DEPARTMENT OF' MARINE BIOLOGY. I4I 



To such a culture a trace of very finely powdered hydrated calcium sul- 

 phate or fine sand was added. This resulted in the formation of a precipitate, 

 which, on microscopical examination, could be seen to consist of finely lami- 

 nated concretions, some of which appeared to have a particle of calcium sul- 

 phate or sand as a nucleus. The concretions were soluble in dilute hydro- 

 chloric acid with evolution of carbon dioxide. These concretions bear a 

 resemblance to those of some oolitic limestones, and the experiment suggests 

 the manner in which such oolites may have been formed. 



The bacteria which cause the formation of these concretions seem to be 

 the same as those found at Tortugas and Jamaica. 



Bacteria from depths ranging from the surface to 80 fathoms from 70 

 miles west-southwest of Ushant, France, on August 20, 1911, appear to be 

 all of one and the same kind, and in cultural reactions to be precisely similar 

 to the Tortugas and Jamaica denitrifying form, with the exception that acid 

 formation does not occur with cane sugar. 



Denitrification was relatively slow, the first trace of nitrite appearing only 

 after 96 hours at 75° F., and all nitrite was never destroyed. 



These observations suggest that the bacterial denitrification in tropical 

 seas is far more active than in temperate regions, and if this can be sub- 

 stantiated by future work it would offer an explanation of the relative scar- 

 city of plankton in the Tropics. As it at present stands, the investigation 

 can at most be considered to offer a mere indication of the part played by 

 bacterial growth in the metabolism of the sea. To obtain a real insight into 

 the question, it would be necessary to make more extensive bacterial and 

 chemical observations in tropical, temperate, and arctic waters, and more 

 especially to make a study of the bacterial flora of the deep sea in some spot 

 where considerable depth, unaffected by currents, could be obtained. The 

 possibility that bacterial growth plays an important part in causing the pre- 

 cipitation of certain bottom deposits in coastal waters, and conceivably also 

 in the deep sea, is a subject well worthy of investigation. 



A point not yet touched upon is the origin of the nitrate supply in the sea. 

 Nitrates are absorbed by diatoms and the phyto-plankton in general in the 

 course of their growth, and are presumably built up into complex nitrogenous 

 compounds within the plant. If these compounds on the death of the plant 

 are broken up and the nitrogen again rendered available for use in the form 

 of nitrates, a series of reactions must be gone through which may well be 

 performed by bacterial agency, and this also applies to the waste nitrogenous 

 products of animal metabolism. 



In addition, it has been shown that nitrates are actually decomposed by 

 the denitrifying bacteria, and they would thus tend to keep the nitrate con- 

 centration down to the level necessary for their own existence, and would 

 come into competition for this essential with other forms of plant life. If 

 the bacteria are successful in liberating free nitrogen from nitrates, as seems 

 probable from the experiments in cultures, it follows that there must be 

 some source of nitrates in order that the concentration in the sea may remain 

 constant. The existence of symbiotic nitrifying bacteria on some of the 

 algae around the shores of the North Sea has been demonstrated, and it 

 seems possible that forms having a similar nitrifying action in the open sea 

 remain to be discovered. 



