Dissolved organic matter DOM

Particulate organic matter is always present in seawater, but in addition, varying quantities of organic compounds are present in solution. This is referred to as dissolved organic matter (DOM) or dissolved organic carbon (DOC) (Jorgensen, 1976; Williams, 1975). The estimation of minute quantities of organic solutes is difficult, but it appears that ocean water commonly contains about 2 mg carbon per litre in dissolved organic forms, in some of which nitrogen, phosphorus, sulphur, iron or cobalt are included. However, recent analyses suggest that the amounts may be higher than this (Toggweiler, 1988). Although these concentrations may seem small, the total quantity in the ocean is very large, and it has been calculated that there are on average about 15 kg of DOM beneath each square metre of the ocean surface. This greatly exceeds the amount of organic matter present in particulate form as either living material or organic debris. The yellow colour sometimes detectable in seawater derives mainly from various organic compounds in solution.

These solutes evidently come from a variety of sources. They are predominantly of biological origin. Some enter the seas via land drainage, some on airborne particles or vapours. The major contribution must certainly derive from the activities of marine organisms in metabolic processes of photosynthesis, feeding, excretion, breakdown of dead tissues, etc. Possibly traces of organic matter are leached from living tissues, and some organisms are known to liberate organic secretions into the water as part of their normal metabolism. Such external metabolites are termed exocrines. In recent years, man-made organic compounds have been produced in increasing amounts, and many of these eventually find their way into the sea (see Chapter 10).

In temperate waters the total quantity of DOM shows seasonal changes with a marked increase following the spring phytoplankton bloom (see page 195), giving high summer values declining to a minimum in later winter. During summer, greater amounts of DOM are found above the thermocline than below it. It is now certain that a significant fraction of primary production is quickly released via various routes as DOM. Some of this material may leach from plant cells, some may be lost during cell division, and much may be routed through the feeding, excretion and egestion of animals. Some zooplanktonts grazing on phytoplankton liberate into the water appreciable quantities of DOM from plant cells by breaking up the frustules prior to ingestion, so-called 'sloppy feeding'.

There is little precise information regarding the quantities and chemical nature of these compounds, but a growing list of substances identified in seawater includes various hydrocarbons, carbohydrates, urea, aminoacids, organic pigments, lipids, alcohols, and vitamins such as ascorbic acid and components of the vitamin B complex, e.g. thiamin and cobalamin.

The way in which organic solutes influence the biological properties of seawater is a matter for speculation, but it is clear that at least some are absorbed by organisms and may have various effects. Artificial seawater made from all known inorganic constituents including the trace elements does not have the same biological quality as natural seawater, and organisms do not thrive in it. If, however, small quantities of organic material are added in such forms as soil extracts, urine, extracts of various tissues or even small volumes of natural seawater, the quality of the solution as a satisfactory medium for marine life is greatly enhanced. The fact that the concentrations of organic solutes in natural seawater are low suggests that they may be continuously absorbed from the water by organisms. The importance of DOM in the organic food cycle in the sea, has only recently been appreciated. The role of DOM in the so-called 'microbial loop' is described in Section 5.1.2.

It is now clear that certain phytoplankton species readily utilize dissolved organic nitrogen (DON) and phosphorus (DOP) compounds, indeed some species may even thrive better on organic solutes than inorganic. Although in temperate waters the concentrations of nitrate and phosphate in the surface layers become markedly depleted following the spring growth of diatoms, with generally low amounts continuing through the summer, the total dissolved nitrogen and phosphorus in both inorganic and organic forms undergoes no corresponding reduction. Evidently many of the inorganic nutrients taken in by the phytoplankton reappear as DOM, and are thus available to maintain production of species which absorb nitrogen and phosphorus in organic form. The seasonal succession of phytoplankton species which occurs during the productive period may partly reflect the seasonal changes in DOM, with spring species flourishing on inorganic nutrients and later dominants utilizing organic solutes which occur in the water following the spring bloom. This may also explain why measurements of production during the summer sometimes indicate higher rates than would seem to be consistent with the low concentrations of organic nutrients if these were limiting factors.

There is also evidence that certain forms of DOM in seawater may become aggregated into particles at air-water interfaces, especially on the surface of bubbles, thereby adding to particulate food supplies in the surface layers.

Much of the dissolved organic matter in seawater is fairly resistant to bacterial degradation, constituting a sort of stable marine humus. The easily degradable small molecules of glucose, aminoacids or fatty acids are found mainly in the euphotic zone or in sediments, where they are presumably produced and quickly consumed. The amount of DOM used as a source of nutrition is therefore probably greater at the surface and bottom than at middle depths. However, occasional reports of large numbers of pelagic saprophytes at deep levels around 3000 to 4000 m suggest that DOM may also be a significant food source for abyssopelagic organisms. Although less in quantity here than at the surface or bottom, its relative importance may be great because other food is lacking.

The role of DOM as a source of food to benthic animals is discussed in Section 6.4.

On land, many animals communicate by means of airborne chemicals such as pheromones. It is now known that various interactions between marine organisms are brought about by external secretions that dissolve in, and are carried around by, the water. The olfactory sense is highly developed in many marine creatures and probably serves several useful functions; for example, the recognition and location of other individuals, the detection of food, and in certain species it is important in connection with navigation and migration.

Various micro-organisms, particularly flagellates and bacteria, are known to produce substances toxic or repellent to other organisms. It has been demonstrated that bacterial respiration is sometimes depressed in the presence of diatoms performing photosynthesis, presumably due to antibiotic substances set free by the plants. Phaeocystis when abundant gives off acrylic acid which depresses the growth of certain other phytoplankton species. This may even be an important aspect of competition, and possibly also of defence. It has also been suggested that zooplankton avoids water containing large numbers of phytoplankton because of distasteful external metabolites produced by the plants (see page 189). Some dinoflagellates contain toxic substances and may cause the death of other marine creatures when present as a bloom or 'red tide' (see Section 2.2.2).

In some cases, organic solutes have a growth-promoting effect, and seem to have a role in nutrition comparable to that of vitamins, auxins or hormones. Experiments with cultures of marine algae indicate that some have specific organic requirements for normal growth, for example cobalamin. Chelating agents in seawater may also be of some importance, favouring plant growth by bringing into solution essential trace metals which occur in particulate form. The enhanced fertility of coastal water for phytoplankton growth may be partly attributable to the large amounts of DOM derived from land drainage. The group of coloured organic solutes broadly termed 'humic substances' are also thought to fulfil a nutrient function for some marine plants.

Early experiments by Wilson and Armstrong in the 1950s on the rearing of invertebrate larvae, for example the sea urchin Echinus and the polychaete Ophelia, demonstrated differences in survival rates of larvae reared in samples of water collected from different areas although there were no obvious chemical or physical features of the water to which the differences could be clearly attributed. The factors responsible for these variations in biological quality of the water could not be determined, but some growth-promoting or growth-inhibiting substances were evidently present. The result of mixing seawaters of different quality suggested that the differences were probably due to the presence of beneficial substances in some samples, rather than to harmful substances in others. It has been suggested that differences in biological properties between seawaters of different areas may depend partly upon their biological histories through the effect of metabolites produced by preceding generations of organisms.

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