Rejuvenation of Melosira Granulata (bacillariophyceae) Resting Cells from the Anoxic Sediments of Douglas Lake, Michigan, Ii. Electron Microscopy 1

Detailed cytological changes that accompany the rejuvenation of resting cells o/'Melosira granulata were studied with the electron microscope. Dormant and viable cells that we previously classified as the condensed state generally contain definable chloroplasts, mitochondria, a nucleus and other cytoplasmic remnants. However, there appears to be a continuous cytoplasmic degradation spectrum and some cells which appear intensely colored with the light microscope have discontinuous chloroplast membranes and few other cytoplasmic remnants. Rejuvenation of viable dormant cells is initially accompanied by the accumulation of both lipids and polyphosphates. In the earliest stages of expansion, these storage products are dispersed throughout the cell. In later stages of expansion, the lipids appear to be coalesced into larger droplets which are easily identified at the light microscope level. The fully ' .Accepted: 27 September 1985. • Address for corresp)ndence and reprints. expanded stage is characterized by the normal complement of organelles and their arrangement at the periphery of the cells and central cytoplasmic bridge. These cells appear both anabolically and catabolically active as ei'idenced by the abundance of endoplasmic reticulum, ribosomes and secretory and lytic vesicles. Prior to cell division, both lipids and polyphosphates a re reduced or absent in the cells. The ultrastructural features of the dormant, condensed state in resting cells of M, granulata are similar to those described for hypnospores. A rejuvenation sequence that produces cytological features common to resting state formation could provide a population of cells which could easily revert should environmental conditions become adverse. Key index words: diatom ultrastructure; Melosira; rcjuvenation; resting cells Although knowledge of the function and fate of resting spores of diatom populations has increased, little attention has been given to cells which function asa vegetative resting state. These resting cells differ RESTING CELL ULTRASTRUCTURE 29 from spores in several aspects. Spores can be more easily distinguished from vegetative cells in the light microscope by their external or frustular morphology. In some genera, resting spores resemble the parent vegetative cells. In others, spores and vegetative cells are morphologically dissimilar (Hargraves 1976, 1979). Generally spores are more heavily silicified than vegetagive cells (Hargraves and French 1983) or may possess frustular modifications such as the internal thecae described by von Stosch and Fecher (1979). Cells which are dormant but viable and have no known frustular modifications have been referred to as "physiologically resting cells" (Hargraves and French 1983). Perhaps because ofthe lack of differentiable morphology, the distribution and fate of resting cells have been studied very little. Malone et al, (1973) found that severa! species of pennate diatoms collected from red clay sediments in the North Atlantic bloomed when exposed to light at sea surface temperatures, Anderson (1975, 1976) studied the cytological and physiological characteristics of Amphora resting cells and found that the alga was capable of forming resting cells under adverse environmental conditions. These cells were also capable of reestablishing themselves under favorable growth conditions. In our previous report (Sicko-Goad et al. 1986) we described with the light microscope cytological changes and '*C uptake during the rejuvenation sequence of Melosira granulata resting cells. The following report is a detailed description of the fine structural changes that delineate this ordered rejuvenation sequence. MATERIALS AND METHODS Ten and 20 cm sediment samples, roughly corresponding to 15 to 30 years burial in the sediment, were collected and processed as previously described (Sicko-Goad et al. 1986). Cell types Were individually distinguished by light microscopy and selected for sectioning. All photographs were taken of sediment samples that had been resuspended in light for less than 104 h. Thin sections of the epoxy-embedded material were cut with a diamond knife, collected on cleaned formvar coated 200 mesh copper grids and stained with aqueous uranyl acetate (Watson 1958). Sections were examined in a JEOL JENf lOOB electron microscope operatmg at 80 KV. Permanent epoxy mounts were also made from the osmicated samples as reference slides,