Sporopollenin in the cell walls of Coccomyxa and Myrmecia phycobionts of various lichens: an ultrastructural and chemical investigation

Abstract

Symbiotic and cultivated Myrmecia phycobionts of Baeomyces rufus and Coccomyxa phycobionts of five different asco- and basidio-lichens were investigated with cytological and chemical methods. The cell wall structure of the free-living type species Coccomyxa dispar was compared with that of the lichen phycobionts.Three different wall layers were observed in all Coccomyxa and Myrmecia cells investigated. An innermost, variably thick layer is amorphous in structure and is built up mainly by Golgi-derived hemicelluloses. An outer wall layer, uniformly thick, appears electron dense in thin sections and exhibits short, probably cellulosic fibrils embedded in an amorphous matrix in freeze-fracture preparations. Beyond these two wall layers is an outermost trilaminar wall layer of uniform thickness in all species investigated. It contains sporopollenin in its electron-transparent, rigid middle part; proteinlike particles are embedded in an amorphous, carbohydrate-containing matrix on its electron-dense inner and outer surfaces. IR spectrophotometry of acetolysis-resistant material yielded data comparable with those of other sporopollenin-containing algal walls, although the Coccomyxa and Myrmecia sporopollenin did not dissolve in chromic acid. The trilaminar layer is not lysed during or after autospore formation. Persistent mother cell walls were detected in all lichen thalli investigated, as well as in the culture medium of isolated phycobionts. Persistent mother cell walls were also found in the gelatinous sheath of Coccomyxa dispar.This type of wall structure affords protection against fungal parasitism and may be a significant factor in the success of some lichens and some other symbiotic systems.