COMMUNITY ORGANIZATION: METABOLISM 



501 



marine bacteria, and this view may be 

 maintained until the same species or 

 strains are found growing naturally in non- 

 marine habitats. Numerous fresh-water 

 bacteria have been found that can develop 

 in salt concentrations higher than those of 

 sea water, although the death rate of many 

 fresh-water bacteria is thought to be liigh 

 in salt water (Burke, 1934). 



Our general conclusion is that bacterial 

 activity is of fundamental importance in 

 the metabohsm of all major communities; 

 that these activities are essentially similar in 

 all major communities; and that these 

 processes are carried out by many ecologi- 

 cally equivalent species of bacteria. 



Ecologists as a group have been more 

 aware of the place of the second key in- 

 dustry, photosynthesis, in the metabohsm 

 of communities than they have been of the 

 role played by bacteria. The photosynthetic 

 process in which chlorophyll synthesizes 

 carbohydrate in the presence of water, car- 

 bon dioxide, and radiant energy from the 

 sun, has been investigated by many plant 

 physiologists and biochemists; its impor- 

 tance has been noted in previous pages. 

 We are concerned now with the more spe- 

 cific community aspects of this funda- 

 mental industry. 



Photosynthetic carbohydrate production 

 is an anaboUc process from the point of 

 view of the metabolism of the whole com- 

 munity. The photosynthetic output is 

 limited chiefly by intensity and wave- 

 lengths of light, cloudy weather, atmos- 

 pheric dust, turbidity, amount of available 

 carbon dioxide, and temperature of the at- 

 mosphere. All these conditions act as a 

 whole to regulate green plant production, 

 growth, and well-being. Where plants com- 

 pete for light or animals reduce the 

 chlorophyll by direct or indirect actions, 

 this productivity, growth, or health is cor- 

 respondingly accelerated or retarded or 

 otherwise afiFected. 



For example, in communities where the 

 plants are relatively fixed, as in forests, the 

 shape of the leaf, thickness of the leaf 

 blade, amount of mesophyll, amount of 

 stem elongation and crown volume are 

 modified by the physical and the biological 

 environment. Intensity and composition of 

 light and direction of the light beams are 

 especially important influences of the oper- 

 ational physical environment (Warming, 



1909; Coulter, Barnes, and Cowles, 1911). 

 Thus a plant species may be, first tolerant 

 or intolerant of sun or shade in various de- 

 grees; second, the total plant population 

 may adjust to the light gradient by posi- 

 tional stratification; and third, the indi- 

 vidual plants may adjust to seasonal and 

 daily permutations of forest illumination. 



On the other hand, in communities 

 where the chlorophyll-bearing organisms, 

 the major "producers" of Thienemann 

 (1926), are not fixed, as in the marine 

 photic zone, the response to reduced fight, 

 as a consequence of increase in population 

 density above them or for other reasons, is 

 a general movement upward by those capa- 

 ble of swimming. Thus the shade species 

 of Ceratium (Graham, 1941) move verti- 

 cally in response to changes in fight inten- 

 sity, and this response is ecologically equiv- 

 alent to the several positional adjustments 

 of the leaves of forest plants. This extends 

 the postulate of Nielsen previously noted 

 (p. 448). 



In aquatic communities the original car- 

 bohydrate "producers" are chiefly floating 

 algae or weakly swimming chlorophyll- 

 bearing flagellates, rooted vegetation, and, 

 to a lesser degree, photosynthetic auto- 

 trophic bacteria. 



The general process will be discussed 

 with respect to the photic zone of the ma- 

 rine community, first, by a brief descrip- 

 tion of the chief groups of nonbacterial 

 "producers," and second, by an analysis of 

 the diatom cycle of the open North Atlantic 

 waters. The marine photic zone holds pro- 

 digious numbers of a few groups of these 

 primary producers composed of a small 

 number of basic types. Five such groups 

 deserve a brief discussion. 



1. The only large seaweed that is free- 

 floating on high seas belongs to species of 

 the brown seaweeds or Sargassiim (Phaeo- 

 phyceae). These algae are broken from 

 their littoral rock habitats and reproduce 

 vegetatively as they are carried by ocean 

 currents. Before the death and disintegra- 

 tion of this alga, it forms the food and shel- 

 ter of many zooplankters, some of which 

 apparentlv may not live elsewhere (Coker, 

 1938, 1947). 



2. Green algae (Chlorophyceae), abun- 

 dant in surface layers of fresh-water, are 

 represented in the sea by a few species that 

 may become locally abundant. An example 



