Responses to desiccation stress by Klebsormidium rivulare (Ulotrichales, Chlorophyta) from a Rhode Island stream

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Abstract
Epilithic populations of Klebsormidium rivulare (Kütz.) comb. nov. in a Rhode Island stream were subjected to prolonged intervals of desiccation during the summer and early fall. Following periods of exposure, filaments contained markedly thickened walls (> 2 μm) and/or unusual H-shaped wall pieces. The cells of these filaments were also filled with abundant lipid and starch granules and therefore can be termed akinetes. A clone of K. rivulare was isolated and subjected to a controlled drying regime in culture. Changes observed after 4 weeks of desiccation included a four-fold increase in wall thickness, accumulation of cytoplasmic lipid globules, a disruption of thylakoid organization by numerous starch granules, a slight decline in pigments and a disappearance of vacuoles. Continued carbon fixation in the desiccated cells was implied by a 1.5-fold increase in dry weight and total carbon. A doubling of carbohydrates, a tripling of lipids and a 10% decline in proteins demonstrated that the carbon metabolism of K. rivulare cells was altered by drying. Water potential regulation in desiccated cells was indicated by a 3.5-fold increase in low molecular weight solutes. Upon rehydration, both field-collected and laboratory-induced akinetes germinated. Cell division and filament elongation pushed thickened cross walls apart, resulting in H-shaped remnants similar to those seen in situ. The akinetes of K. rivulare have many features in common with a variety of stressed plant cells, indicating a similarity in stress responses and tolerance mechanisms.