Chapter V — 77 — Distribution in the Sea 



microbiological variations in such a constant environment. The frag- 

 mentary observations which have been made show nothing to the con- 

 trary. 



Effect of other organisms : — Prof. W. E. Allen of the Scripps Institu- 

 tion has kept a daily record of the abundance of diatoms and dinoflagel- 

 lates in samples of local sea water during the last 25 years. A comparison 

 of these phytoplankton records with the bacterial plate counts during 

 the last ten years shows that, in general, the largest bacterial populations 

 are found in water containing the most phytoplankton. This is shown 

 better from an inspection of the daily or weekly averages than from the 

 monthly averages, because the abundance of phytoplankton fluctuates 

 within rather wide limits from day to day as does the bacterial population 

 of sea water. It is for this reason that Allen (1941) stresses the necessity 

 of continuous observations over a long period of time. 



Curves for the vertical distribution of bacteria follow very closely the 

 curves for the vertical distribution of diatoms and other phytoplankton, 

 as is illustrated by Figure 7 on page 67. 



Since phytoplankton constitute the principal source of food of bac- 

 teria in the sea, as well as providing solid surfaces for attachment, it is not 

 surprising that the abundance of bacteria is closely related to the abun- 

 dance of phytoplankton. The total quantity of organic matter in sea 

 water varies very little with depth until the bottom of the sea is reached. 

 Actually there is often somewhat more organic matter in water below 

 than in the euphotic zone in spite of the activities of photosynthetic 

 organisms. Nevertheless, bacteria are usually most abundant where 

 there is particulate organic matter. 



In studying the relation of marine bacteria to plankton diatoms in 

 which bottles filled with sea water were suspended in the sea. Gran (1933) 

 observed that the abundance of bacteria increased and decreased with the 

 diatoms. While there is nothing in Gran's experiments to indicate 

 whether the bacteria were attached to the diatoms or whether the dia- 

 toms provided the saprophytic bacteria with organic matter for growth, 

 the observation provides supplementary evidence of the close relationship 

 between bacteria and diatoms in the sea. 



Waksman et al. (1933c) analyzed sea water and plankton tows from 

 the same water for the presence of bacteria. A number 20 silk net was 

 used, which until the pores become clogged would not collect many nan- 

 noplankton or bacteria. However, large numbers of bacteria were found 

 in the plankton tows, presumably because the bacteria were attached to 

 the plankton. From 500 to 2,270 times as many bacteria were found per 

 unit volume of diatom plankton as were found in the unstrained water. 

 These workers concluded that this indicates that a definite parallelism 

 exists between the bacteria and the plankton content of the sea. The 

 actual existence of such a relationship is substantiated by the fact that the 

 bacteria from the diatom tow showed a considerable abundance of agar- 

 liquefying organisms. While one encounters only infrequently such or- 

 ganisms in the free water, they were found to make up 5.7 to 6.7 per cent 

 of the total bacterial flora in plankton tows, as determined by the plate 

 method. 



According to Waksman et al. (1933c) bacteria occur only to a very 

 limited extent floating free in water, most of them being attached to 

 plankton organisms. Bacteria may live upon dead plankton or upon the 



