Turnover of hexitols in the marine macroalga Himanthalia elongata (Phaeophyta, Fucales)

C-13 nuclear magnetic resonance spectroscopy (NMR) has been used to study the metabolic flux of carbon through the intracellular pools of the isomeric hexitols D-altritol and D-mannitol in Himanthalia elongata. Natural abundance C-13 NMR spectra of freshly collected plant material showed altritol as the dominant intracellular low-molecular-weight organic solute, with mannitol present at less than 20% of the altritol concentration. plant material incubated in seawater medium with added C-13-enriched bicarbonate showed a rapid increase in the C-13 signal due to mannitol, with an increase of more than 12-fold in under 48 h, in contrast to the altritol signal, which increased by approximately 2-fold over the same period. The intracellular mannitol signal decreased rapidly when C-13-enriched plant material was transferred to a non-enriched medium, while altritol showed a slower decline. These results are consistent with previous observations on the effects of salinity on the intracellular hexitol pools of H. elongata, suggesting that mannitol is more rapidly metabolised than altritol. Estimates of the half-time for tracer exchange support this view, with a half-time for the turnover of altritol (> 400 h) which is an order of magnitude greater than that for mannitol turnover (approximate to 50 h).