Lorica Production in Choanoflagellates–A Model System for Investigating the Mechanics, Controls and Dynamics of Secretion

BIOMINERALIZATION is the process by which living organisms assemble structures from naturally occurring inorganic compounds. Mineral deposition is common and widespread amongst Protozoa and in most instances the mineralized structures provide skeletal support and protection for softer organic parts 10]. The 2 most common minerals to be deposited by Protozoa are silica and calcium carbonate. Groups of Protozoa that deposit silica, which we are concerned with here, include the diatoms, chrysophytes, choanoflagellates, Radiolar-ia, Heliozoa and testate amoebae 10]. In the majority of silica-depositing protista, silica is taken up from the medium in the form of monomelic orthosilicic acid Si(OH)₄ (soluble reactive silicate) and deposited as amorphous, polymerised biogenic silica or opal within membrane-bounded vesicles known as silica deposition vesicles (SDV). Often biogenic silica is characteristically patterned and ornamented and for most protozoan groups the morphology of silicified parts is of prime taxonomic importance. By far the most extensively studied group of silica-depositing organisms are the diatoms 1, 12, 13]. To date most of our knowledge of silica metabolism in protists has been based on investigations into this group. Diatoms require silica for the production of their frustules. Uptake and deposition of silica occurs within a closely denned portion of the cell cycle, between nuclear division and cell separation. It occupies about ± of the cell cycle and without an adequate supply of silica diatoms are unable to produce new frustule valves with the result that cell division cannot be completed. Diatoms, therefore, have an obligate requirement for silica and without this nutrient they cease to grow 11]. In contrast to diatoms a number of other silica-depositing protistan groups, such as loricate choanoflagellates and certain chrysophytes, have a facultative requirement for silica. In the past decade the ultras true ture, physiology and ecology of loricate choanoflagellates have been extensively studied by a number of different workers 7] and the significance of these studies to our understanding of the mechanisms, controls and dynamics of silica secretion is summarised and discussed here.