The marine coccolithophorid phytoplankton species Hymenomonas carterae (class Prymnesiophyceae) produces both dimethylpropiothetin (DMPT) and dimethylsulfide (DMS) in axenic cultures. The rate of DMS production is closely regulated by the cell; it remains independent of environmental sulfate concentration down to levels of 2.5% of the seawater value. Below this sulfate level, DMS production decreases with decreasing sulfate concentration, but significant amounts of DMS are released even under conditions of sulfate-limited growth. Hymenomonas carterae can grow on sulfite, thiosulfate, and cysteine as sulfur sources, but not on methionine. The rate of DMS output is similar for the different sulfur sources.Both the intracellular concentration of DMPT and the rate of output of DMS by H. carterae increase with increasing salinity of the medium. This increase is observed when either salt or sucrose is used to control the osmolarity of the growth medium. Variations in DMPT levels and DMS output were observed within hours after transferring cells to a medium of different osmotic pressure. The intracellular DMPT concentration is of the order of 0.3 mole per liter and contributes significantly to the osmotic pressure in the cell. These results suggest that DMPT plays an important role in osmoregulation by H. carterae.
Organic nitrogen incorporated into geomacromolecules (e.g., humic substances, kerogen) represents a major reservoir of nitrogen on the earth's surface, accounting for more than 90% of the total nitrogen in soils, sediments, and aquatic environments. Its primary source is biochemical nitrogen from dead plant and animal residues (predominantly proteinaceous substances), which undergo a complex series of transformations, mediated by microbes and abiotic processes, ultimately resulting in the incorporation of the nonmineralized fraction into geomacromolecules. Simultaneously,the biochemical N is thought to be extensively altered structurally, forming more stable structures (such as heterocyclic forms), although the type of changes in chemical speciation, their timing, and mechanisms are not clear. It is important to have this knowledge because the type of N formed influences not only its reactivity and fate (e.g., the release of bioavailable N in soils) but also the physical and chemical characteristics of the associated macromolecular organic matter. We used nitrogen K-edge XANES spectroscopy (a selective, sensitive, and nondestructive method) to gain new insights into the speciation of this macromolecular nitrogen. Our results verified amide N as being the dominant type in humic substances and sediments but revealed that pyridinic N also is a significant component of the total N (approximately 20-30%), with a subfraction consisting of its oxidized derivatives. An unidentified form of highly oxidized N was present mainly in sediments. While amide N represents residues of original biochemical molecules, pyridinic N probably is generated abiotically. Our results imply that the abiotic formation of pyridinic N sets in during the early stages of organic matter transformations thereby stabilizing organic N, although such processes generating heterocyclic structures may continue much longer.
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