MARINE ORGANISMS AND ENVIRONMENTAL INFLUENCES 283 



gested by Braarud (1937) in the case of Thalassiosira norden- 

 skioeldi. 



The main objection to the technique used may be that the 

 cultures were not bacteria-free. In such a case two effects may be 

 responsible for the different growth rates of a culture at different 

 temperatures: (1) the response of the alga to the temperature 

 factor, the response which we are looking for, and (2) the response 

 of the bacterial population in the culture which may, indirectly, 

 influence the rate of growth of the algal population. As long as we 

 have no parallel temperature experiments with and without bac- 

 teria as associates, we are unable to judge how important the 

 latter effect may be, but the recent discovery of the vitamin re- 

 quirements of the plankton algae lends some support to such a 

 view. It would not seem unreasonable to suggest that the dif- 

 ference between the temperature optima observed in the experi- 

 ments with strains of Peridinium trochoideum and Prorocentruni 

 micans from widely separated localities might be due to the dif- 

 ference in the bacterial populations of the two cultures, since the 

 latter may presumably provide vitamins necessary for growth. 



The other alternative, that temperature is not a decisive en- 

 vironmental factor for the abundance of a species such as Thalas- 

 siosira nordenskioeldi at low temperature and its absence at higher 

 temperature, as indicated by the experiments, finds some support 

 in the fact that the rise in temperature in spring in northern At- 

 lantic waters coincides with a decline in the supply of inorganic 

 nutrients. Species demanding fairly high concentration of nutrients, 

 therefore, might show a similar seasonal occurrence as a cold 

 water species. Only new experiments with bacteria-free cultures 

 can give us a clearer picture of the actual temperature effect upon 

 growth. 



From the experiments on the salinity and temperature factors 

 it may be concluded that the tolerance of the various species to 

 extreme conditions should be studied with special care. 



The two remaining environmental factors of this group, light 

 and nutrient supply, can be dealt with quickly as very few ob- 

 servations are available for a discussion of specific differences. 

 Photosynthesis experiments with cultures of diatoms suspended 



