Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?

Previous studies of the subsurface biosphere have deduced average cellular doubling times of hundreds to thousands of years based upon geochemical models. We have directly constrained the in situ average cellular protein turnover or doubling times for metabolically active micro-organisms based on cellular amino acid abundances, D/L values of cellular aspartic acid, and the in vivo aspartic acid racemization rate. Application of this method to planktonic microbial communities collected from deep fractures in South Africa yielded maximum cellular amino acid turnover times of ~89 years for 1 km depth and 27 °C and 1-2 years for 3 km depth and 54 °C. The latter turnover times are much shorter than previously estimated cellular turnover times based upon geochemical arguments. The aspartic acid racemization rate at higher temperatures yields cellular protein doubling times that are consistent with the survival times of hyperthermophilic strains and predicts that at temperatures of 85 °C, cells must replace proteins every couple of days to maintain enzymatic activity. Such a high maintenance requirement may be the principal limit on the abundance of living micro-organisms in the deep, hot subsurface biosphere, as well as a potential limit on their activity. The measurement of the D/L of aspartic acid in biological samples is a potentially powerful tool for deep, fractured continental and oceanic crustal settings where geochemical models of carbon turnover times are poorly constrained. Experimental observations on the racemization rates of aspartic acid in living thermophiles and hyperthermophiles could test this hypothesis. The development of corrections for cell wall peptides and spores will be required, however, to improve the accuracy of these estimates for environmental samples.

Apoptosis in rat gastric antrum: evidence that regulation by food intake depends on nitric oxide synthase

The turnover of the epithelium of the gastrointestinal tract is regulated by a balance between cell multiplication and cell loss. We examined the effects of starvation on apoptosis in endocrine and other epithelial cells of rat antropyloric mucosa. Apoptosis was determined by the TUNEL reaction combined with immunocytochemical staining for gastrin and somatostatin. Apoptotic cell morphology was determined by bisbenzimide staining for DNA. Both gastrin and somatostatin cells showed a significantly lower apoptotic index than the general epithelium. This agrees with the longer turnover kinetics of gastric endocrine cells. On starvation, the apoptotic index of the general epithelium and of the gastrin but not of the somatostatin, cells increased significantly. This was prevented by the nitric oxide synthase (NOS) inhibitor L-NAME but not by its inactive stereoisomer D-NAME. Immunoreactive neuronal NOS was present in somatostatin cells, in nonendocrine cells predominating in the surface and pit epithelium, and in rare nerve fibers. Endothelial cell NOS was present in vessels, whereas the inducible isoform was barely detectable. Thus, endogenous NOS isoforms participate in regulating antropyloric epithelial apoptosis during starvation. The close paracrine relation between somatostatin cells and gastrin cells suggests that the former regulates apoptosis of the latter through release of NO.

Surface Tension Measurements of Surface Freezing in Liquid Normal Alkanes

Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30 degrees C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n </= 50. The measured carbon number and temperature dependence of the surface tension is interpreted within a simple thermodynamical model based on known bulk latent heat data and surface energy considerations. The vanishing of the surface phase for n </= 14 is a possible transition from surface freezing to surface melting behavior.