Bryophyte C:N:P stoichiometry, biogeochemical niches and elementome plasticity driven by environment and coexistence.

Ecological stoichiometry and studies of biogeochemical niches have mainly focused on plankton and vascular plants, but the phenotypically closest modern relatives of early plants, bryophytes, have been largely neglected. We analysed C:N:P stoichiometries and elemental compositions (K, Na, Mg, Ca, S, Fe) of 35 widely distributed bryophyte species inhabiting springs. We estimated bryophyte C:N:P ratios and their biogeochemical niches, investigated how elementomes respond to the environment and determined whether they tend to diverge more for coexisting than non-coexisting individuals and species. The median C:N:P was 145:8:1, intermediate between Redfield's ratio for marine plankton and those for vascular plants. Biogeochemical niches were differentiated amongst species and were phylogenetically conserved. Differences in individual and species-specific elementomes increased with coexistence between species. Our results provide an evolutionary bridge between the ecological stoichiometries of algae and vascular plants and suggest that differences in elementomes could be used to understand community assemblages and functional diversity.

Monocarboxylate transporter 1 is up-regulated in Caco-2 cells by the methionine precursor DL-2-hydroxy-(4-methylthio)butanoic acid.

The methionine precursor, DL-2-hydroxy-(4-methylthio)butanoic acid (HMTBA), is a synthetic source of dietary methionine, which is widely used as a poultry nutritional supplement. In the intestinal epithelium, HMTBA transport across the apical membrane is mediated by monocarboxylate transporter 1 (MCT1). The first step in biological utilisation of this methionine precursor is the stereospecific conversion of HMTBA to the corresponding keto acid. In the present study, the regulation of trans-epithelial HMTBA transport was investigated in Caco-2 cell monolayers. Differentiated Caco-2 cells were maintained under control conditions (apical compartment: 0.2 mmol/L L-methionine) or in a HMTBA-enriched medium (2 mmol/L HMTBA). The effect of culture on HMTBA transport was evaluated from apical and basolateral kinetic parameters. MCT1 and MCT4 immuno-localisation and gene expression were investigated by confocal microscopy and real-time quantitative RT-PCR, respectively. The results indicated that apical MCT1 was up-regulated by exposure to HMTBA (1.4-fold increase in Vmax without changes in Km). Moreover, total monolayer MCT1 immunoreactivity increased 1.8-fold in HMTBA-supplemented cultures, this effect mainly being localised at the apical membrane. Functional and immuno-localisation data suggest involvement of MCT1 and MCT4 in basolateral HMTBA transport, although, in this case, no effect was observed for HMTBA-enrichment. Molecular analysis confirmed MCT1 mRNA up-regulation (1.8-fold), with no effect on MCT4 mRNA expression. Thus, exposure to HMTBA up-regulates the trans-epithelial transport of this methionine precursor by increasing the expression and the transport capacity of apical MCT1.