mamtolo 60
Determination of endogenous respiratory rate
m Polve v3.a and F o soiraliso sooooooooooeoaoot^oo 60 Respiratory carbon losses by Pelvetia during
6L103COILOÏ1o
starvation of emersed Pelvetia in the dark,.**, 66 3o The possible role of mannitol in tlie maintenance of
Introduction
The occurrence, physiology and biochemistry of low molecular weight carbohydrates in the Phaeophyceae is moderately well documented© Touster and Shaw (I962) have
considered metabolic aspects of polyols, and Lewis and Smith (1967) have dealt extensively with the distribution, physiology and metabolism of polyols in plants© It is here proposed to bring together information concerning the Phaepphyceae©
Of the various low molecular weight carbohydrates found in the Phaeophyceae, the acyclic polyols, and particularly mannitol, are quantitatively of the greatest significance©
Lewis and Smith (196?) have listed twenty~seven genera in which mannitol has been found, and indeed there is only one report
of the definite absence of this polyol in members of Phaeophyceae (Kylin, 1944)©
Quantitative estimations reveal that mannitol may
contribute a significant proportion of the dry weight of brown algae, and this proportion has repeatedly been demonstrated
to be controlled largely be environmental factors, particularly "seasonal change"© The results of Black (I960) suggest that for Laminaria hyperborea day length controls mannitol levels; he reports for the Orkneys, an annual minimum of 8% in March and a. summer maximum of 669^ dry weight, whereas further South, In Cullipool* the same species had a summer maximum of Z G % o
42
In each case the polysaccharide laminarin showed a parallel variation© In South Africa, where seasonal variation is less marked, Holdt et al# (1966) demonstrated winter mannitol
maxima and an insignificant laminarin change©
Reproductive status of the plants has also been shown to affect mannitol levels© Moss (1948, I960) showed that the negative gradient of mannitol content from apex to base in
vegetative thalli of Fucus was lost at the onset of reproductive activity* The mannitol content of receptacles diminished with maturity, and Moss (I960) suggested that the mature gametes do not store this polyol©
Bidwell (1968) demonstrated that mannitol was the major soluble product of photosynthesis in seven genera of Phaeophyceae®
l4
From data obtained in experiments in which mannitol-l- C was offered to Fucus vesiculosus, Bidwell and Ghosh (1962)
concluded that mannitol was not, however, the respiratory substrate© Explanation of this anomaly in terms of the vast difference between the internal and external specific activities
of mannitol has been offered by Lewis and Smith (1967), and in response to work by Yamaguchi et al© (1966) on Eisenia bicyclis, Bidwell (1967) has re**appraised the situation and come to
agree with the contemporary opinion that mannitol is the respiratory substrate in these algae©
Mannitol has also been implicated in other roles, such as the maintenance of high internal osmotic pressure (Lewis and Smith, I967)# which is usually somewhat higher than that
of sea-water (Guillard, 1962)© Also, Bourne (1968) has suggested that polyols might function in the protection of injured tissue©
In tissues other than those of marine algae certain
other features of polyol metabolism have been established© Edson (1963) has demonstrated that co«enzymes and polyol dehydrogenases mediate the interconversion of sugar isomers via polyols in male accessory glands® This system has also been described in fungi, but not in the algae®
Since polyols are more highly reduced than the
corresponding sugars they are eminently suitable as storage
compoundso It has been proposed by Touster and Shaw (I962) that where polyols are not serving specifically in metabolism they might function in the transfer of hydrogen on co-enzymes, and thereby in the control of the various metabolic pathways*
In addition to mannitol two other polyols are known to occur in the Phaeophyceae® One of these, volemitol, was found by Lindberg and Paju (1964) in Pelvetia canaliculata, the only brown alga known to contain a heptitol® The other, laminitol, is a cyclic polyol (cyclitol), and has been found in L# hyperborea (Lindberg and McPherson, 1964), F® spiralis
44
and Desmarestia aculeata (Bouveng and Lindberg, 1964)@ Laminitol is present only in trace amounts, and like volemitol it has no known metabolic role®
Besides the free polyols, Lindberg and his colleagues have isolated and identified polyol glucosides and acetates from various brown algae, and Lindberg and McPherson (1964)
have made positive identification of the free reducing sugars larainarobiose and laminarotriose in L© hyperborea® These might have been present as the biosynthetic precursors of laminaran or products of its autolytic degradation© The presence of glucose and fructose is attributed to the hydrolysis of sucrose, and trehalose has been tentatively identified in F© vesiculosus by Lindberg (1966)® These data are summarised in table 3;1®
- absent
T trace amount present
1. Lindberg and McPherson (1934)