The processes of return of plant nutrients to the water following the degradation of organic compounds are termed regeneration. Regeneration is 'direct' when the products set free into the water by metabolism are directly utilized by plants, which is the case with most of the excretory products of marine animals. Phosphorus is excreted mainly as phosphate with some soluble organic phosphorus (Marshall and Orr, 1961). In most marine animals, nitrogen is excreted mainly as ammonia. Teleosts excrete part of their waste nitrogen as trimethylamine oxide (see page 114). Some animals also excrete aminoacids, uric acid or urea. All these compounds are utilizable in varying degrees by plants. However, the greater part of the nutrients taken in by plants is probably regenerated by 'indirect' processes involving bacterial activity.
Bacteria are an essential part of the organic cycle, necessary for the decomposition of particulate organic matter from faecal pellets and the bodies of dead organisms. After death, the tissues of plants and animals become converted by degrees into soluble form. Dissolution may be initiated by autolysis, the tissues being broken down by the dead organisms' own enzymes, but decomposition is brought about mainly by bacterial action. Bacteria are abundant on the surface of organisms and detritus and are specially numerous in the uppermost layers of bottom deposits. Bacterial metabolism converts solid organic matter into organic solutes, and eventually into an inorganic form.
Regeneration of phosphorus is mainly to phosphate, although to some extent plants can also absorb certain dissolved organic phosphorus compounds. Following the death of marine organisms, much of the phosphorus in their tissues returns very quickly to the water as phosphate, indicating that decomposition of organic phosphorus compounds is probably largely by autolysis and hydrolysis. Particulate organic phosphorus is acted on by bacteria producing various solutes, e.g. glucosephosphate, glycerophosphate, adenosine phosphate, some of which may be utilized by plants via their phosphatase enzymes, or further degraded by bacteria to phosphate. Some links of the phosphorus cycle are illustrated in Figure 5.3.
Nitrogenous organic materials are broken down more slowly than phosphorus compounds, mainly by bacterial action. Particulate and dissolved organic nitrogen are converted by bacteria first to ammonium ions and then further oxidized to nitrite and finally to nitrate. Plants absorb nitrogen mainly as nitrate, also as nitrite, ammonium and various simple organic solutes. Certain marine plants (mainly blue-green algae) and bacteria (Azotobacter, Clostridium, Desulfo-vibrians, etc.) are capable of fixing dissolved elemental nitrogen, but this is probably not a major addition to nitrogenous compounds in the sea because of the high energy intake required for this reaction. Some bacteria in anaerobic conditions obtain energy from organic carbon compounds by oxidations involving the reduction of nitrate to elemental nitrogen i.e. nitrogen-freeing. Aspects of the very complex nitrogen cycle of the sea are shown in Figure 5.4.
Sulphur-containing compounds are regenerated mainly as sulphate. Bacterial decomposition of organic sulphur commonly yields hydrogen sulphide, which can be oxidized to elemental sulphur (by Beggiatoa) and thence to sulphate (by Thiobacillus). The nitrogen and sulphur cycles are closely interconnected by a variety of bacterial reactions involving both groups of compounds.
In the course of these various processes of regeneration and recycling, bacteria themselves grow and multiply, and constitute an important component of the food supply. Bacteria therefore perform two major functions in marine food cycles:
(a) the breakdown of dead organic matter into soluble forms, mainly inorganic ions, which can be utilized by plants, and
(b) the transformation of dead organic matter into bacterial protoplasm which is directly utilizable as food by some animals.
There are continuous losses of organic material from the euphotic zone due to sinking and to movements by animals down to deeper levels after feeding. Some of this material may reach the bottom and become lost from the cycle by being permanently incorporated in the sediment. The greater part is regenerated in deep layers of water or on the bottom, and nutrients therefore accumulate below the euphotic zone. The continuation of production depends upon the restoration of nutrients from the deep to the surface layers by vertical water mixing (see page 183).
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