Biochemical characterization of mannitol metabolism in the unicellular red algaDixoniella grisea(Rhodellophyceae)

Abstract

The mannitol cycle has been verified in a unicellular red alga (Rhodellophyceae) for the first time. All four enzymes involved in the cycle (mannitol-1-phosphate dehydrogenase, Mt1PDH: EC 1.1.1.17; mannitol-1-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.

Mannitol-1-phosphate dehydrogenase activity in Ectocarpus siliculosus, a key role for mannitol synthesis in brown algae

“…Activity was detected only in the direction of F6P reduction, and not for the reverse reaction. A similar observation was made in the green alga Platymonas (Richter and Kirst 1987), while both activities were measured from cell-free extracts of S. pacificum (Ikawa et al 1972) and red algae (Karsten et al 1997;Eggert et al 2006). It is difficult to compare the level of F6P reduction activity in Ectocarpus with values determined for other algae in part because, in contrast to previous reports, we neither carried out any fractionation steps by ammonium sulphate treatment, nor purified M1PDH proteins before conducting our analysis.…”

“…Among marine photosynthetic organisms, mannitol has been reported in some green microalgae of the class Prasinophyceae (Kirst 1975(Kirst , 1989Hellebust 1976), in a few red algae (Rhodophyceae) such as the pluricellular seaweeds of the genus Caloglossa (Karsten et al 1992, Karsten andWest 1993;Mostaert et al 1995) and the unicellular Dixoniella grisea (Eggert et al 2006(Eggert et al , 2007, and in several brown algae (Phaeophyceae). In the latter organisms, mannitol can compose up to 20-30% of the dry weight, although its levels vary among species Zubia et al 2008).…”

“…In red algae, occurrence of the four enzymatic activities of the mannitol cycle was proven in the eulittoral LGZZ indicates the occurrence of the gene in one of the 34 linkage groups of the genetic map produced for this alga (Heesch et al 2010) macroalga Caloglossa leprieurii by Karsten et al (1997), and M1PDH and M1Pase were purified to homogeneity from another species of Caloglossa, C. continua (Iwamoto et al 2001(Iwamoto et al , 2003. Both enzymatic activities were also measured later in the microalga Dixoniella grisea (Eggert et al 2006(Eggert et al , 2007. In brown seaweeds, mannitol metabolism is not well understood.…”

Characterization of mannitol metabolism genes inSaccharinaexplains its key role in mannitol biosynthesis and evolutionary significance in Laminariales

“…Mannitol is one of the most widely occurring sugar alcohols in nature and is produced by a variety of living organisms, including bacteria, fungi, terrestrial plants, and algae (Iwamoto and Shiraiwa, 2005; Rousvoal et al ., 2011). The presence of mannitol has been reported in primary endosymbiotic algae, such as those belonging to Chlorophyta (Dickson and Kirst, 1987; Dittami et al ., 2011) and a few species of Rhodophyta (Karsten et al ., 1997; Eggert et al ., 2006), as well as in secondary endosymbiotic Ochrophyta algae, such as brown algae (Ji et al ., 1980; Karsten et al ., 1991; Gylle et al ., 2009) and diatoms (Hellebust, 1965). As one of the main primary photosynthetic products and storage compounds in Laminariales (Kremer, 1980; Wei et al ., 2013; Xia et al ., 2016), mannitol can represent up to 15–26% of the dry weight of the organism (Black, 1948; Reed et al ., 1985).…”

“…Later, a more comprehensive assessment across various algal lineages proved that this gene may have been present in nonphotosynthetic eukaryotic host cells involved in endosymbiosis (Tonon, 2017). Native M1PDH and M1Pase activity has previously been characterized in cell-free extracts from red algae, namely Dixoniella grisea (Eggert et al ., 2006), Caloglossa continua (Iwamoto et al ., 2001; Iwamoto et al ., 2003), and Caloglossa leprieurii (Karsten et al ., 1997), and from brown algae, namely Spatoglossum pacificum, Dictyota dichotoma, Platymonas subcordiformis , and Laminaria digitata (Ikawa et al ., 1972; Grant and Rees, 1981; Richter and Kirst, 1987); however, the genes encoding these enzymes have not yet been identified. Recent structural and functional genomics research on E. siliculosus has analyzed mannitol metabolism, and recombinant Ectocarpus M1PDHcat (EsiM1PDHcat; containing only the catalytic domain) and M1Pase2 (EsiM1Pase2) have been characterized (Rousvoal et al ., 2011; Groisillier et al ., 2014; Bonin et al ., 2015).…”