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

Abstract

Ash, mannitol, alginate content, and inclusion levels of 27 inorganic elements in the brown algae Macrocystis integrifolia and Nereocystis luetkeana were monitored monthly over a period of 2 years. In addition, the composition and viscosity of the alginate were determined. Potassium, sodium, calcium, magnesium and phosphorus were the major elements present. Others, in decreasing amounts, included iodide, strontium, boron, iron, arsenic, aluminum, zinc, barium, manganese, copper, and cobalt. 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 potassium and phosphate, with high levels in winter and low levels in summer. Boron and iodide showed a reverse trend. Among the major elements, sodium, calcium, and magnesium showed no seasonal variations. Most of the inorganic elements, except potassium, were more abundant in the blade compared with the stipe. Leaching experiments with organic and inorganic solvents suggested that the alkali metals, sodium and potassium, were present mostly as inorganic salts, while divalent cations, calcium, magnesium, and strontium, 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 potassium in winter. Nereocystis showed higher values of ash than 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.