The extracellular calcification of zoospores ofPhacotus lenticularis(Chlorophyta, Chlamydomonadales)

Abstract

In nature the zoospores of Phacotus lenticularis usually have a conspicuous, bivalved lorica. It consists of two clearly distinguishable layers: an outer calcite layer formed of rings of staggered calcite crystals with electron-opaque rod-like components between the crystals, and an underlying electron-opaque layer. Underneath the lorica a mucilaginous sheath is present. The ultrastructural development of these extracelluIar layers was studied by scanning electron microscopy and transmission electron microscopy of flat-embedded specimens from the natural habitat, Lake Feldberger Haussee (Baltic Lake District, Germany). During vegetative propagation, the two mother shells separate but remain attached to the mucilaginous zoosporangium which is formed between the shells. Usually two longitudinal cell divisions result in four naked zoospores in the sporangium. These begin to form new extracellular material, consisting initially of an electron-opaque layer that becomes detached by the formation of a mucilaginous inner sheath. During this stage the dictyosomes are highly active and the cells contain vesicles with rod-like structures resembling the electron-opaque material between the crystals of the adult Iorica. Zoospores are motile in the sporangium and the outermost extracellular layer gradually calcifies with small crystals developing on the electron-opaque layer in a regular manner and separated by equal distances (spotted sphere stage). The growing crystals develop in a pattern resembling a tiled roof and merge to form the final, massive calcite layer of the lorica. The zoosporangium ruptures between the former mother shells and the zoospores are released. Several possible nucleation and calcification mechanisms are discussed.

Calcite production by the calcifying green alga Phacotus lenticularis

“…The rim interspace seemed to be filled with a volatile substance, which under an electron beam under vacuum conditions slowly disappears. This substance might represent the mucilaginous sheath (Hepperle and Krienitz, 1996), previously called "gelatinous mother cell wall" (Giering et al, 1990). We observed that the pore channels seem to be coated or filled with this same volatile organic substance.…”

Phylogenetic Position of the Phacotaceae Within the Chlamydophyceae as Revealed by Analysis of 18S rDNA and rbcL Sequences

“…These layers are known to develop during sporulation within the zoosporangium (cf. Hepperle and Krienitz 1996). At the apical cell pole, the central layer extended inward and formed the flagellar channels.…”

“…curs in mucilaginous sporangia that are formed between parts of the mother lorica. Release of the daughter zoospores occurs after (9) complete formation of the lorica by (10) lysis of the sporangium (Hepperle and Krienitz 1996). (11) These species overcome environmental limitations by formation of palmellae.…”

“…According to Preisig et al (1994) loricae are considered to be cell walls fitting loosely over the body proper of the organism and not exhibiting defined chemical compositions. At least the organic compounds of the loricae of the Phacotaceae are supposed to be of dictyosomal origin and seem to assemble on the cell surface after their excretion Dunlap 1994, 1995;Hepperle and Krienitz 1996). A subsequent, unknown mechanism leads to mineralization of the loricae with metallic compounds, such as calcium carbonate in the case of Phacotus (Hepperle and Krienitz 1997) or iron salts in Dysmorphococcus (Porcella and Walne 1980).…”

“…Most authors consider the lorica of the Phacotaceae to be a conserved, apomorphic characteristic (e.g., Walne and Dunlap 1994). Nevertheless, a comparison of lorica morphology and metal composition reveals extreme differences between the genera (Dunlap and Walne 1993;Walne and Dunlap 1994;Hepperle et al 1994;Hepperle and Krienitz 1996). This indicates that the family might be polyphyletic and thus an artificial conglomerate.…”