12 Papers from the Marine Biological Laboratory at Tortugas. 



reinoculated at intervals. In my experiments I was able to obtain similar 

 results in cultures kept at 30° C. after 8 days; in cultures kept at 15° C. the 

 first formation of nitrite was detectible in from 5 to 6 days, but denitri- 

 fication never proceeded beyond this stage. 



Baur (i) showed that the optimum temperature for growth and denitri- 

 fication of the species described by him lay between 20° and 25° C. when 

 the bacteria were grown in fluid culture media containing peptone. 



The most important work on the distribution of marine bacteria is that 

 of Fischer (8) in 1886, 1889, and 1893, but he does not enter into the chemical 

 activities of the species found, so that the observations do not throw much 

 direct light on problems of the metabolism of the sea. The variations in 

 the number of bacteria found in different surface samples from positions in 

 mid-ocean are somewhat surprising and difficult to account for. Deeper 

 samples were taken by means of a water-bottle made of brass, but in view 

 of the now well-known bactericidal action of metals, and of copper in 

 particular, I do not consider that any great value can be attached to these 

 observations. With the exception of Fischer's work little seems to have 

 been published on the general distribution of marine bacteria. 



A point that has not yet been considered is the origin of the nitrate supply 

 in the sea. Nitrates are absorbed by diatoms and the phyto-plankton in 

 general, 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 actu- 

 ally decomposed by the denitrifying bacteria, which would thus tend to 

 keep the nitrate concentration 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 decomposing nitrates 

 to the extent of entirely removing the nitrogen from all chemical combi- 

 nation, 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 nitrifying bacteria, which 

 are capable of absorbing and combining with the free nitrogen of the air 

 and eventually giving rise to nitrates, has been shown by Keding (10) and 

 Keunter (11), but these have so far only been found on the bottom close to 

 shore, or apparently living in symbiosis with algae or plankton organisms. 

 Similarly Thomsen (16) has demonstrated the presence on the bottom of 

 inshore waters of bacteria which are capable of forming nitrites from 

 ammonium salts, and others which can convert nitrites into nitrates. It 

 would seem possible that similar bacteria having a nitrifying action remain 

 to be discovered in the open sea. 



The precipitation of calcium carbonate in the sea by bacterial agency is 

 apparently a line of investigation that has not previously been suggested or 



