Ecology of Algae, Protozoa. Fungi and Viruses 31 



dark under large ice masses. In the laboratory, Joyce Lewin has 

 demonstrated that a number of naviculoid diatom species can 

 grow in the dark with glucose as a carbon source, but she failed 

 to grow many of the centric forms in this way. This is difficult 

 to reconcile with Bursa's findings, so it looks as though hetero- 

 trophic growth in phytoplankton may occur by metabolic path- 

 ways not yet known. I regard Dr. Lewin's findings as very im- 

 portant. 



Osnitskaya (Chapter 33) has demonstrated heterotrophic 

 growth among the purple sulfur bacteria, and has shown tliat 

 a Chromatium could utilize lactic and propionic acids with 

 minimal illumination. She suggests that these organisms use a 

 photoheterotrophic nutrition when light becomes poor. This 

 would explain the occurrence of puiple sulfur bacteria below 

 deep algal mats in estuaries. 



From the papers given in this symposium, we must recognize 

 facultative heterotrophy as a common phenomenon in the sea, 

 spread over a wide range of photosynthetic organisms. The ques- 

 tion is to what extent does heterotrophy occur in the sea among 

 chlorophyll-bearing organisms, and as Provasoli suggested to me, 

 do they forni at times a metabiotic system in which several hetero- 

 trophic organisms combine to produce a system which is autotro- 

 phic overall? Parker and Bold (5) have shown a carbon-dioxide- 

 oxygen interchange between Chlamydomonas and Streptomijces 

 which could suggest a mechanism for this. One might get some 

 leads from thermo-dynamic considerations, especially as hydro- 

 static pressure may considerably alter energy relationships. Osnits- 

 kaya's work raises the tliought that photoheterotropliic assimila- 

 tion of carbon might take place in the algae as well as in tlie pur- 

 ple sulfur bacteria, especially in regions of minimal light intensity. 



PRODUCTIVITY STUDIES 



The study of what is called "productivity" is made more 

 difficult by a lack of precision in our terminology. Apart from 

 tliat, the measurement of productivity is extreme]\- difficult as so 

 many illunderstood factors are involved. Tlie most modern con- 

 cept of measurement is "carbon assimilation" using added C^^ 

 to estimate this. Chlorophyll determination still has its adherents, 



