Text by: Kjell R. Bj°rklund and Giuseppe Cortese (2002)
Polycystine radiolarians, as well as some phaeodarian species, are found in different areas at different times throughout the Quaternary.
The Quaternary Period encompasses the time from 1.8 million yrs. B.P. till today, and is subdivided into the Pleistocene Epoch (1.8 million yrs. B.P. to 10.000 yrs. B.P.) and the Holocene Epoch (10.000 yrs. B.P. till present).
Biogeographical studies of recent radiolarians are based on both living (plankton) and subfossil (surface sediment) material. The former category of studies is usually carried out by radiolarists with a biological background, while the latter is worked on by micropaleontologists. The taxonomic identification of polycystine radiolarian species follows the classical Haeckelian taxonomy, based on similarities and differences in skeletal elements and structures as species level identification criteria.
However, M. Cachon, J. Cachon and A. Hollande in the 1960's and 70's used biological criteria, based on soft tissue structures, to build their taxonomic system. Similarities in skeletal structures between distantly related species and changes in these structures over the lifetime of a single individual, led to the conclusion that a new taxonomic system was needed (Hollande and Cachon-Enjumet, 1960; Cachon and Cachon, 1968, 1982, 1985a, b; and Petrushevskaya et al., 1976). This system was based on the cytology and the fine structure of the axiopod/axioplast microtubular system.
The taxonomy based on these cytological criteria has only been worked out on a few of the many hundreds of living polycystine radiolarian species. This is, however, a taxonomical system that has a future potential, especially if coupled with genetic molecular biology, as this will probably be a tool for a better understanding of evolutionary lineages within radiolarians.
Molecular genetics and gene sequencing are still in their infancy, and some of their tenets must still be proven. Some of the problems in the application of these techniques are:
- These studies are currently based on very sharply defined populations of single species, and do not address (or are not even able to address) the problem of how much the gene sequences of individuals belonging to widely separated populations of the same species differ between each other. Oceanographic expeditions sampling widely separated populations of the same species from the same ocean are now starting to be launched to test this variability (Fock, pers.comm., 2000). Implication: Intra-specific variation can be huge in geographically separated populations of the same species, sometimes even more than between a "sample population" and another species.
- The amount of gene sequence variability is species dependent: some species display a big variability, others are more conservative in their gene sequence.
Implication: It is still difficult to assess a "cut-off value" for the difference in the gene sequence to be able to separate between species A and species B, even between closely related species.
- Relatively huge differences in the gene sequences can sometimes show off as structural differences in the skeleton, but other times they do not. It usually depends on whether the skeletal structure has an evolutionary meaningful function or not (i.e. if it has a lot of selection pressure applied to it or not).
Implication: Genotypic variability is not necessarily coupled with Phenotypic variability (i.e. the amount of variability in the genes is not necessarily reflected in the same amount of variability in the external appearance of a species).
- Fossil material can hardly be studied by these techniques, with the exceptions of very special fossilization processes (amber, tar pits, etc., etc.) that can preserve organic material from some taxa groups.
Conclusion: Gene sequencing is a very interesting field of molecular biology which could lead to major contributions towards solving taxonomic problems. However, the results obtained so far (limited to its taxonomic applications) are inconclusive, and more work is needed to test the genetic variability of natural populations.
The distribution of polycystine radiolarians in the world ocean is affected by a special set of ecological conditions, which characterize the water masses they live in. Due to their planktonic nature, radiolarians are easily transported to, and distributed over areas where the dominant ecological conditions allow the species to live and reproduce. The present day distribution of radiolarians in the surface sediments of the world ocean therefore depicts the general patterns of the major water masses, current systems and production zones. This is clearly demonstrated in Lisitzin (1971a, b, 1985) where he illustrates the relationship between the annual production of silica in g/m3/year and the amount of amorphous silica, basically made up of diatom frustule and radiolarian tests, in surface sediments expressed in weight percent of dry sediment. As a consequence of this, the study of fossil assemblages permits to trace the position of water masses and current systems through time.
Only a few radiolarians are cosmopolitic in their distribution, the majority being provincial in nature, more regional than local. Species diversity is highest at the equator, with a decreasing number of species towards the poles (see Boltovskoy 1998, Fig. 8 ). Provincialism was more strongly developed after Central America emerged and closed the connection between the Atlantic and Pacific oceans, and especially when the northern hemisphere glaciation started, about 2.5 million yrs B.P. At this time even stronger meridional temperature gradients got established, causing stronger hydrographic boundaries than in pre-glacial times, and local and regional polycystine radiolarian assemblages developed.
The distribution and concentration of radiolarian skeletons in the sediments depend mainly on these factors:
1) Production of radiolarians in the water column;
2) Energy level at the site of sedimentation;
3) Masking of radiolarian skeletons by other sedimentary components;
4) Dissolution of biogenic opal.
Only a few percent of the opal produced in the surface waters reach and are incorporated in the bottom sediments. Apart from the dissolution taking place during the settling of the skeletons through the water column, further dissolution takes place at the water-sediment interface and in the upper few cm of the sediment. The rate of dissolution at the bottom of the ocean is very dependant on the locality. Normally, pelagic sediments have a good opal preservation, while in low sedimentation areas the preservation is bad, or opal does not preserve at all, e.g. in the red clay province of the Pacific Ocean.
In highly energetic areas, as on the continental shelves, the preservation of opal is also bad (on the shelf off Namibia silica is deposited and displays good preservation). This may be a result of low productivity on the shelves, lateral transportation by currents, mechanical breakage and chemical dissolution of the opal tests.
Johnson (1976) concluded that the preservation of siliceous microfossils is most strongly influenced by the relative sedimentation rates of biogenic material and certain detrital silicate minerals, especially minerals depleted in silica by intense chemical weathering in tropical regions. If the ratio between the sedimentation rate of biogenic opal and detrital silica is low, a large sink for dissolved interstitial silica is created by silicate reconstitution reactions. Biogenic opal present in these sediments dissolves to replace the dissolved silica taken up by these reactions and, if the silica sink is sufficiently large, all of the opal ultimately disappears.
On the contrary, Aluminum uptake from the water column by living diatoms and its replacement of Silicium in opal tests have been demonstrated to have a positive effect on the preservation of diatom frustules (van Bennekom, pers.comm. 2000), and could play a similar role in radiolarians.
The distribution patterns of polycystine radiolarians in the surface sediments of the world ocean may therefore be severely altered by intensive and even complete dissolution of the biogenic opal microfossils.
The Norwegian Sea is a good example of this:
While its southern Norway Basin is characterized by high amounts of opal microfossils, its northern Lofoten Basin contains strongly diluted, mechanically broken and chemically dissolved opal microfossils in its bottom sediments, as a result of heavy turbidite activity in the area. The same is the situation in the Norway Basin, along the path of the Storegga slumps (Bugge, 1981), where radiolarian skeletons are few and badly preserved.
The biomass of polycystine radiolarians is usually very little compared
to the total plankton biomass. Polycystine radiolarians have therefore to be separated from the remaining plankton material by using a strong oxydizing agent or a low temperature asher where the organic material is being burned off. The residue is then mounted by Canada Balsam on regular microscopic slides, or on stubs for SEM studies. Plankton material, living or oxidized, may also be studied by means of an inverted microscope where the material has settled on the bottom of small cells.
Quaternary sediments are normally unconsolidated and it is fairly easy to separate the radiolarian skeletons from the sediment. A normal procedure will be:
1) Boil a sample in water until it disintegrates;
2) Sieve sample over a 45 Ám screen to get rid of most of the fine fraction and organic matter;
3) Boil the residue one more time, add hydrogen-peroxide to the water (step 2 will prevent the samples from boiling over);
4) Sieve sample over a 45 Ám screen;
5) Add diluted hydrochloric acid to dissolve carbonate;
6) Mount residue on microscopic slides, let dry, add a drop of xylene, embed the skeletons in Canada Balsam, and finally add a cover slip. If unwanted bubbles are trapped under the cover-slip, boil gently on a hotplate, and put slide on a cold tile or steel surface. For SEM studies the residue is glued on stubs and covered with gold-palladium.
The study of the recent siliceous microfossil distribution is of particular interest for paleotemperature estimates, providing information on the operation mode and temperature distribution of past oceans and therefore setting the boundary conditions for General Circulation Models (GCMs) and Ocean Atmosphere Coupled Models (OACMs), widely used tools nowadays for climate prediction and analysis.
Industrial applications of siliceous microfossils is only associated with terrestrial outcrops of siliceous oozes. In most cases these are classified as diatomites, but with a considerable content of other siliceous microfossils such as silicoflagellates, radiolarians and sponge spiculae. When the siliceous oozes are of good quality, they have been applied in industry as addition to toothpaste as the polishing medium, in dynamite production, as insulators in electrical appliances, and put on the market as "cat sand" to mention a few areas. Recently, protists at large have started to be grown in cultures in order to extract unsaturated fatty acids (Omega-3) from their cytoplasm, as an integrator for human diets (as they have similar nutritional properties to cod-liver oil).
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