Photosynthesis in a sub-Antarctic shore-zone lichen

•  Photosynthetic responses to moisture, light, temperature, salinity and inorganic nitrogen fertilization are reported for a shore-zone lichen Turgidiusculum complicatulum (formerly Mastodia tesselata), a possible recent introduction to sub-Antarctic Marion Island. •  Optimum moisture contents for net photosynthesis were 225-346% (ash free, dry mass). Net CO₂ exchange was dominated by a strong temperature dependence of respiration rate. Net photosynthetic rate responded sharply to increasing PPFD and saturated below 300 µmol m^-2  s^-1 , but electron transport rate (ETR) increased up to approx. 900 µmol m^-2  s^-1 PPFD suggesting that gross photosynthesis responded to light to this level. Nonphotochemical fluorescence quenching increased rapidly with PPFD up to approx. 400 µmol m^-2  s^-1 and thereafter more slowly. Even at high PPFD (1050 µmol m^-2  s^-1 ) most PSII centres were open. •  Salinity did not significantly influence CO₂ assimilation rate; however, NH₄ NO₃ significantly depressed net photosynthesis rate at all salinities except 100% seawater. ETR and dark respiration rate were increased by NH₄ NO₃ . •  The response of T. complicatulum to light and temperature enables high rates of CO₂ assimilation under the island's microclimatic regime; if sufficiently hydrated, the lichen would exhibit near maximal photosynthesis rates for approx. 75% of the photoperiod over the year.