Seasonal variation in the chemical constituents of the brown algae Macrocystis integrifolia and Nereocystis luetkeana

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Abstract
Ash, mannitol, alginate content and inclusion levels of 27 inorganic elements in the brown algae M. integrifolia and N. luetkeana were monitored monthly over a period of 2 yr. In addition, the composition and viscosity of the alginate were determined. K, Na, Ca, Mg, and P were the major elements present. Others, in decreasing amounts, included I, Sr, B, Fe, As, Al, Zn, Ba, Mn, Cu and Co. Some of these elements were accumulated to high concentrations in the thallus when compared with their concentrations in seawater. Marked seasonal variations were observed for K and phosphate, with high levels in winter and low levels in summer. B and I showed a reverse trend. Among the major elements, Na, Ca and Mg showed no seasonal variations. Most of the inorganic elements, except K, were more abundant in the blade compared with the stipe. Leaching experiments with organic and inorganic solvents suggested that the alkali metals, Na and K, were present mostly as inorganic salts, while divalent cations, Ca, Mg and Sr, were probably bound to alginic acid. Organic solvents did not leach appreciable quantities of the inorganic elements. The ash content in both algae was minimum (24%) in summer and maximum (49%) during winter, mainly because of the higher concentration of K in winter. Nereocystis showed higher values of ash thean Macrocystis, and ash content was slightly higher in the stipe than in the blade tissues. The mannitol content ranged from 2 to 16% and alginate content from 19 to 31% of dry weight, both compounds showing higher values during the summer. Far more mannitol was present in the blade than in the stipe tissue, whereas the alginic acid content was slightly higher in the stipe than in the blade tissue. The mannuronate-guluronate ratio of alginate varied between 1.2 and 1.7 and the viscosity between 800 and 2000 cps.