Cellulose synthesizing complexes in some giant marine algae

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Abstract
The structure of putative cellulose synthesizing complexes (TCs) has been studied in giant marine algae and is discussed in relation to the assembly of cellulose microfibrils. Including previous work, 14 species belonging to nine genera in the Siphonocladales and two species in the Cladophorales are known to have linear TCs on both E- and P-fracture faces of the plasma membrane. Species studied in the present paper included Boodlea composita, Dictyosphaeria cavernosa, Ernodesmis verticillata, Siphonocladus tropicus, Struvea elegans, Valoniopsis pachynema and Chaetomorpha aerea. Contrary to their fairly consistent width (30–36 nm), TCs have a ‘wide distribution of length among individual species and at various stages of development in the same species. Most of the TCs have a random arrangement of subunits, but sometimes they are arranged in three rows. The mean TC length is greater during secondary wall synthesis than in primary ‘wall synthesis in all of the following species: Boodlea composita, Dictyosphaeria cavernosa, Siphonocladus tropicus, Valonia macrophysa, Valonia ventricosa and Chaetomorpha aerea. These results support previous results suggesting that the linear TCs increase their length during cell wall development. The size of TC subunits, ranging from 7.3 to 8.9 nm, was smaller than the structural membrane particles on the plasma membrane in all of the species examined. It is suggested that the spacing between individual glucan chains will be reduced to half after crystallization of cellulose microfibrils, on the basis of evidence that the width of microfibrils is as wide as that of TC. The width of microfibrils ranged from 11.2 to 23.6 nm, while most of the species had microfibrils with a width in the range 14 to 16 nm. The width of microfibrils in Boergesenia was the largest among the giant marine algae. The formation of TCs from subunits, which are transmembrane particles, is characteristic of Siphonocladales in spite of their varying cell morphology.