The Nitrogen Cycle
The main features of the nitrogen cycle must be considered because of the outstanding importance of nitrogen to all forms of life and because of the great general interest it holds in illustrating the successive ecological implications with respect to bacteria.
It has been pointed out (chapter VI) that nitrogen exists in the sea in combination with other elements, for example, in ammonia (NH3), and as oxides of nitrogen in nitrite ion (NO2), and nitrate ion (NO3). Nitrogen enters into the composition of all living things. It is one of the building blocks (nutrients) used by plants in forming the complex protein molecules of their bodies from which animals must derive their nitrogen. However, not all forms of nitrogen can be used by plants, hence the complex nitrogenous compounds found in both plants and animals must, upon death of the organisms, be decomposed, along with their products of excretion, to chemically simpler compounds utilizable by the plants. This decomposition, we know, is accomplished mainly by the activity of proteolytic bacteria.
The process of decomposition involves a series of steps in which specifically different bacteria are concerned. The early stages of the transformations are not fully known, but amino acids result and the nitrogen compounds—ammonia, nitrites and nitrates—are a part of this process. It is these inorganic compounds and also, to some extent, the amino acids that can be used directly by the plants for their supply of nitrogen.
The main store of nitrogen in the sea has been considered elsewhere (pp. 181 and 242). We need to be concerned here only with the main features of the chemical circulation of this element as activated by organisms including the plants, animals, and bacteria living within the sea. We may begin with the large and complex protein molecule of plant or animal tissue which is broken down into more simple products containing nitrogen. The cycle is composed of six transitions known as ammonification, nitrification, nitrogen assimilation, denitrification, and nitrogen fixation.
Ammonification. It is well known that ammonia is a product associated with decay of organic material. There are a number of stages and different products involved in this simplification of the molecule, among which products are the amino acids, considered present in the sea only by inference (Cooper, 1937). The bacterial process of deaminization results in splitting off the NH2 group in the amino acids, and there are many types of bacteria in the sea which are endowed with the ability to carry on this process. In weak solutions the ammonia may be intercepted at this point and the nitrogen assimilated directly by diatoms, as is known to occur also with higher plants. ZoBell (1935) and Cooper (1937) review the literature, in which there is much evidence that ammonia nitrogen constitutes one of the important sources of nitrogen utilized by unicellular algae. Ammonia is present usually only in small quantities and its distribution and concentration appear to indicate the place and intensity of organic decomposition (Redfield and Keys, 1938).
Nitrification. It was formerly suggested by some investigators that the sea water must derive its nitrates through drainage from land or through electrical discharges and photochemical processes. The nitrifying bacteria occurring near shore were thought to have washed in from land and not to live normally in the sea. In more recent years investigations have concluded from bacteriological and chemical studies that this type of bacteria is also beyond doubt truly marine. Carey (1938) summarizes briefly the development of our knowledge of these microorganisms in the sea.
The complete process of nitrification includes the formation of nitrates from ammonia and nitrites (NH3 → NO2 → NO3). The organisms responsible for converting the ammonia to nitrite are called Nitrosomonas and Nitrosococcus. They may live in the absence of organic material, obtaining their carbon through assimilation of carbon dioxide and their energy to carry on life processes through the oxidation of ammonia to nitrites. Investigations by Carey indicate that this takes place mainly in two regions, on or near the bottom in coastal areas and in the water in association with plankton at moderate depths, though some nitrites may also be formed at mid-depths. Oxidation of ammonia to nitrite may also occur photochemically (p. 256).
Following the conversion to nitrites, another group of bacteria oxidizes the nitrites to nitrates. The source of carbon for the nitrateformers is apparently also the carbon dioxide, and the energy is derived through oxidation of nitrities.
Nitrogen Assimilation. The process of nitrogen assimilation is primarily a function of the plants, the phytoplankton and benthic algae, but bacteria also assimilate nitrogen. Nitrogenous nutrients such as amino acids, ammonia, nitrates, or nitrites are the available sources of the nitrogen used by the plants in building up the amino-nitrogen of the protoplasm. It is not clear which of the last three mineralized compounds are preferred by phytoplankton as a source of nitrogen, but field observations provide evidence that they may be used simultaneously. During periods of low plant production the process of nitrification results in the storage of nitrogen as nitrates, and subsequent outbursts of diatoms then draw heavily upon this supply. After assimilation these plants may die and be dissociated directly by bacteria, or what is perhaps the most usual occurrence, they may be eaten by animals which require organic nitrogen from the plants directly or indirectly. In any event, a return to the inorganic nitrogen compounds is eventually effected by the bacteria, although animals preying directly upon each other may keep the organic nitrogen within their own cycle for some time. However, due to constant losses, this cycle cannot be self-perpetuating, and there must therefore be a large group of herbivorous animals to serve as food for the carnivorous forms.
Nitrate Reduction and Denitrification. The nitrogen cycle in the sea is more involved than is indicated by the above discussion. Gran (1901) and later Baur (1902) showed that denitrifying bacteria also exist in the sea. These denitrifiers and nitrate-reducers produce the effect just opposite to nitrification. Accordingly, nitrates are reduced to nitrites by splitting off a part of the oxygen, and other bacteria, the true denitrifiers, may carry the process even further with complete reduction of the nitrites and evolution of free nitrogen (NO3 → NO2 → N2). This last step, which is apparently not of great importance in the sea, constitutes a loss to the cycle in the sea unless the elementary nitrogen can be reclaimed by nitrogen-fixing bacteria (see below). The relatively uniform distribution of dissolved nitrogen in the sea also suggests that true denitrifiers and nitrogen fixers are not important in the general economy of the sea (Hamm and Thompson, 1941).
It has been held that the process of reduction of nitrates and nitrites by bacteria is characteristic of an environment lacking or poor in oxygen. Under these anaerobic conditions and in the presence of organic matter the bacteria find their source of needed oxygen in the molecules of these compounds. Rather little is known about the extent of denitrification in the sea, but since most sea waters have at least a small supply of
Nitrogen Fixation. In the terrestrial environment, the fixation of free nitrogen by bacteria is an important factor. To what extent similar nitrogen fixation occurs in the sea through the activity of bacteria is not well known. However, nitrogen fixers are reported from coastal areas, some in symbiosis with algae, and the extent to which they function in fixing free nitrogen depends upon environmental conditions involving the amount of available nitrogen compounds at their disposal, since nitrogen fixation is not an obligatory process (Benecke, 1933).