Photosynthetic efficiency, desiccation tolerance and ultrastructure in two phylogenetically distinct strains of alpine Zygnema sp. (Zygnematophyceae, Streptophyta): role of pre-akinete formation

Two newly isolated strains of green algae from alpine regions were compared physiologically at different culture ages (1, 6, 9 and 15 months). The strains of Zygnema sp. were from different altitudes (‘Saalach’ (S), 440 m above sea level (a.s.l.), SAG 2419 and ‘Elmau-Alm’ (E-A), 1,500 m a.s.l., SAG 2418). Phylogenetic analysis of rbc L sequences grouped the strains into different major subclades of the genus. The mean diameters of the cells were 23.2 μm ( Zygnema S) and 18.7 μm ( Zygnema E-A) but were reduced significantly with culture age. The photophysiological response between the strains differed significantly; Zygnema S had a maximal relative electron transport rate ( rETR max ) of 103.4 μmol electrons m −2 s −1 , Zygnema E-A only 61.7 μmol electrons m −2 s −1 , and decreased significantly with culture age. Both strains showed a low-light adaption and the absence of strong photoinhibition up to 2,000 μmol photons m −2 s −1 . Photosynthetic oxygen production showed similar results ( P max Zygnema S, 527.2 μmol O 2 h −1 mg −1 chlorophyll (chl.) a , Zygnema E-A, 390.7 μmol O 2 h −1 mg −1 chl. a ); the temperature optimum was at 35 °C for Zygnema S and 30 °C for Zygnema E-A. Increasing culture age moreover leads to the formation of pre-akinetes, which accumulate storage products as revealed by light and transmission electron microscopy. Desiccation at 84 % relative air humidity (RH) lead to a reduction of the effective quantum yield of photosystem II (PSII) (Δ Fv /Fm′) to zero between 90 to 120 min ( Zygnema S) and between 30 to 60 min ( Zygnema E-A), depending on the culture age. A partial recovery of Δ Fv /Fm′ was only observed in older cultures. We conclude that pre-akinetes are crucial for the aeroterrestrial lifestyle of Zygnema .