Light micrographs of the strains pre-cultivated on agar medium for 9 weeks.

<p><b>A–D:</b> cultures grown on regular BBM medium (A BBM); <b>E–H:</b> cultures grown on BBM without nitrate (A BBM-N). The images were taken prior to the desiccation experiments; a <i>Zygnema</i> akinete with a distinct brown mesospore is marked with an asterisk. Scale bars: 10 µm.</p

Transmission electron micrographs of pre-akinetes following desiccation for 2.5 hours at 86% rh.

<p><b>A–C:</b><i>Zygnema</i> sp. B; <b>D:</b><i>Zygnema</i> sp. C; <b>E</b>–<b>F:</b><i>Zygnema</i> sp. E; and <b>G:</b><i>Zygnemopsis</i> sp. L. Note the following features: <b>A:</b> dense structure of the chloroplast and fusions of lipid bodies; <b>B:</b> dense structure of Golgi body and chloroplast; <b>C:</b> nucleus with less electron-dense areas of heterochromatin (arrow); <b>D:</b> accumulation of lipid bodies in the cell periphery; <b>E:</b> starch grains, lipid bodies in the cell periphery and the cell wall covered by a fibrillose mucilage layer; <b>F:</b> accumulations of ribosomes (arrows) next to the nucleus; and <b>G:</b> accumulation of vesicles and small electron translucent compartments next to the lipid bodies. Abbreviations: Chl: chloroplast; G: Golgi body; L: lipid body; M: mitochondrion; ML: mucilage layer; N: nucleus; S: starch. Scale bars: A–D and G: 1 µm; E: 2 µm; F: 0.5 µm.</p

Recovery of effective quantum yield (<i>Φ</i>_PSII) during rehydration after 24 hours desiccation.

<p>Each panel compares the performance of the same culture desiccated under different conditions; no plots were constructed for those regimes that were lethal for all of the cells. Samples were collected from cultures pre-cultivated on agar media: <b>A</b>–<b>D:</b> cultures grown on regular agar medium (A BBM); <b>E</b>–<b>H:</b> cultures grown on BBM without nitrate (A BBM-N). Results are means ± standard deviations of four independent experimental replicates.</p

Effective quantum yield (<i>Φ</i>_PSII) under different desiccation scenarios.

<p><b>A</b>–<b>D:</b> rapid desiccation, approximately 10% relative humidity (rh); <b>E</b>–<b>H:</b> slow desiccation, 86% rh; <b>I</b>–<b>L:</b> very slow desiccation, 86% rh plus additional moistening of samples with 10 µl BBM. All strains shown were pre-cultivated on regular agar medium (A BBM) or on medium without nitrate (A BBM-N). Results are means ± standard deviations of four independent experimental replicates.</p

Viability of cultures pre-cultivated on agar following 48 hours of rehydration in water (A BBM and A BBM-N cultures).

<p>−: No living cells observed; +: <5% living cells; ++: 5–50% living cells; +++: 50–100% living cells.</p><p>Viability of cultures pre-cultivated on agar following 48 hours of rehydration in water (A BBM and A BBM-N cultures).</p

Transmission electron micrographs of pre-akinetes prior to desiccation.

<p><b>A:</b><i>Zygnema</i> sp. B. Large lipid bodies and chloroplast with starch grains are indicated. <b>B:</b><i>Zygnema</i> sp. C. Lipid bodies and electron-dense particles (arrow) are indicated. <b>C:</b><i>Zygnema</i> sp. E. Lipid bodies in the cell periphery, electron-dense particles (arrows) and cell walls with a fibrillose mucilage layer are marked. <b>D:</b><i>Zygnemopsis</i> sp. L. Chloroplast lobes with starch grains, electron-dense particles (arrows), and lipid bodies are marked. All cultures were cultivated on agar medium without nitrate (A BBM-N) for 9 weeks. Abbreviations: Chl: chloroplast; L: lipid body; M: mitochondrion; ML: mucilage layer; PG: plastoglobules; S: starch. Scale bars: 1 µm.</p

Phylogenetic tree of Zygnematophyceae showing the positions of the strains investigated in this study.

<p>A midpoint-rooted Bayesian tree of <i>rbc</i>L sequences is shown. Values at the branches indicate Bayesian posterior probabilities (BI PP), maximum likelihood (ML), and maximum parsimony (MP) bootstrap values (BS). Asterisks indicate BI PP = 1.00, and ML and MP BS = 100; dashes indicate BI PP<0.8, and ML and MP BS<50. Strains used in this study are in bold.</p

Schematic illustration of the experimental design.

<p>Four combinations of pre-cultivation conditions, four desiccation regimes (a–d; d applied only on samples pre-cultivated on agar) and final recovery after rewetting. For details see methodology section of the text.</p

Relative values of the effective quantum yield during recovery of algal samples pre-cultivated on agar (A BBM and A BBM-N cultures).

<p>The given percentages were computed as ratios between initial values of <i>Φ</i>_PSII and the values measured immediately after rehydration (t = 0) or after 48 hours of rehydration (t = 48 h). No data are shown for those regimes that were lethal for all of the cells (ND). Values shown are means of four, independent experimental replicates.</p><p>Relative values of the effective quantum yield during recovery of algal samples pre-cultivated on agar (A BBM and A BBM-N cultures).</p

Morphology and ultrastructure of <i>Interfilum</i> and <i>Klebsormidium</i> (Klebsormidiales, Streptophyta) with special reference to cell division and thallus formation

<div><p>Representatives of the closely related genera, <i>Interfilum</i> and <i>Klebsormidium</i>, are characterized by unicells, dyads or packets in <i>Interfilum</i> and contrasting uniseriate filaments in <i>Klebsormidium</i>. According to the literature, these distinct thallus forms originate by different types of cell division, sporulation (cytogony) versus vegetative cell division (cytotomy), but investigations of their morphology and ultrastructure show a high degree of similarity. Cell walls of both genera are characterized by triangular spaces between cell walls of neighbouring cells and the parental wall or central space among the walls of a cell packet, exfoliations and projections of the parental wall and cap-like and H-like fragments of the cell wall. In both genera, each cell has its individual cell wall and it also has part of the common parental wall or its remnants. Therefore, vegetative cells of <i>Interfilum</i> and <i>Klebsormidium</i> probably divide by the same type of cell division (sporulation-like). Various strains representing different species of the two genera are characterized by differences in cell wall ultrastructure, particularly the level of preservation, rupture or gelatinization of the parental wall surrounding the daughter cells. The differing morphologies of representatives of various lineages result from features of the parental wall during cell separation and detachment. Cell division in three planes (usual in <i>Interfilum</i> and a rare event in <i>Klebsormidium</i>) takes place in spherical or short cylindrical cells, with the chloroplast positioned perpendicularly or obliquely to the filament (dyad) axis. The morphological differences are mainly a consequence of differing fates of the parental wall after cell division and detachment. The development of different morphologies within the two genera mostly depends on characters such as the shape of cells, texture of cell walls, mechanical interactions between cells and the influence of environmental conditions.</p></div