468 BULLETIN OF THE BUREAU OF FISHERIES 



growth, and in the long run probably the supply of nitrogenous compounds chiefly 

 determines the regional richness and poverty of the phytoplankton as a whole. 



Allen and Nelson's (1910) experiments on rearing marine diatoms corroborate 

 this, for they found it necessary to increase the concentration of nitrates, and 

 apparently also of phosphates, above that of normal sea water in order to produce 

 active multiplication. Fritz's (1921a) work along this line is especially pertinent' 

 here, for she experimented at St. Andrews on the culture of planktonic marine 

 diatoms of Gulf of Maine species with similar results, being unable to obtain any 

 considerable and persistent growth without the addition to the normal sea water of 

 the nutrient salts — nitrates and phosphoric acid — employed by Allen and Nelson. 

 With these, however, she obtained flourishing cultures of Thalassiosira nordensJcioldi, 

 SJceletonema costatum, Asterionella japonica, NitscJiia closterium, Melosira Ti/yperborm, 

 and various other planktonic species. 



NITROGEN 



It has long been known that sea water absorbs nitrogen so readily from the air 

 that the surface strata are usually saturated with this element, but it is still question- 

 able whether any of the planktonic plants are able to utilize elemental nitrogen first 

 hand. It has long been the commonly-accepted belief, also, supported by experiments 

 on land, that no chlorophyllous plants can do so, unless, like the Leguminosas, in 

 symbiosis with nitrogen-fixing bacteria; but that all others — terrestrial or marine, 

 unicellular or multicellular — are dependent on nitrogen compounds elaborated by 

 some other means for their food supply of this essential element. 



In 1920, however, Moore and Webster (1920) published the results of experiments 

 which seemed to demonstrate that certain green unicellular alga? do possess the abihty 

 to obtain, and to fix by a process of photosynthesis, elemental nitrogen dissolved by 

 the water from the air. A year later Moore, Whitley, and Webster (1921) carried out 

 further experiments on a marine green alga, which they grew in measured volumes 

 of sea water, finding that at the end of the experiment the amount of fixed nitrogen 

 in plant and water combined exceeded the nitrite present at the beginning. From . 

 this they concluded that the excess must have come from the elemental nitrogen 

 dissolved in the water, and so, in turn, from the air. These experiments, however, 

 were not conclusive, no precaution having been taken to exclude the nitrogen-fixing 

 bacteria which Reinke (1904) and Keding (1906) found on the fronds of several 

 species of fixed algse at Helgoland, and which, therefore, were probably present on 

 the algal fronds used by Moore, Whitley, and Webster in their experiments, or to 

 determine their presence or absence. And although Moore and his associates adduce 

 several reasons why they think it improbable that the value of their experiments 

 is detracted from by this "loophole" in technique, it remains an open question 

 whether the increase in the amount of fixed nitrogen, which they demonstrated, 

 did actually result from photosynthesis by the algal fronds experimented upon or 

 from activity on the part of bacteria living symbiotic upon them. 



So far as I am aware, the ability of marine phytoplankton to synthesize ele- 

 mental nitrogen has not actually been tested by critical experiment directed to this 

 definite end. But it has repeatedly been found that very much richer cultures of 



