The fine structure of an algal mat from a freshwater maritime antarctic lake

The three-dimensional microstructure of Tolypothrix mats from the bottom of a maritime antarctic lake of Signy Island, South Orkneys, was examined. Samples from mats at two depths, 4 and 6 m, within the lake were taken by scuba divers and frozen (−80 °C) in March 1987. The samples were freeze-fractured and examined by ambient and low temperature scanning electron microscopy (LTSEM). The mats shared a similar structure consisting of a compact lower zone of prostrate filaments and an upper zone of loose vertical filaments. An outer layer of extremely loose spreading filaments was only found by LTSEM, leading to the conclusion that some collapse of the mat structure occurred during dehydration for ambient SEM. Fine detail of the mat matrix such as the attachment of epiphytes and associated microfauna to the filaments was often obscured by mucilage. Fast-particle etching was used to remove this mucilage and also the organic coat that covers uncleaned diatoms, thereby allowing the identification of attached cells in situ. A variety of attachment strategies were observed although sessile forms were most common. Further fast-particle etching of the epiphytic diatom assemblages revealed that many of the attached diatoms were devoid of cell contents. Together with the epiphytic diatoms a variety of microfauna were identified, both incorporated within the mat and on the mat surface. The implications of these observations are discussed.

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