Resting Spores of the Freshwater Diatoms <Emphasis Type="Italic">

Journal of Paleolimnology 9: 55-61, 1993. © 1993 Khtwer Academic Publishers. Printed in Belgium. 55 Resting spores of the freshwater diatoms Acanthoceras and Urosolenia Mark B. Edlund & E. F. Stoermer Center for Great Lakes and Aquatic Sciences, University of Michigan, 2200 Bonisteel Blvd, Ann Arbor, MI 48109-2099, USA Key words: diatoms, Acanthoceras, Attheya, Rhizosolenia, Urosolenia, cysts, resting spores Abstract Diatom resting spores are a widespread, but sometimes misconstrued component of siliceous micro fossil assemblages. We illustrate and discuss resting spore morphology found in populations of Acanthoceras and Urosolenia, two widely distributed freshwater genera. Taxonomic status of these genera and the potential paleolimnologic interpretation of resting spores are discussed. Introduction (Syvertsen, 1979). They have a higher chlorophyll content and faster sinking rates than vegetative Resting spores are especially common in temper- cells (French & Hargraves, 1980), and are rich in ate, neritic, marine centric diatoms. In most taxa, storage products (Hargraves & French, 1983). they are an asexual stage in the diatom's life his- Three types of resting spores have been described, tory and formed under conditions of nutrient based on the association of the spore to the stress. Recent articles by Hargraves & French mother cell; exogenous, semi-endogenous, and (1983) and Garrison (1984)review the character- endogenous (Anonymous, 1975). istics and ecological importance of resting spores. The occurrence of diatom resting spores in in- In short, resting spores are thought to function as land waters is limited to a few centric and pennate a resistant stage during periods of environmental taxa (von Stosch & Fecher, 1979). The centric extreme, but may also function for grazing resis- representatives belong to groups oftaxa that have tance and as a means of increasing species dis- classically been placed in marine genera, includ- persal potential. While unable to survive years of ing Chaetoceros Ehrenb., Rhizosolenia Brightwell, dormancy, resting spores have greater viability and Attheya West. Chaetoceros is widely distrib- than vegetative cells under conditions of dark- uted in inland saline lakes and brackish waters ness, temperature extreme, and desiccation. Ex- (Rushforth & Johansen, 1986). Its resting spores cystment usually occurs when spores are resup- are commonly preserved in saline lacustrine sedi- plied with light and nutrients. Also known as ments and have proven to be excellent paleolim- statospores, statocysts, or Dauersporen, the terms nologic indicators of higher salinity levels (Fritz resting spores or hypnospores are now most & Battarbee, 1988). In Rhizosolenia and Attheya, widely accepted. recently proposed nomenclatural changes have Spores are characterized by two heavily silici- separated the marine and freshwater taxa. Fresh- fled valves, termed primary and secondary, and water Rhizosolenia have been transferred to the may or may not have copulae or girdle bands genus Urosolenia Round and Crawford (Round 56 et al., 1990) and Attheya zachariasii J. Brun. was AuPd, and viewed at an accelerating voltage of transferred to the monotypic genus Acanthoceras 25 kV in a JEOL JSM-T100 SEM. Honigmann (Simonsen, 1979). These changes have taken into account ecological, cytological, Results and frustular differences between freshwater and marine species. As these two freshwater genera A note on taxonomy of Attheya/Acanthoceras and also form resting spores (Huber-Pestalozzi, Urosolenia/Rhizosolenia 1942), we fek that a better understanding of this aspect of their life history and a presentation of Since there have been a number of nomenclatural resting spore morphology might provide taxo- changes involving taxa discussed here, and fur- nomic assistance and information on their pale- ther changes apparently required, a brief discus- olimnologic interpretation. sion of historical nomenclature and current taxonomic status is in order. Simonsen (1979) pointed out that the freshwa- Materials and methods ter taxon commonly reported as Attheya zachariasii Brun differs morphologically from marine Nearsurface plankton containing populations species of Attheya, and is better referred to as identified as Urosolenia eriensis (H. L. Smith) Acanthoceras Honigmann. Acanthoceras is based Round and Crawford (=Rhizosolenia eriensis on A. magdeburgense Honigmann, an apparent H. L. Smith) and Acanthoceras zachariasii (Brun) later specific synonym ofAttheya zachariasii Brun. Simonsen ( = Attheya zachariasii J. Brun) was col- Acanthoceras zachariasii (Brun) Simonsen is lected from East Lake Okoboji, Dickinson widely distributed and occasionally abundant but County, Iowa, USA, on 31 July 1991. East Lake ephemeral, in alkaline, eutrophic, lakes, ponds, Okoboji is a shallow (10 m), alkaline, hyper- and rivers in North America and northern Eu- eutrophic lake (Bachmann & Jones, 1974). A rope (Beaver, 1981). population identified as Rhizosolenia long&eta Za- We should also point out that further nomen- charias producing resting spores was found in clatural changes affecting this taxon may be re- plankton collected on 02 July 1992 from East Pike quired. Acanthoceras Honigmann 1909 appears to Lake, Cook Co., Minnesota, USA. East Pike be a later homonym of Aeanthoceras K~itzing Lake is a shallow (14 m), circumneutral, softwa- 1842, a name applied to a red alga (Rhodophyta). ter, moderately productive lake (Minnesota De- A new generic name may eventually be required partment of Natural Resources, 1990). (P. Comp6re, pers. comm.). In addition to whole plankton burn mounts, Round & Crawford (Round etal., 1990) formalin-preserved samples were prepared for erected the genus Urosolenia to include certain microscopy by oxidizing the organic content in freshwater taxa formerly assigned to the primarily fuming HNO3 for one hour or overnight in cold marine genus Rhizosolenia Brightwell. Round & 30 % H202. Oxidation byproducts were removed Crawford based their new genus on Rhizosolenia with six successive rinses with distilled water, al- eriensis H. L. Smith. Urosolenia eriensis (H. L. lowing six hours between rinses for settling. Smith) Round & Crawford is widely reported Cleaned diatoms were dried onto coverslips. from an extreme diversity of habitats in Europe Samples for light microscopy were mounted in and North America. Another commonly recorded Hyrax TM and the slides deposited in the Center freshwater taxon, Rhizosolenia longiseta Zachar- for Great Lakes and Aquatic Sciences Diatom ias, was not formally transferred to Urosolenia by Herbarium (Dr E. F. Stoermer, University of Round et aI. (1990) or, so far as we are aware, by Michigan). For scanning electron microscopy, subsequent authors. Being of the opinion that the pieces of coverslips with cleaned diatoms were taxon commonly reported in the literature as attached to aluminum stubs, coated with 20 nm Rhizosolenia longiseta has the morphological char- 57 Plate L Scanning electron micrographs of diatom resting spores. Scale bars = 2.0 #m, 1 ° = primary spore valve, 2 ° = secondary spore valve. Fig. 1. Association ofAcanthoceras zachariasii spore with mother frustule. Fig. 2. Primary valve face ofA. zachariasfi spore. Fig. 3. Primary valve face of Urosolenia eriensis spore with ring of short spines. Fig. 4. Secondary valve face of U. eriensis spore with small spines sometimes present (arrow). Fig. 5. Primary valve face of U. longiseta spore devoid of spines. Fig. 6. Secondary valve face of U. longiseta spore. 58 35 0 0 )oo' ........ 30 29 33 31 32 Plate H. Light micrographs and line drawings of diatom resting spores. Scale bars = 10 #m. All light micrographs at same mag- nification (scale bar in Fig. 7). a= apical axis, p= pervalvar axis, 1 °= primary spore valve, 2 °= secondary spore valve. Figs 7-15. Acanthoceras zachariasii resting spores. Fig. 7. Girdle view of spore still associated with half of mother frustule. Figs. 8-10, 12-15. Girdle views of Acanthoceras spores showing size and morphological range. Fig. 11. Valve view. Figs. 16-25. 59 acteristics of Urosolenia and not those of Rhizo- mented and oval, resembling diatom girdle bands solenia, we propose the new combination: (Fig. 11). In our material, we were unable to dis- cern any girdle bands in the Acanthoceras spore, and there was no evidence of pores or portulae in Urosolenia longiseta (Zacharias) comb. nov. the spore wall (Figs 1, 2). Spore shape and lack of ornamentation are distinguishing characters in Rhizosolenia longiseta Zacharias, Forschungsber. the light microscope. Biol. Stat. PlOn, Bd. L S. 38, Fig. 7. I893. In making this new combination, we recognize Morphology of the Urosolenia resting spore that the current taxonomy of this group is ex- tremely uncertain, and liable to extensive further The Urosolenia specimens from East Lake change. Specifically, Rhizosolenia longiseta may Okoboji were identified as U. eriensis (Huber- prove to be a later synonym of Rhizosolenia gra- Pestalozzi, 1942). Urosolenia eriensis produced cilis H. L. Smith. Generally, the extreme range of one endogenous resting spore per mother cell physical and chemical habitat types to which (Fig. 16). A range of cell sizes were able to form some of the taxa referable to Urosolenia have been spores, and correspondingly, spores also had a reported (Beaver, 1981), leads to suspicion that small range of sizes. Spores were usually seen in either the taxonomy of this group has not been girdle view on prepared mounts and varied in size sufficiently resolved,

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