FINE-STRUCTURE OF A MARINE PROTEOMYXID AND CYTOCHEMICAL CHANGES DURING ENCYSTMENT

A proteomyxid (Biomyxa vagans) isolated from Sargassum sp. was maintained in laboratory culture with a bacterial food source. The life cycle consists of four stages: (1) resting cysts formed during unfavorable growth conditions, (2) a dispersal stage following excystment when active growth resumes, (3) generative growth characterized by large plasmodial cells which give rise to numerous daughter cells, and (4) a recruitment stage in which solitary cells become aggregated during progressively unfavorable growth conditions and eventually produce clusters of resting cysts. No sexual reproduction was observed. The fine structure of active cells shows that they are multinucleated, possess a thin envelope of fibrillar material surrounding the cell, and contain digestive vacuoles filled with bacteria and detritus. Encysting cells exhibit lipid autophagy as shown by cytochemical staining and biochemical analysis of the lipid content of encysting cells compared to active cells. The cysts have a thickened cell coat, contain smaller nuclei than the active cells, possess fewer and smaller digestive vacuoles, and exhibit less secretory activity at the periphery of the cell. The nutrition and life history of Biomyxa vagans are discussed in relation to its surface-dwelling habit within a pelagic community. © 1979 Academic Press, Inc.