are either long-term incubation of water samples in bottles 

 containing the suspected limiting elements, correlative inference 

 from proximate chemical analyses, or mathematical modeling. The 

 findings from such studies are often contentious. Most ocean 

 ecosystem models relate carbon fixation to the availability of 

 inorganic nitrogen; this seems to be applicable in most of the 

 oligotrophic open ocean, where the concentration of inorganic 

 nitrogen is vanishingly low. However, in the Subarctic Pacific and 

 the Southern Ocean, there is excess inorganic nitrogen and 

 phosphate; temperature, light and, more recently, iron have been 

 suggested to be limiting factors. Despite extensive observations, 

 ecologists are often still unable to unequivocally identify which 

 of these potential factors is the rate-limiting one. 



Several laboratory studies suggest that unique molecular 

 markers, diagnostic of specific limiting factors, may be used to 

 indicate the factor limiting growth in situ , and perhaps the degree 

 of limitation. For example, in cyanobacteria, a set of membrane- 

 bound proteins is synthesized in iron-deficient cells. In diatoms, 

 iron deficiency appears to alter the migration of the small subunit 

 of the carboxylation enzyme ribulose 1, 5-bisphosphate carboxylase. 

 Some phytoplankton species synthesize a chlorophyll protein (CP) 

 complex similar to CP43, but lacking a 100 amino acid domain 

 localized in the membrane lumen. The gene for the protein (which 

 is highly conserved in all oxygenic photoautotrophs) is controlled 

 by an iron-regulated promoter, which appears to be recognized 

 specifically under iron deficiency. In situ measurements of these 

 types of responses will provide a basis for examining the degree to 

 which phytoplankton photosynthesis may be limited by iron in the 

 ocean. Similar markers are known from laboratory studies on 

 nitrogen and phosphorus deficiency, so that a sense of in situ 

 molecular marker measurements may resolve which factor is rate 

 limiting. 



The growth rates of microbes in nature appear to be much lower 

 than their potential capabilities. For example, estimated growth 

 rates in aquatic systems are at least one to two orders of 

 magnitude lower than those for laboratory cultivated microbes. 

 Microbial activity is important for the regeneration of many of the 

 substrates for photosynthetic organisms. Determining which factors 

 limit microbial growth rates is essential to understanding the 

 fluxes of many biogeochemically important elements. In laboratory 

 studies, specific proteins and membrane lipids are induced in gram- 



