Production, Chemical Structure, Metabolism, and Biological Function of the (1→3)-Linked, β3-D-Glucans in Diatoms

Abstract

Glucans act as food-reserve materials in all living organisms. The (1→3)- linked β-D-glucans, often called chrysolaminaran, perform this function in at least six of the 11 recognized algal phyla, including the Bacillariophyta, or diatoms. In culture their content of glucan is usually low in the exponential phase of growth, but it increases markedly in the stationary phase, when N or P is in limited supply. Light and temperature also affect the content of glucan, but salinity does not. The higher the N/P ratio in the medium, the lower the net production of glucan in the diatoms. It is easy to determine chrysolaminaran quantitatively by a simple acid-extraction procedure, followed by determination of the total carbohydrate in the extract, or a specific determination of D-glucose after acid hydrolysis. The glucan can be isolated in pure form by extraction, dialysis, and freeze-drying. The ratio of protein to glucan in diatoms is a sensitive indicator of nutritional status, both in laboratory cultures and in natural phytoplankton populations. A ratio of > 1 normally indicates nutrient sufficiency, but it varies with the species, and there are also diurnal variations. Chrysolaminaran is not, so far as we know, synthesized by marine bacteria, and it is therefore uniquely a product of phytoplankton in the open sea. The chemical structure of chrysolaminaran is that of a (l→3)-linked β-D-glucopyranan with occasional branching through positions 2 and/or 6. The molecular weight varies from 6000 to 13,000. There have been no biosynthetic studies, but [^l4C]NaHCO₃ is quickly incorporated into the glucan in the light. A degradative enzyme, exo-(I→3)- β-D-glucanase, has been isolated from marine diatoms, and conversion of the glucan into CO₂ by respiration has been observed in marine populations. Chrysolaminaran seems to act as a reserve polysaccharide on both a short-term and a long-term basis. Its level in the cells seems to be regulated by the nitrogen supply.

Keywords:

• β-(1 → 3)-D-glucans
• biological function
• metabolism
• chemical structure
• diatoms
• production
• ecological aspects