MARINE BACTERIA AND THEIR RÔLE IN THE CYCLE OF LIFE IN THE SEA

Abstract

1. A preliminary study has been made of the occurrence and activities in the sea of bacteria concerned in the cycle of nitrogen in marine life. 2. Sea water, especially in the upper layers, is practically free from nitrifying bacteria or contains only very few cells. The activities of these organisms are limited almost entirely to the sea bottom. 3. In order to demonstrate the presence of these organisms even in the bottom material, it is essential to use a medium containing a layer of sand or mud, so as to make conditions favorable for their development. 4. It is comparatively easy to demonstrate the presence in the sea bottom of bacteria oxidizing ammonium salts to nitrite. It is somewhat more difficult to establish the presence of nitrate-forming bacteria; this can be done, however, if the sensitiveness of these organisms to the laboratory conditions is taken into consideration, especially nutrient concentration and period of incubation. 5. Sea water and sea bottom contain an abundance of nitrate-reducing bacteria. Most of these organisms, especially those present in the water, are able, however, to reduce the nitrate only to nitrite, but not to atmospheric nitrogen. 6. Bacteria that bring about complete reduction of the nitrate are present largely in the sea bottom. In order to reduce the nitrate, an available source of energy must be present. The marine humus cannot be used to any great extent as such a source of energy. The plankton residues are used largely by those bacteria which consume the nitrate and transform it into organic nitrogenous compounds. 7. The conclusion is reached that, even if denitrifying bacteria are present in the sea, their activities are so limited under marine conditions as to render them in most instances of little importance in limiting the nitrate supply of the sea. 8. The sea contains an abundant population of nitrogen-fixing bacteria. The ærobic Azotobacter is largely found in the water and in the surface layer of the marine bottom, while the anærobic Clostridium is present in the sea bottom. 9. In the presence of a favorable source of energy, the marine nitrogen-fixing bacteria are capable of fixing appreciable quantities of nitrogen. It still remains to be determined, however, to what extent this process actually takes place in the sea. 10. The bacteria concerned in the transformation of nitrogen can be tentatively distributed among the plankton and the sea bottom, the two major seats of bacterial activities in the sea, as follows: the nitrifying bacteria live largely on the sea bottom; of the nitrate-reducing bacteria, those forms which reduce nitrate to nitrite live both in the water and in the bottom, while the true denitrifying bacteria live largely on the bottom; of the nitrogen-fixing bacteria, the ærobic forms live both in the water and on the bottom, while the anærobic forms live largely in the bottom material.