Bacterial Modes of Life
For our immediate study of the relationship of bacteria to other organisms and to the chemical cycles of the sea, it is necessary to take into consideration the vital implications of their activities as a part of the dynamic energy cycle within the marine population of which they are a part. The investigations of marine bacteria are therefore concerned chiefly with their physiological processes as applied to the sea and its chemical and biological problems. Their indispensable function in the economy of the sea is primarily one concerned with transformation of organized substances and not with accumulation or storage of organic matter.
In order to understand better the activities of bacteria, it is necessary to distinguish between certain of the different modes of life. These are concerned especially with the source of nutrition and the oxygen supply.
Autotrophic Bacteria. These resemble green plants in their ability to build carbohydrates and proteins out of the simple substances carbon dioxide and inorganic salts. Some of these, known as photosynthetic, possess coloring material, or bacteriochlorin, and use radiant energy in building up protoplasm, while others, known as chemosynthetic, derive their energy from the oxidation of various inorganic compounds such as H2S, S, or NH4.
The amount of organic material synthesized in the sea by bacteria is small when compared to that produced by the chlorophyll-bearing plants. It is not fully known to what extent the chemosynthetic forms contribute to organic material on the sea bottom where depths are so great that there is insufficient light for photosynthesis, but the concentration of benthic animals suggests correlation mainly with the pelagic and benthic algae as the source of primary food and not with the bacteria.
However, autotrophic purple sulphur bacteria have been on several occasions reported as being so abundant in isolated inshore situations as to impart a red tint to the water, to the surface of algae, or to the bottom (Benecke, 1933).
Heterotrophic Bacteria. These obtain their energy by the oxidation of organic compounds. Hence they live as saprophytes or parasites. Most bacteria of the sea are of this type.
We have learned that in the cycle of organic material in the sea it falls to the phytoplankton, in particular, but also to other algae to synthesize organic substance from such inorganic raw materials as carbon dioxide, nitrates, phosphates, and others. But the store of certain of these materials may be exhausted in the euphotic layer and become bound as part of the substance of organisms. However, over a long period of time organisms die at the same rate as they are born, and thus a continuous return of the raw material is possible if a transforming agency is provided. Such an agency exists in the heterotrophic bacteria, whose enormous task is to perpetuate this phase of the cycle through mineralization of organic matter. Animals take part in the general cycle, but owing to the fact that as a group they are neither producers nor transformers (in the sense that plants and bacteria are), a reduced cycle could go on without them; indeed, considerable quantities of plants must at times by-pass the animals and be directly reduced to inorganic state by bacteria (fig. 248, p. 926). The interposition of animals no doubt functions to smooth out the cycle so that bacterial activity goes on at a more even rate throughout the year. There appears to be no evidence that the number or kind of bacteria show any marked seasonal cycles (Lloyd, 1930, ZoBell, 1938). During seasons of plant production, rapidly growing animals incorporate large quantities of plant substance into their own structure and, owing to the longer life cycles of many animals, much of this material may become more gradually available to bacteria at periods of minimum plant production. Illustrative of this is Lohmann's study of the cycle of plankton organisms over the whole year at Kiel. According to his observations the plants for the year averaged 56 per cent of the volume of the total pxsankton However, it was found that from December to February plants formed scarcely a third of the total plankton. At Plymouth the winter production of plants is set at about one fourth of the summer rate. These rates apply only to calculations based on given diatom populations during twenty-four hour periods of production during these seasons and may vary with other species and conditions (Harvey et al, 1935).
Utilization of Dissolved Organic Matter. In discussing the chemistry of the sea, it was pointed out (p. 248) that an appreciable quantity of organic matter is present in solution in sea water. The concentrations per unit volume of water are very much smaller than those
It was believed by Pütter (1907) that many marine animals are able to absorb dissolved organic matter through their gills and integuments and may thus obtain a portion of their food through utilization of the organic matter in solution in the sea. There is little evidence that other forms than bacteria make any considerable use of this supply of nutriment (Krogh, 1934b, Bond, 1933).
According to some authorities the low concentration and uniform distribution of dissolved organic matter in the sea may result from the action of heterotrophic bacteria. Sea water may be looked upon as a dilute culture medium in which the upper limit of concentration of dissolved organic substance is a threshold value maintained by the bacteria at a level below 10 mg/l. In apparent contradiction it must be mentioned that this may appear too dilute for bacterial growth, since for some non-marine bacteria 10 mg/l of different carbon compounds is the minimum concentration in which successful growth occurs (Stephenson, 1939). This may also be true for marine bacteria suspended free in the water. But when the bacteria and other organic material are adsorbed to solid surfaces a greater efficiency in utilization of dilute organic matter is possible, and growth occurs although concentrations may be much lower than 10 mg/l.
Experimentally it has been shown by ZoBell and Anderson (1936a) that in normal filtered sea water bacteria may flourish but do so best when grown in receptacles providing the greatest solid surface area per unit volume of sea water. Increased surfaces were obtained by introducing sterile beads, siliceous sand, and so forth. Increasing the ratio of solid surface to water leads to increased bacterial activity only when the nutrient medium is of low concentration such as that prevailing in the sea. It is explained that the increased surfaces offer places for attachment of greater numbers of periphytic bacteria (that is, those which grow attached to solid surfaces). Clean glass microscope slides submerged in the sea also quickly show a concentration of bacteria following adsorption of organic food and provision of solid surface.
The efficiency of the activities of periphytic bacteria is believed to be enhanced owing to adsorption of organic matter to solid surfaces and to the reduction of diffusion at the immediate surfaces of bodies and within the interstices at the tangent of the bacterial cell and the solid surfaces. Thus these micro-volumes serve as concentrating foci for bacterial
In nature the solid surfaces are provided by all types of particulate matter, animate or inanimate, on the bottom, in the plankton, and on the nekton.
Oxygen Relations. It is convenient to classify the bacteria further on the basis of their different oxygen requirements or tolerances. There are obligate aerobes which use free oxygen, obligate anaerobes which function in the total absence of free oxygen, facultative forms which may live in either type of environment, and microaerophiles which require a reduction of free oxygen but not to the point of anaerobic conditions. Most marine bacteria are said to be of this type.
Bacterial Chemical Transformations. To facilitate study of the cycle of substances in the sea, it is necessary further to classify the marine bacteria into several convenient general groups, namely nitrifying, denitrifying, nitrogen fixing, sulphur, and iron bacteria, and so on, based upon their metabolic activities in the transformation of these substances. To describe these groups is to describe the role of bacteria in the chemical and biological cycles of the sea.