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ScienceWeek
2004 18 June B2 EVOLUTIONARY BIOLOGY: ON THE RISE OF RHIZOSOLENID DIATOMS
The following points are made by J.S. Sinninghe Damste et al (Science 2004 304:584):
1) In modern oceans, diatoms are the most important group of phytoplankton, responsible for almost half of the marine primary productivity (1,2), but their ecological dominance occurred relatively recently in geological time (3). The fossil record of marine diatom tests reveals that diatoms only evolved in the Jurassic and became more common in the mid-Cretaceous (4). This indicates that in the past 100 million years (Ma) or so there has been a substantial shift in the relative importance of the different phytoplankton groups, with enormous consequences for the biogeochemical cycling of elements (e.g., carbon, nitrogen, phosphorus, and silicate) in the ocean (3).
2) Unfortunately, the fossil record of diatoms is punctuated and not very reliable, because the silica tests of diatoms are prone to dissolution during early diagenesis. Therefore, our present knowledge of the advent of the diatoms and of the consequences for biogeochemical cycling is rather limited. Conservative 18S rDNA gene sequences of extant diatom species can be used as an alternative tool to reconstruct the rise of the diatoms (5). However, for proper interpretation, the molecular clock rate needs to be calibrated against the often-incomplete fossil diatom record.
3) An alternative approach to reconstructing the evolution of the diatoms is to use molecular fossils: characteristic organic components that may survive for hundred of millions of years in sediments and even in petroleum. Probably the most specific group of diatom markers are the highly branched isoprenoid (HBI) alkenes. These components differ in terms of biosynthesis from most common acyclic and cyclic isoprenoid natural products because their skeletons are characterized by a distinctive "T branch". In the C25 HBI alkenes, the T branch is formed by attachment of a C10 isoprenoid unit to a C15 isoprenoid unit at C-7, most likely requiring an unusual set of enzymes. In the C30 HBI alkenes, a C15 instead of a C10 isoprenoid unit is used. C25 and C30 HBI alkenes have been identified in the diatom genera Rhizosolenia, Haslea, Navicula, and Pleurosigma, but not in any other organisms.
4) In summary: The 18S ribosomal DNA molecular phylogeny and lipid composition of over 120 marine diatoms demonstrates that the capability to biosynthesize highly branched isoprenoid (HBI) alkenes is restricted to two specific phylogenetic clusters, which independently evolved in centric and pennate diatoms. The molecular record of C25 HBI chemical fossils in a large suite of well-dated marine sediments and petroleum revealed that the older cluster, composed of rhizosolenid diatoms, evolved 91.5 +- 1.5 million years ago (Upper Turonian), enabling an accurate dating of the pace of diatom evolution that is unprecedented. The rapid rise of the rhizosolenid diatoms probably resulted from a major reorganization of the nutrient budget in the mid-Cretaceous oceans, triggered by plate tectonics.
References (abridged):
1. P. Treguer et al., Science 268, 375 (1995)
2. C. B. Field, M. J. Behrenfeld, J. T. Randerson, P. Falkowski, Science 281, 237 (1998)
3. P. G. Falkowski, R. T. Barber, V. Smetacek, Science 281, 200 (1998)
4. F. E. Round, R. M. Crawford, D. G. Mann, The Diatoms: Biology and Morphology of the Genera (Cambridge Univ. Press, Cambridge, 1990)
5. W. H. C. F. Kooistra, L. K. Medlin, Mol. Phylogenet. Evol. 6, 391 (1996)
Science http://www.sciencemag.org
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Related Material:
MARINE BIOLOGY: A BIOLOGICAL FUNCTION FOR CADMIUM
Marine microorganisms are largely responsible for the cycling and distribution of many nutrients in the sea, and it is the availability of these nutrients that in turn controls oceanic creation of organic matter (biomass) via photosynthesis or chemosynthesis (the creation process known as "primary production"). In addition to being true for major nutrients, this relationship is also true for essential trace elements. In oceanic systems, the vertical distribution (water column distribution) of many biologically important trace metals is thus similar to that of the major nutrients phosphate, nitrate, and silicate. At the ocean surface, where photosynthetic activity depletes algal nutrients, essential trace metals are present at extremely low concentrations, presumably as a result of biological uptake. Like those of major nutrients, these metal concentrations increase at depth because of decomposition of organic matter and remineralization.
The oceanic distribution of cadmium closely follows that of major nutrients such as phosphate, but the reasons for this "nutrient-like" distribution are unclear, since cadmium does not have an apparent biological function and is generally toxic to biological cells.
Diatoms, also called bacillariophytes, are microscopic unicellular eukaryotic algae differentiated into approximately 10,000 different species. (The term "eukaryotic" is applied to biological cells that have internal membrane-bound organelles such as a nucleus.)
Carbonic anhydrase (carbonate dehydratase) is an enzyme that catalyzes the reversible hydration of carbon dioxide to carbonic acid (or to bicarbonate ion at certain pH values). The enzyme is found in a wide range of living systems in various forms (isozymes). It is an intracellular enzyme with zinc as a cofactor.
The following points are made by T.W. Lane and F.M.M. Morel (Proc. Natl. Acad. Sci. 2000 97:4627):
1) The authors report evidence of a biological role for cadmium in the marine diatom Thalassiosira weissflogii under conditions of low zinc, a typical situation in the marine surface environment. The authors make the following points:
2) Addition of cadmium to zinc-limited cultures enhances the growth rate of T. weissflogii, particularly at low concentrations of carbon dioxide. This increase in growth rate is reflected in increased levels of cellular carbonic anhydrase activity, although the levels of TWCA1, the major intracellular zinc-requiring isoform of carbonic anhydrase in T. weissflogii remains low. Isotope cadmium label [(sup109)Cd] comigrates with a protein band that shows carbonic anhydrase activity and is distinct from native TWCA1. The levels of the cadmium protein are modulated by carbon dioxide in a manner consistent with a role for this enzyme in carbon acquisition. Purification of the carbonic anhydrase-active fraction leads to the isolation of a cadmium-containing protein of 43 kilodaltons. The authors suggest it is now clear that T. weissflogii expresses a cadmium-specific carbonic anhydrase, which, particularly under conditions of zinc limitation, can replace the zinc enzyme TWCA1 in its carbon-concentration mechanism.
3) The authors conclude: "It is very likely that substitution of metals in active metalloproteins and replacement of metalloenzymes by others containing other metals or none -- as observed for carbonic anhydrase -- are the mechanisms by which oceanic microorganisms have adapted to an environment of extremely low metal concentrations. It is thus not surprising that we should find common enzymes such as carbonic anhydrase with uncommon metal requirements. What we are observing is the unique biogeochemistry of the oceanic environment reflected in the unique biochemistry of its flora."
Proc. Nat. Acad. Sci. http://www.pnas.org
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EARTH SCIENCES: ON MUDROCK QUARTZ AND MARINE PLANKTON
In geology, the term "silt" refers to a sediment with particles in the size range 4 to 62.5 microns, and the term "mudrock" (mudstone) refers to a silt that has been transformed into rock ("lithified rock")
Mudrocks are an important archive of the history of Earth, containing the *detritus of the physical and biogeochemical cycles that shape and regulate global systems. A major constituent of mudrocks is quartz [silica = SiO(sub2)], one of the most abundant rock-forming minerals and the most resistant to weathering. The consensus view has been that the quartz found in ancient sediments came from the erosion and fragmentation of quartz crystals from pre-existing rocks. That view in now being challenged.
The following points are made by J. Scheiber et al (Nature 31 Aug 00 406:981):
1) The authors report an analysis of quartz silt from *black shales in the eastern US, the material dating back to the late Devonian period (approximately 370 million years ago). The authors make the following points:
2) Using *backscattered electron imaging and *cathodoluminescence imaging, the authors report their results indicate that up to 100 percent of the quartz silt in their samples does not originate from the continental crust. Instead, it appears to have precipitated early in *diagenesis in algal cysts and other pore spaces, with silica derived from the dissolution of *opaline skeletons of *planktonic organisms such as *radiolaria and *diatoms.
3) The authors suggest that transformation of early diatoms into _in situ_ quartz silt might explain the time gap between the earliest fossil occurrence of diatoms approximately 120 million years ago and molecular evidence for a much earlier appearance between 266 and 500 million years ago. Furthermore, the authors suggest that if many other mudstone *successions show similarly high proportions of _in situ_ precipitated -- rather than detrital -- quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for estimates of paleoproductivity as well as for perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
detritus: In this context, the term "detritus" refers to material containing loose finely divided rock and/or finely divided remains of animal and/or plant tissues.
black shales: Black/dark grey mudrocks rich in organic carbon and generally formed in bottom waters poor in oxygen.
backscattered electron imaging: In general, in this context, "backscattering" is the deflection of radiation by scattering processes through angles greater than 90 degrees with respect to the original angle of travel.
cathodoluminescence imaging: The term "cathodoluminescence" refers to luminescence produced when high-velocity electrons bombard a material in vacuum, vaporizing small amounts of material in an excited state, the vapor emitting radiation characteristic of the material.
diagenesis: In general, the term "diagenesis" refers to all the changes that occur in a sediment at low temperature and pressure after deposition. With increasing temperature and pressure, diagenesis grades into "*metamorphism".
metamorphism: In general, the process of changing the characteristics of a rock in response to changes in temperature, pressure, or volatile content. Most metamorphic changes do not include bulk chemical changes, but merely the crystallization of new mineral phases. Examples of the transformation of sediments through diagenesis and metamorphism are sand to sandstone and peat to coal.
opaline skeletons: In general, "opal" [SiO(sub2).nH(sub2)O] is a hydrated variety of silica consisting of spheres approximately 300 nanometers in diameter. It is normally deposited at low temperatures from silica-bearing waters.
planktonic organisms: The term "plankton" is a general designation for various drifting microscopic aquatic organisms in the upper regions of the oceans.
radiolaria: A group of symmetrically shaped marine protozoans with radiating thread-like pseudopodia and a starburst-shaped silicaceous external wall ("test"). The tests are practically indestructible, and upon death of the animal, the test (on average, 50 to 100 microns in diameter) settles to the ocean bottom.
diatoms: Also called bacillariophytes, diatoms are microscopic unicellular eukaryotic algae differentiated into approximately 10,000 different species. (The term "eukaryotic" is applied to biological cells that have internal membrane-bound organelles such as a nucleus.) The outstanding feature of diatoms is a two-part transparent silicaceous cell wall (called a "test"), which may be circular or elongated.
successions: In this context, groups of rock units or strata that succeed one another in chronological order.
ScienceWeek http://scienceweek.com
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