1989), marine Vibrio species (Nystrom et al., 1990), and other 

 gram-negative bacteria (Grimes et al., 1986) suggests that the 

 identification of growth states might provide an alternative to 

 measurement of growth rate. Organisms exposed to the stress of 

 severe starvation exhibit so many new proteins that their 

 adaptation is reminiscent of sporulation; that is, bacteria may 

 have a complex developmental response to nutrient limitation 

 involving more than one state. This developmental process results 

 in a dormant form of the organism that is more energy-efficient and 

 less susceptible to heat and other environmental insults (Chesbro 

 et al., 1990; Morita, 1985). Thus, the growth state of an organism 

 in a natural population may provide clues not only to the nature of 

 the stress (carbon, nitrogen, or iron limitation) , but also to the 

 extent and duration of the stress. 



Identifying Dominant Metabolic Pathways in an Organism 



The importance of identifying the dominant metabolic pathways 

 can be seen from two examples: 



Many phytoplankton in the chrysophyte/prymnesiophyte line live 

 in close association with their bacteria and cannot be maintained 

 in axenic culture. The nature of the association is not yet clear. 

 In some cases, the bacteria may be vitamin sources; in others they 

 are alternate carbon sources. The method of ingestion is 

 phagotrophy; that is, the phytoplankton ingest (and presumably 

 digest) whole bacteria. While at first glance this life style may 

 seem inefficient, in a fluctuating environment (delta light and 

 nutrients) , it makes good sense. The phytoplankton photosynthesize 

 when conditions are optimal, but rely on phagotrophy when they are 

 not, with symbiosis optimizing DOC transfer or respiration back to 

 CO2. Thus, the unit of selection (unit of persistence in the 

 environment) is the association; similarly, with regard to fluxes 

 of energy and carbon, the unit again is the association and not the 

 individual. 



Possibly there is a metabolic switch from photoautotrophy to 

 phagotrophy, analogous to a switch from the active to the dormant 

 stage. Here also, we need to identify pathways and products that 

 would provide biomarkers. A comprehensive integrative method for 

 large-scale studies depends on multiple stable isotope technigues 

 to trace autotrophic and heterotrophic inputs with natural 



II-2 



