Dinoflagellates
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DINOFLAGELLATES Dinoflagellates are a highly diverse group of flagellated protists consisting of both photosynthetic and nonphotosynthetic (heterotrophic) taxa in equal proportions (Taylor et al., 2008). Dinoflagellates are an important component of the modern plankton and have been common to abundant in marine and freshwater environments since the Mesozoic. More than 2,000 extant and 2,500 fossil species have been described. Being mostly biflagellated, swimming unicells of typically 10–100 µm in diameter, they can position themselves in the water column and take full advantage of the available light and nutrients. Many genera are indicative of (paleo-) environmental conditions such as water salinity, nutrient levels, and oceanic temperature zones. In modern and ancient sediments, dinoflagellate-derived organic matter contributions can be specified by distinctive steroid biomarkers (dinosterane and related compounds, see entry “Biomarkers (Molecular Fossils)”). Some dinoflagellates, called zooxanthellae, are capable of forming symbioses with a phylogenetically wide range of marine protists and invertebrate animals. For further information, see entries “Algae (Eukaryotic)” and “Protozoa (Heterotroph, Eukaryotic).”
Bibliography Taylor, F. J. R., Hoppenrath, M., and Saldarriaga, J. F., 2008. Dinoflagellate diversity and distribution. Biodiversity and Conservation, 17, 407–418.
DIVALENT EARTH ALKALINE CATIONS IN SEAWATER Anton Eisenhauer IFM-GEOMAR, Christian Albrechts Universität zu Kiel, Kiel, Germany
Synonyms Divalent cations in seawater Definition The most abundant divalent earth alkaline cations in seawater are Mg2þ, Ca2þ, and Sr2þ are simply termed the “divalent cations.” These ions and their dynamic change play a major role for the modern oceans. On long geological time scales dynamic changes of divalent cation concentrations in the oceans influenced the evolution of live and the climate evolution in the past. Divalent cations in modern ocean water Magnesium Magnesium is a chemical element with the symbol Mg, the atomic number 12, and an atomic mass of 24.31.
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Magnesium is the ninth most abundant element in the universe by mass. It constitutes about 2% of the Earth’s crust by mass, and it is the third most abundant element dissolved in seawater and it is the most abundant divalent cation followed by Ca and Sr in seawater (Brown et al., 1992). Mg ions are essential to all living cells, and is the 11th most abundant element by mass in the human body. In seawater (S = 35 psu), Mg is present at approximately 53 mM (mM ¼ millimolar) (Brown et al., 1992). Only the monovalent sodium (Naþ, 469 mM) is present at higher concentration. Magnesium is about five times more abundant than the third most abundant cation Ca (10 mM). Magnesium concentration is significantly lighter than calcium, so when compared on a weight basis, it is only about 3 times as concentrated (1285 vs. 420 ppm). Magnesium is present in seawater as a free divalent cation with water and other molecules attached to it (Brown et al., 1992; Kastner,
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