The unicellular, acido- and thermophilic red alga Galdieria sulphuraria is unique in its ability to grow heterotrophically on at least 27
different sugars and polyols. The enzymatic machinery necessary for metabolizing this variety of compounds is constitutively expressed in
the alga. The uptake system, however, has to be induced. From in vivo studies we conclude that the uptake system consists of several
transporters with a partly overlapping substrate specificity. These transporters can be grouped into hexose, polyol and pentose
transporters. The mode of induction depends on the substrate supplied for heterotrophic growth. When autotrophic cells were supplied
with hexoses, hexose and polyol transporters were induced from the onset of heterotrophic conditions. Depletion of substrate in the
medium triggered the induction of other transporters. Dulcitol, as well as other polyols and pentoses, led to a co-induction of all
transporters as judged from induction and competition experiments. The heterotrophic capabilities of Galdieria are intriguing, because the
natural habitat of the alga contains only traces of organic carbon. The substrates for heterotrophic growth apparently originate from
decaying cells releasing hydrolysed cell material. Trace amounts of glucose were sufficient for the induction of sugar uptake in Galdieria.
The co-induction of transporters obviously enables the cells in endolithic mats to import a wide spectrum of organic carbon in order to
survive periods of light limitation.