Fine structure of the plastids retained by the foraminifer Elphidium excavatum (Terquem)

Previous studies showed that, when specimens of Elphidium excavatum harvested from field collections are fed diatoms, they retain more chloroplasts and survive longer than they do when fed diets of other types of algae. Does the host retain the chloroplasts other types of algae? To answer this question, foraminifera were fed diets of either a chlorophyte or a dinoflagellate and then studied by transmission electron microscopy (TEM). The algae used Amphora coffeiformis (diatom), Amphidinium sp. (dinoflagellate), and Dunaliella salina (green alga) have chloroplasts which are quite distinct from each other at the fine structural level. The experimental foraminifera were harvested directly from the field and thus already had a population of chloroplasts within their cytoplasm when the experiment started. Regardless of the type of alga used in the experimental diet, the ultrastructure of the retained plastids were typical of diatoms: 2-3 thylakoid stacks; a simple internal pyrenoid with a thylakoid crossing it; and a girdle lamella surrounding the outside of the chloroplast. Some of the plastids were undergoing digestion, because they had lost organization and we observed fusion of vesicles (presumed to be lysosomes) with the symbiosome vacuole. Cytoplasm within the inter-chamber canals of the foraminifera contained numerous vacuoles and vesicles, some of which contained algae in the process of digestion, others had isolated plastids. The plastids inside the chambers had protrusions on their edges corresponding to the positions of cannalicular pores connected to the intercameral canals. These pores were plugged by an electron dense material and no cytoplasmic connection was observed between the pores in the test and the intercameral cytoplasm.