Biological Sciences
Rhizaria
Rhizaria is a diverse group of unicellular eukaryotes characterized by the presence of thread-like pseudopodia. They are primarily found in marine environments and include organisms such as foraminifera, radiolarians, and cercozoans. Rhizaria play important ecological roles as primary producers and consumers in marine food webs.
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3 Key excerpts on "Rhizaria"
eBook - ePubMarine Microbiology
Ecology & Applications
- Colin Munn, Colin B. Munn(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
Figure 6.2 ). Hundreds of species have been described, including specifically marine lineages. Planktonic amoebae are sparsely distributed in the open-ocean water column, so their contribution to energy flow via preying on bacteria, microalgae, and other protists within pelagic food webs has largely been overlooked. However, they can reach high densities in more turbid estuarine and coastal waters and are particularly associated with surfaces such as flocs and marine snow particles, where the density can exceed that of the ciliated protists most commonly associated with bacterivory. Their plastic cell shape may facilitate removal of firmly attached bacteria by enabling them to penetrate crevices inaccessible to other grazers. Some bacteria have evolved the ability to survive the killing mechanisms in the phagocytic vacuole and can multiply within host amoebae, forming stable symbiotic or parasitic associations with their host.Radiolarians and foraminifera have highly diverse morphologies with mineral shells
These two groups are classified as members of the supergroup Rhizaria (Figure 6.2 ). They are characterized by an amoeboid body form, using pseudopodia for locomotion and feeding. The cells are usually less than 1 mm in size, but some species are the largest protists known, with diameters up to several cm.Radiolarians have existed since the beginning of the Paleozoic era about 600 MYA and produce a great diversity of beautiful, intricate shapes (Figures 6.1 , 6.10A ) used in identification of more than 4000 species. Cells are typically 0.1–0.2 mm. Two main groups known as the Polycystina and Acantharea are recognized. They are characterized by stiff, needle-like pseudopodia arranged in radial symmetry (from which they derive the name radiolarian) and internal skeletons made of silica. Larger species are often associated with surfaces and may contain algal symbionts that provide some nutrients to the cell, while the smaller types occur throughout the water column and in deep-sea sediments. Densities vary greatly, ranging from 104 cells cm-3 in some parts of the subtropical Pacific Ocean to less than 10 cells cm-3
eBook - ePubThorp and Covich's Freshwater Invertebrates
Ecology and General Biology
- James H. Thorp, D. Christopher Rogers(Authors)
- 2014(Publication Date)
- Academic Press(Publisher)
Anderson, 1983 ).Acantharians can reach relatively high abundances (up to 25–35/l), and they are often the most abundant of the planktonic shelled amebae. Nonetheless, relatively little is known about live individuals because as soon as they are collected the fragile cell membrane breaks, and the strontium sulfate skeleton dissolves. Radiolarians are important stratigraphic tools, and skeletal morphology is the main characteristic used to identify species; hence, much information is available on the diversity of radiolarian skeletons.The heliozoans are predominantly a freshwater group. They resemble functionally the radiolarians, especially with respect to their “diffusion” feeding. They span a wide size range. Some (e.g., Actinosphaerium ) can be >1 mm, but most (e.g., Actinophrys ) are 40–100 μm. Depending on their size, different species feed with little apparent selectivity on algae, flagellates, ciliates, and rotifers. Small heliozoan species account for most of the biomass of ameboid protozoa in the freshwater plankton. Most, however, are attached to or loosely associated with sediment and other submerged surfaces (Page and Siemensma, 1991 ).Slime molds may be regarded as protozoa because they spend most of their active lives as amebae or as naked ameboid (and often macroscopic) plasmodia (Figure 7.2 ). They have been known by many names, including mycetozoa (“fungus animals”). This results from the observation that they never develop hyphae, but they produce fruiting bodies (sporangia) supported on cellulose-rich stalks. There are two large groups: the cellular slime molds (dictyostelids), such as Dictyostelium , and the acellular slime molds (myxomycetes), such as Physarum . Two smaller groups are the acrasids (e.g., Acrasis ) and the protostelids (e.g., Cavostelium
eBook - ePubFreshwater Ecology
Concepts and Environmental Applications
- Walter K. Dodds, Walter Dodds(Authors)
- 2002(Publication Date)
- Academic Press(Publisher)
Chara can be abundant in the benthos of some oligotrophic lakes but may also be an important component of more productive wetlands. Many species of charophyte are sensitive to nutrient enrichment and distribution of the stoneworts has been used to indicate nutrient pollution.Additional Algal Groups
Additional groups are found in freshwaters and include the Crypto-phyceae, the Tribophyceae, and the Phaeophyceae. Members of these groups can occasionally be important in freshwaters. However, detailed description is left to phycology courses and the comprehensive phycological texts (South and Whittick, 1987 ; Graham and Wilcox, 2000 ).Protozoa
Protozoa are found in all aquatic habitats. Some of the species classified as protozoa are also considered algae. Many types are heterotrophic and survive by ingesting particles or absorbing dissolved organic carbon. They are very important predators of bacteria in aquatic environments. There are also important parasites in this group. Some of the smallest protozoa are only slightly larger than bacteria and can ingest virus-sized particles. The largest are visible to the unaided eye. This is a very diverse group (Figs. 8.5D , 8.5E , 8.8A–8.8C , 8.9B , 8.9G , and 8.10 ), and includes the most complex single-celled organisms known.FIGURE 8.10 Selected protozoa: (A) Khawkinea, a zooflagellate; (B) Amoeba; (C) Vorti-cella, a colonial ciliate; and (D) Stentor, a solitary ciliate(reproduced with permission from Thorp and Covich, 1991b ).Biography 8.1 RUTH PATRICKDr. Ruth Patrick (Fig. 8.7
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