Phototrophic organisms

As we began this chapter, we saw that photosynthesis traditionally is equated with the process of COg fixation, that is, the net synthesis of carbohydrate from COg. Indeed, the capacity to perform net accumulation of carbohydrate from COg distinguishes the phototrophic (and autotrophic) organisms from het-erotrophs. Although animals possess enzymes capable of linking COg to organic acceptors, they cannot achieve a net accumulation of organic material by these reactions. For example, fatty acid biosynthesis is primed by covalent attachment of COg to acetyl-CoA to form malonyl-CoA (Chapter 25). Nevertheless, this fixed COg is liberated in the very next reaction, so no net COg incorporation occurs. 

It is important to underscore the fact that carbon dioxide is required not only for the growth of strictly phototrophic and lithotrophic organisms. Many heterotrophic organisms that are heterotrophic have an obligate requirement for carbon dioxide for their growth. Illustrative examples include the following ... 

T. roseopersicina is a phototrophic purple sulphur bacterium the strain marked BBS has been isolated from the cold water of the North Sea. Its anaerobic photosynthesis uses reduced sulphur compounds (sulphide, thiosulfide, or elementary sulphur), but it can also grow on organic compounds (sugar, acetate) in the dark. The bacterium contains a nitrogenase enzyme complex, thus it is capable of fixing atmospheric N2, a process accompanied by H2 production [Vignais et al., 1995],... 

For all phototrophic organisms exposed to UV radiation for substantial parts of their life cycles, strategies that passively screen UV radiation will contribute to preventing UV-induced direct and indirect damage to essential biomolecules. In addition, UV-screening may also save metabolic energy by reducing the need for constantly active avoidance and repair processes. 

Other physiological functions of MAAs in phototrophic organisms such as organic osmolytes have been suggested, because very high concentrations can be found in cyanobacteria living in hypersaline environments (Oren 1997). However, salt shock experiments with the marine cyanobacterium Microcoleus chthono-plastes did not indicate ary major involvement of MAAs in the process of osmotic acclimation (Karsten 2002), and hence their proposed function as osmolytes has to be questioned. 

Phototroph An organism that derives its energy for growth from photosynthesis. 

Although phototrophs are restricted to the surface ocean, photosynthesis is the ultimate source of almost all the organic matter that supports heterotrophic activity in the sea. As shown in Figure 8.1, this organic matter is transferred through the marine food web by the feeding activities of heterotrophs including bacteria, protozoans, and animals. 

---