Respiration, and growth-efficiency of coastal prokaryote communities in continuous cultures under different growth rates and temperatures.

Organotrophic prokaryotes in aquatic environments account for about half of community respiration in surface oceans and are key trophic links in the plankton food web connecting dissolved organics and higher trophic levels. The transfer efficiency is partially characterized by the ratio of prokaryote respiration rates (r, day^-1) to growth rates (m, day^-1) and the resulting growth efficiency (Y). Much literature has been published about the response of these parameters to temperature in monospecific cultures, but little is known about the response of a community of pelagic prokaryotes were the sum of the genotypes and phenotype define the physiological potential. We inoculated 10 turbidostats and 39 chemostats with coastal bacteria and measured CO₂ production, carbon biomass and cell abundance, with m ranging from 0.05 to 62 day^-1 between 10 and 26^oC. Under substrate limited conditions, common in the ocean, r showed no significant trend with temperature and was proportional to m implying constant Y. Under temperature-limited, nutrient replete growth the m of coastal prokaryote communities increased with temperature but r decreased (Q₁₀: 0.4), resulting in an increase of Y with temperature (Q₁₀: 2.5). The carbon demand rate (b, fmol C (cell day)^-1) of turbidostat cultures showed a very high Q₁₀ of 8.4. Casting the data in the framework of the metabolic theory of ecology (MTE), the physiological rates normalized to cell carbon showed no significant changes with temperature using either respiration or carbon demand as a proxy for physiological rate. Our results suggest that physiological patterns related to temperature are very different under nutrient limited or replete conditions and under neither condition it followed the pattern expected by MTE.