Cycles, biological carbon

Anabolism and catabolism are complementary to one another, and may be compared with the biologically mediated cycling of carbon (and other materials) in the environment (Fig. 5.2). However, there is an area of metabolism where the two types of metabolism coincide and become indistinguishable. These pathways are known as intermediary metabolism and are the central part of cellular metabolism where the basic requirements for growth are met. At this metabolic crossroads, materials are either rearranged into synthetic precursors or are oxidised to generate energy. 

What physical, biological, chemical, and social processes are basic to regulation of the cycles of carbon, nitrogen, sulfur, water, and other elements both in space and in time ... 

Inputs of agricultural fertilizers are having more than one impact on the biological pump. A shift in the NO PO4 Si(OH)4 of natural waters is causing a shift in the phytoplankton community structure that should impact the biological cycling of carbon in aquatic systems (Conley et al, 1993). Additionally, a recent shift in the C N P ratios of deeper waters and an increase in export production have been observed for the northern hemisphere oceans (Pahlow and Riebesell, 2000). 

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.

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). 

Biological calcification and the biological cycle of carbonates control the availability of CO2 and its storage in sediments and rocks. These in turn have a strong influence on climatic and other properties of the natural environment. 

The biological carbon cycle in the ocean is essentially short-circuited. Despite the high turnover rate it has little effect on the atmosphere, because... 

The return of such elements into the earth s biological systems is rather complex and depends on processes such as, for example, erosion. The sedimentation cycles are less perfect than others (e.g. the cycles of carbon or nitrogen) and, therefore, they are more easily disturbed by man for example, he accelerates the losses of phosphorus to such an extent that this element may become a decisive factor in the functioning of the biosphere in the future. 

Fig. 4.15. The biological carbon cycle. Thick arrows — biological processes, thin arrows — non-biological processes (modified from ref. [68])... 

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