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. Recent advances in fluorescence imaging facilitateactualistic studies of organisms used for palaeoceanographicreconstructions. Observations of cytoskeleton organisation and dynamics inliving foraminifera foster understanding of morphogenetic andbiomineralisation principles. This paper describes the organisation of aforaminiferal actin cytoskeleton using in vivo staining based on fluorescentSiR-actin. Surprisingly, the most distinctive pattern of SiR-actin stainingin foraminifera is the prevalence of SiR-actin-labelled granules (ALGs)within pseudopodial structures. Fluorescent signals obtained from granulesdominate over dispersed signals from the actin meshwork. SiR-actin-labelledgranules are small (around 1  µ m in diameter) actin-rich structures,demonstrating a wide range of motility behaviours, from almost stationarilyoscillating around certain points to exhibiting rapid motion. These labelledmicrostructures are present both in Globothalamea (Amphistegina, Ammonia) and Tubothalamea(Quinqueloculina). They are found to be active in all kinds of pseudopodial ectoplasmicstructures, including granuloreticulopodia, globopodia, and lamellipodia, aswell as within the endoplasm. Several hypotheses are set up to explaineither specific or non-specific actin staining. Two hypotheses regardingtheir function are proposed if specific actin labelling is taken intoaccount: (1) granules are involved in endocytosis and intracellulartransport of different kinds of cargo, or (2) they transport prefabricatedand/or recycled actin fibres to the sites where they are needed. Thesehypotheses are not mutually exclusive. The first hypothesis is based on thepresence of similar actin structures in fungi, fungi-like protists, and someplant cells. The later hypothesis is based on the assumption that actingranules are analogous to tubulin paracrystals responsible for efficienttransport of tubulin. Actin patches transported in that manner are mostlikely involved in maintaining shape, rapid reorganisation, and elasticityof pseudopodial structures, as well as in adhesion to the substrate.Finally, our comparative studies suggest that a large proportion ofSiR-actin-labelled granules probably represent fibrillar vesicles andelliptical fuzzy-coated vesicles often identified in transmission electron microscope images.

SiR-actin-labelled granules in foraminifera: patterns, dynamics, and hypotheses