STUDIES ON THE BIOLOGY AND CHEMISTRY OF THE GULF OF MAINE

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Abstract
A bacteriological survey has been made of the waters and bottom sediments in the Gulf of Maine and Georges Bank. The samples of water, plankton tow, and sedimentary material were taken under sterile conditions and subjected immediately to bacteriological analysis as soon as brought on board the "Atlantis." The agar-plate method was used for the enumeration of the numbers of bacteria. This was supplemented to a limited extent by the dilution method. Various specific media were used to determine the distribution and relative abundance of certain groups of bacteria which are believed to take part in important marine processes. The results obtained demonstrated the fact that the bacterial population of the sea can be divided into three groups on the basis of their habitat: (1) those forms which live in the sea bottom, especially in the surface layers; (2) those bacteria which live in the free water, this being possible only when the water contains in solution organic and inorganic substances which can serve as nutrients for the bacteria; (3) those bacteria which live largely upon or in association with the plankton organisms. Sea water is a rather poor medium for the growth of bacteria, while the marine bottom is comparatively richer in the total number of bacteria capable of developing on the plate and in solution media. Mud bottoms contain more living bacterial cells than sand bottoms. However, the waters above the sandy bottom were found to contain many more bacteria than the waters above the mud bottoms. This may be due to the greater abundance of plankton organisms, especially diatoms, in the shallower seas with the sandy bottom, to the greater mixing of the waters, or to the greater absorption of bacterial cells by the mud bottom material than by the sand bottom. The numbers of bacteria obtained by the plate method represent only a part of the bacterial population of the sea. This was shown by the fact that the dilution method, using gelatin as a medium for bacterial development, gave higher numbers than the plate method. A decided parallelism was observed between the abundance of diatoms in the sea and abundance of bacteria. A comparatively large number of these bacteria were agar-liquefying organisms. In the artificial culture of diatoms, the numbers of bacteria were found to increase with the development of the diatoms. However, when the latter reached a maximum and began to die out, bacterial numbers did not diminish very rapidly; these bacteria seemed to be largely limited to a few specific types, the importance of which in marine processes still remains to be determined. These results seem to point definitely to the fact that the development of phytoplankton in the sea is accompanied closely by bacterial development. The bacteria feed upon the excretion products of the diatoms, algæ, and animal forms and probably upon these plankton forms themselves as soon as they die, thus bringing about their rapid disintegration and liberation of the nutrient elements in an available form. Anærobic bacteria were found abundantly in the marine mud; these bacteria were present in the mud even at considerable depths. Their presence points to continued decomposition of the plant and animal debris of the ocean on and in the ocean bottom, even with an insufficient supply of oxygen. The bacterial population of the sea was found to consist of a number of types, some of which take part in well-known processes which are of great importance in the metabolism of the sea, such as nitrogen-fixing, nitrite- and nitrate-forming, nitrate-reducing, cellulose-decomposing, agar-decomposing, chitin-decomposing, and many others. The importance of many common bacteria, both ærobic and anærobic, mostly non-spore-forming but also spore-forming, in marine processes still remains to be determined. The presence of nitrogen-fixing bacteria, comprising both the ærobic Azotobacter and the anærobic Clostridiunm, has been definitely demonstrated. One strain of the first group was isolated in pure culture and was found to be Azotobacter chroococcum, which fixed considerable quantities of nitrogen when grown on artificial culture media, with various carbon sources. Bacteria capable of oxidizing ammonium salts to nitrites were found in the sea bottom, but only seldom in the sea water. For demonstrating the presence of these organisms in the sea, a medium containing a layer of sand and CaCO3 and covered with a shallow layer of liquid containing an ammonium salt and the necessary minerals was used. Bacteria capable of reducing nitrate were found abundantly both in the water, especially at the surface layers and the zone of photosynthetic activities, and in the sea bottom. However, these bacteria were able to reduce nitrate only to nitrite and not to nitrogen gas; only in one instance, namely, in the mud from the first station taken on this expedition, was there present an organism capable of bringing about the last process. The cycle of nitrate in the sea is explained as follows: The nitrogenous constituents of the plant and animal residues in the sea are decomposed with the liberation of ammonia, largely in the sea bottom. The ammonia is oxidized in the sea bottom to nitrite and later to nitrate. The latter gradually diffuses into the waters. It is not reduced in the sea bottom or lower layers of water, largely because of a lack of available energy material necessary for the activities of the nitrate-reducing bacteria. In the zone of maximum photosynthetic activity such energy is available, hence nitrate-reduction may take place, but only to nitrite, since bacteria capable of reducing it to nitrogen gas are lacking under those conditions. The nitrite thus formed may also be assimilated by the phytoplankton and does not represent any loss of nitrogen in the sea. The marine humus (total organic matter) content of the mud bottom in the Gulf of Maine is found to be more or less uniform in composition. This humus is best calculated from the total organic carbon. It was found that determinations based upon one sample may not give absolute evidence, since different samples may give considerable variation. The humus content usually decreases with the depth of the mud. In some stations, however, an increase in humus content with depth was observed. There is also a widening of the carbon-nitrogen ratio of the humus with an increase in depth. This points to a greater decomposition of the humus, especially of the nitrogenous complexes, with an increase in depth. An increase in humus content with depth of mud found in some stations merely points to the fact that the rate of deposition of the humus may vary considerably with different periods. A definite parallelism was observed between the numbers of bacteria and abundance of humus in the mud. A detailed review of the literature bearing upon the results presented in this report, as well as a detailed description of the methods used and a correlation of these results with the results of bacteriological investigations of the sea near shore, is reserved for a future publication.

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