Journal
EUROPEAN JOURNAL OF PHYCOLOGY
Volume 41, Issue 4, Pages 405-413Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1080/09670260600919831
Keywords
Dixoniella; mannitol cycle; osmolyte; Rhodellophyceae
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The mannitol cycle has been verified in a unicellular red alga (Rhodellophyceae) for the first time. All four enzymes involved in the cycle (mannitol-l-phosphate dehydrogenase, Mt1PDH: EC 1.1.1.17; mannitol-l-phosphatase, Mt1Pase: EC 3.1.3.22; mannitol dehydrogenase, MtDH: 1.1.1.67; hexokinase, HK: 2.7.1.1.) were detected and characterized in crude algal extracts from Dixoniella grisea. These enzymes, with the exception of Mt1Pase, were specific to their corresponding substrates and nucleotides. The activities of enzymes in the anabolic pathway (fructose-6-P reduction by Mt1PDH and mannitol-6-P reduction by Mt1Pase) were at least 2- to 4-fold greater than those of the catabolic pathway (mannitol oxidation by MtDH and fructose oxidation by HK). There appears to be, therefore, a net carbon flow in D. grisea towards a high intracellular mannitol pool. The mannitol cycle guarantees a rapid accumulation or degradation of mannitol within algal cells in response to changing salinity in natural habitats. Moreover, the demonstration of the mannitol cycle within the Rhodellophyceae provides evidence that this metabolic pathway is of ancient origin in the red algal lineage.
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