136 REPORTS ON INVESTIGATIONS AND PROJECTS. 



year's crop of eggs, a premise which I beHeve to be true, then in the case of 

 Littorina (probably all species) one would expect the period of copulation to 

 be nearer that of oviposition than it is among those prosobranchs which 

 have motile nurse-cells (oligopyrene spermatozoa), since here the nurse-cells 

 function in the sperm-duct for at least the greater part of the winter. 



Report of Preliminary Investigations on the Marine Denitrifying Bacteria, 

 made at Port Royal, Jamaica, and at Tortugas during May and June 

 ipii, by G. Harold Drew, Christ's College, Cambridge, England. 



It appears to be well established that the plankton of tropical and sub- 

 tropical seas is far less in quantity than that found in colder waters. 



The zoo-plankton depends ultimately for its food on the phyto-plankton ; 

 hence any factor limiting the growth of the phyto-plankton which was capa- 

 ble of exercising its influence in tropical and not in temperate or arctic waters 

 might offer an explanation of this phenomenon. It has been shown by vari- 

 ous investigators that this factor is not temperature, light, or salinity, and it 

 has been suggested that the explanation may lie in the relative deficiency in 

 tropical seas of the nitrates or nitrogenous compounds which are so essential 

 for all plant life. A matter of common observation in support of this view 

 is the remarkable scarcity of algal growth in the shallow waters of tropical 

 shores as compared with that in temperate regions, and the fact that in the 

 Tropics, wherever sewage or other nitrogenous waste is poured into the sea, 

 a free growth of alg?e is found. 



At present no reliable and accurate chemical method of estimating the 

 combined nitrogen in sea-water exists ; hence the above theory can not be 

 directly put to the test. On the other hand, the existence of denitrifying 

 bacteria in temperate waters has long been known, and it would seem a fair 

 deduction that should this bacterial destruction of nitrates take place with 

 greater intensity and completeness in tropical than temperate waters, an ex- 

 planation of the relative scarcity of phyto-plankton in the former would be 

 offered. It was with the object of investigating this question that the pres- 

 ent work was undertaken. 



In Jamaica a measurement was made of the rate of denitrification in fluid 

 culture media inoculated with samples of sea-water, but isolation of the 

 bacteria on solid media was not attempted. The following method was 

 employed : 



Samples of sea-water were collected in sterilized stoppered bottles, from 

 the surface and from depths of 3 and 6 fathoms, from positions about 5 

 miles from shore, where, from a consideration of the wind and tide, the 

 water was probably under truly oceanic conditions and unaffected by the 

 neighboring land. 10 c.c. of these samples were added to 1,000 c.c. of a 

 modification of Gran's medium (see "Studien iiber Meeresbacterien," by 

 H. H. Gran, Bergens Museums Aarbog, No. 3, 1901). This culture fluid 

 was sterilized in glass flasks, and has the following composition : 



Potassium nitrate (KNO3) 0.5 gram. 



Sodium phosphate (Na2HP04, 12H2O) 0.25 gram. 



Calcium malate (C2H3(OH) i<QQQ>Ca) about 5.0 grams. 



Sea-water 1,000 .0 c.c. 



Calcium malate is only slightly soluble in water, so can be added in excess. 



The reduction of the nitrate to a nitrite was tested for by the addition of 



5 c.c. of 10 per cent sulphuric acid and 2 c.c. of a i per cent solution of meta- 



