Images of effective quantum yield (Ф PSII ) revealed that the Ф PSII of algae embedded in the sea ice was approximately 0.5. Furthermore, Ф PSII decreased slightly with distance from the ice–water interface. The response of Antarctic sea ice algae to changes in irradiance and salinity, and the effects of slowly warming and melting the ice block sample were examined using this system. The Ф PSII of bottom-ice algae decreased as irradiance increased and salinities decreased. Bottom-ice algae appear to be most vulnerable to changes in their environment during the melting process of the ice, and this suggests that algae from this region of the ice may not be able to cope with the stress of melting during summer. Chlorophyll fluorescence imaging provides unprecedented imagery of chlorophyll distribution in sea ice and allows measurement of the responses of sea ice algae to environmental stresses with minimal disruption to their physical habitat. The results obtained with this method are comparable to those obtained with algae that have been melted into liquid culture and this indicates that previous melting protocols reveal meaningful data. In this chlorophyll imaging study, rapid light curves did not saturate and this may prevent further use of this configuration. © 2011 Elsevier B.V. All rights reserved. 1. Introduction At its greatest extent, Antarctic sea-ice covers 19 million km 2 (Arrigo and Thomas, 2004). The phytoplankton, protists and bacteria growing within sea-ice exert a strong influence in the Antarctic marine environment, and the bottom or interstitial, communities can reach biomass levels of over 300 mg Chlorophyll-α m -2 in the austral summer (Palmisano and Sullivan, 1983; Kirst and Wiencke, 1995). Their contribution to the primary production of the area is substantial (McMinn et al., 2000) and the overall contribution of ice algae to total primary production in ice-covered regions of the Southern Ocean is estimated at ~25% (Arrigo et al., 1997). The physicochemical conditions for the sea ice microbial commu- nity (SIMCO) are highly variable. There are strong gradients of light, temperature, salinity, and nutrient concentration within the ice column (Arrigo and Sullivan, 1992) and while microbes are found throughout the ice profile, bottom-ice communities are dominant in fast ice around most of Antarctica (Palmisano and Sullivan, 1983; McMinn and Ashworth, 1998; Trenerry et al., 2002; Fiala et al., 2006; Ryan et al., 2006). This is not the case for some 15–30% of the pack ice around Antarctica where surface communities can dominate (Arrigo and Thomas, 2004). The SIMCO must endure increased salinities during the freezing process where salinities can exceed 100‰ (Vargo et al., 1985; Gleitz and Thomas, 1992; Ralph et al., 2007), and temperatures may decrease well below the freezing point of sea water (Thomas et al., 2008). Annual fast ice often reaches over 2 m thickness at the end of winter and the light levels for the bottom-ice algal community, even at midday rarely reach N 15 μmol photons m -2 s -1 (Ryan and Beaglehole, 1994). The bottom-ice algal community may be amongst the most shade adapted photosynthetic organisms on earth (Thomas and Dieckmann, 2002). Photosynthesis has been recorded in these organisms at irradiances of less than 1 μmol photons m -2 s -1 (Palmisano and Sullivan, 1983; McMinn et al., 2003). Their E k (onset of light saturation) is often b 15 μmol photons m -2 s -1 , and they generally become photoinhibited at b 20 μmol photons m -2 s -1 (Palmisano et al., 1985; Kirst and Wiencke, 1995; McMinn et al., 2003) although this light level is rarely achieved under annual fast ice. They are also able to rapidly acclimate to diurnal changes in irradiance (McMinn et al., 2003). These data suggest that the bottom-ice algae in the Ross Sea region may never Journal of Experimental Marine Biology and Ecology 399 (2011) 156–161 ⁎ Corresponding author. Tel.: +64 4 463 6083; fax: +64 4 463 5331. E-mail address: ken.ryan@vuw.ac.nz (K.G. Ryan). 0022-0981/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2011.01.006 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe