Distribution of Bacteria in the Sea

The greatest numbers of marine bacteria have been found in the coastal waters where the greatest abundance of plant and animal life is also produced. In vertical distribution we find two main centers, on the bottom a few millimeters below the mud-water interface, and in the pelagic zone attached to the floating plants and animals and other particulate matter (fig. 247). Reference to table 100, from ZoBell and Anderson (1936b), will illustrate typical vertical distribution off the California coast. It should be noted that even though one center of distribution is in the pelagic zone, associated with the plankton, the existence of bacteria is apparently not truly planktonic but sessile upon organisms, and must therefore also closely coincide vertically and horizontally with the maximum distribution of these organisms.

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It has been indicated by Carey and Waksman (1934) and others that bacteria are also present in relatively small numbers in the water even to a depth of 5000 m, and to that extent must be considered planktonic. But since the favorite habitat is one of attachment, it is not possible at present to know how normal the true planktonic existence is. Many individuals may be there through accident and be able to survive and multiply only to the degree that they are capable of utilizing the dilute organic matter that is in solution in the water. The advantages of a periphytic habit were discussed elsewhere (p. 912). A planktonic distribution must, however, be of great significance in the dispersal of these microorganisms.

The greatest density by far of bacterial populations is found on the bottom, where as many as 420,000,000 cells per gram of wet mud may occur (ZoBell and Anderson, 1936b). Such dense populations are possible mainly because in a thin layer representing an interface between mud and water there is concentrated a large portion of the organic detritus, dead bodies of plants and animals, which is constantly sinking and forming an abundant rich food supply for bacteria as well as for bottom-dwelling animals which compete with them. Most bacteria occur in the first few millimeters of bottom ooze and there is a gradual diminution of numbers with the increasing thickness of the bottom deposits. Viable

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bacteria have been taken from a depth of over 350 cm in marine sediments (Rittenberg, 1940). In general, greater numbers occur in the fine sediments than in the coarser ones.

In fig. 247 is shown the vertical distribution of bacteria and its relation to the diatom population. It will be noted that the large bacterial population of the pelagic region coincides roughly with the vertical range of floating plants and animals. This apparently results from the periphytic habit of many bacteria. The plankton organisms—diatoms, copepods, and so forth—living most abundantly in the euphotic zone, serve as surfaces for attachment and offer a favorable environment for multiplication (p. 912). The presence of a large periphytic bacterial population in the plankton results in prompt bacterial decomposition of large quantities of dead organisms before they have sunk to great depths; thus a large portion of mineralized plant nutrients is regenerated within or only a little below the euphotic zone.