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 rbcL 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) (ΔFv/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 ΔFv/Fm′ was only observed in older cultures. We conclude that pre-akinetes are crucial for the aeroterrestrial lifestyle of Zygnema.