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Chapter 37 : Protistan Community Structure
Protistan assemblages of aquatic ecosystems are the focus of extensive research in aquatic ecology. One stimulus for this work has been the long-standing recognition that phototrophic protists (the unicellular algae) constitute a major fraction of the primary productivity within aquatic ecosystems. We have learned a great deal about the taxonomic composition and trophic structure of aquatic protistan communities through the application of traditional approaches of morphological analysis and culture. Nevertheless, the tremendous diversity of protistan assemblages and the varied methods required for identifying protistan species and their abundances, biomass, and trophic activity continue to hamper in-depth understanding of the structure and function of these communities. The success of using molecular (genetic/immunological) signatures for assessing the community structure of natural protistan assemblages will ultimately depend on linking these signatures to classical (morphological) species descriptions and to the physiological abilities of protistan phylotypes. Ultimately, molecular approaches, in combination with classical methods, will provide new tools for studying the emergent physiological, ecological, and biogeochemical processes that are created and/or affected by protistan community structure. Probably the most distinct difference between freshwater and marine protistan communities is the restriction of the larger sarcodines (acantharia, radiolaria, and foraminifera) to brackish and marine ecosystems. Modern molecular biological approaches have revealed unexpected, and as yet largely uncharacterized, protistan diversity in a wide variety of ecosystems.
Approximate sizes (longest dimension) of the planktonic protistan species listed in Table 1 . Commonly employed size class designations are shown on the right. Note that the sizes of these protists span >3 orders of magnitude. The arrow running under Globigerinoides sacculifer indicates that the group of five species enclosed by the arrow can be larger than 1,000 µm.
Abundance (A) and biovolume (B) relationships of the protistan assemblage listed in Table 1 . The species have been grouped according to size class and trophic mode (phototrophic, mixotrophic, or heterotrophic).
Box model approximating the major trophic interactions within the protistan assemblage listed in Table 1 . The species have been grouped according to known or presumed size-dependent and trophism-dependent relationships. Arrows indicate the directions of energy flow in predator-prey interactions.
Conceptual framework for using culture-independent and -dependent methods to study protistan community structure. Arrows indicate the order in which this goal can be accomplished (see the text for details).
T-RFLP pattern for an artificial protistan community. The 18S rRNA genes of six known species were PCR amplified and digested with a single restriction enzyme. The unique T-RFLP fragment size for each of the species could be recognized in the mixed sample (as indicated).
Relative contributions (percentages of total number of clones) of various protistan taxa to a theoretical clone library (n = 27) created for the protistan assemblage presented in Table 1 .
Species list showing the range of protistan diversity of an oligotrophic oceanic environment a
Planktonic protistan species list from Table 1 , arranged according to trophic category a