The homeostatic potential (i.e. the relative invariability of intracellular composition, in spite of the changes occurring in the environment) of organisms in relation to the type and intensity of environmental perturbations is of paramount importance because it contributes to determine the ecological and evolutionary success of an organism. The hypothesis underlying this study is that there are circumstances in which homeostasis, rather than acclimation, represent a favorable physiological strategy.
This may occurr when environmental stresses are limited in time or when they exceed the limits of the physiological plasticity of cells.
The effects of prolonged darkness (non-acclimatable stress) and high external C/N ratio (shortterm, acclimatable stress) on growth, metabolism and resource allocation were studied in the dinoflagellate Protoceratium reticulatum and in the cyanobacterium Phormidium autumnale.
Cells of P. reticulatum showed relatively small (1 order of magnitude smaller than the C/N increment imposed to the growth medium) or null changes in growth rates, photosynthesis, respiration, cell composition and enzymatic activities, as a result of the increased concentrations of CO2. Dinoflagellates are one of the largest components of Ocean phytoplankton and a crucial component of coral endosymbiotic associations; it follows that the relationship between dinoflagellates growth and atmospheric CO2 increase is likely to play a central role in the evolution of the global C cycle in the next decades. The responses of P. autumnale to 3-4 weeks of complete darkness were strictly homeostatic. In spite of the light deprivation and the absence of any other external energy source, cells maintained constant respiration rates and balanced (i.e. in the same proportion as their initial content in the cells) macromolecular cell composition. Active metabolism and balanced cell composition were mantained using intracellular resources, as suggested by the 50% size decrease which occurred over 4 weeks of light deprivation. It is obvious that homeostatic responses based on consumption of intracellular resources cannot continue indefinitely. The limits of homeostatic response represent a fundamental biological question and further investigations are required to attain a thorough comprehension of this matter and its ecological and evolutionary consequences.
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