Organic carbon biological cycle

In this cycle the reaction of H2 + C02 to give reduced carbon-based fuels such as sugar or oil with oxidation back to H20 and C02 could be useful. It is then precisely the same as the biological cycle. Note that at present mankind and organisms utilise... 

The abundance and ratios of important elements in biological cycles (e.g., C, H, N, O, S, and P) provide the basic foundation of information on organic matter cycling. For example, concentrations of total organic carbon (TOC) provide the most important indicator of organic matter since approximately 50% of most organic matter consists of C. As discussed in chapter 8, TOC in estuaries is derived from a broad spectrum of sources with very different structural properties and decay rates. Consequently, while TOC provides essential information on spatial and temporal dynamics of organic matter it lacks any specificity to source or age of the material. 

The dynamics of marine organic carbon can be described as a set of three nested cycles in which the biological pump is the cycle with flux and reservoir of intermediate magnitude (Figure 3). The cycle with the shortest timescale, which operates within the surface ocean, is composed of net primary production by phytoplankton... 

Figure 2 A diagram of the biological carbon cycle. The conversion from inorganic to organic carbon requires fight or chemical energy and an electron donor (e.g., H2O, H2S, Fe(ll)), and is the process of autotrophy. The reverse reaction, in which organic carbon is oxidized to CO2, releases energy while reducing an electron acceptor (e.g., O2, S04 /S°, Fe(lll)). This part of the cycle is referred to as heterotrophy.

The reduction is typically limited by the availability of organic carbon and often occurs in shallow waters at continental margins. Thus, global sulfide production would be dependent on the availability of biological productive areas over geological time. Sulfur-isotope data can be used to constrain simple models of the sulfur cycle over geological time and establish the size of the reservoirs as shown in Figure 5(b). 

The interaction between the carbonate cycle and the organic carbon cycle takes place under a variety of circumstances. At one end of the spectrum, carbonate chemistry may be under direct enzymic control (see Chapter 2.2). It may take place within cells, within organisms, or within micro-environments in close contact with living tissues (e.g., molluscan mantle). At the other extreme, where products of metabolic activities modify the overall chemistry of the environment, carbonate dissolution or precipitation may be influenced indirectly. The closer the contact between the organism and the substrate, the more specific are the biogenic dissolution and crystallization patterns that remain as traces of biological activity in sediments. 

Once inside the cell, HCO3 is converted to CO2 by the enzyme, carbonic anhydrase. CO2 is then fixed by carboxydismutase and OH is excreted to maintain ionic balance. Carbonic anhydrase is also associated with the extracellular carbonate dissolution by boring organisms (Schneider, 1976) and with the C02-transfer system for intracellular calcification. It represents a key enzyme in the biological cycling of carbonate (Degens, 1976 Raven, 1974). 

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