Abstract
Abstract. Recent advances in fluorescence imaging facilitate actualistic studies of organisms used for palaeoceanographic reconstructions. Observations of cytoskeleton organisation and dynamics in living foraminifera foster understanding of morphogenetic and biomineralisation principles. This paper describes the organisation of a foraminiferal actin cytoskeleton using in vivo staining based on fluorescent SiR-actin. Surprisingly, the most distinctive pattern of SiR-actin staining in foraminifera is the prevalence of SiR-actin-labelled granules (ALGs) within pseudopodial structures. Fluorescent signals obtained from granules dominate over dispersed signals from the actin meshwork. SiR-actin-labelled granules are small (around 1 µm in diameter) actin-rich structures, demonstrating a wide range of motility behaviours, from almost stationarily oscillating around certain points to exhibiting rapid motion. These labelled microstructures are present both in Globothalamea (Amphistegina, Ammonia) and Tubothalamea (Quinqueloculina). They are found to be active in all kinds of pseudopodial ectoplasmic structures, including granuloreticulopodia, globopodia, and lamellipodia, as well as within the endoplasm. Several hypotheses are set up to explain either specific or non-specific actin staining. Two hypotheses regarding their function are proposed if specific actin labelling is taken into account: (1) granules are involved in endocytosis and intracellular transport of different kinds of cargo, or (2) they transport prefabricated and/or recycled actin fibres to the sites where they are needed. These hypotheses are not mutually exclusive. The first hypothesis is based on the presence of similar actin structures in fungi, fungi-like protists, and some plant cells. The later hypothesis is based on the assumption that actin granules are analogous to tubulin paracrystals responsible for efficient transport of tubulin. Actin patches transported in that manner are most likely involved in maintaining shape, rapid reorganisation, and elasticity of pseudopodial structures, as well as in adhesion to the substrate. Finally, our comparative studies suggest that a large proportion of SiR-actin-labelled granules probably represent fibrillar vesicles and elliptical fuzzy-coated vesicles often identified in transmission electron microscope images.
Highlights
Since foraminifera were firstly recognised by science in the beginning of the 19th century, thanks to the work of d’Orbigny (Lipps et al, 2011), they have been the subject of extensive study
Fluorescent SiR-actin labelling has revealed three considerably different patterns of staining in Ammonia sp., i.e. (1) weak but non-uniform staining following all membranous surfaces of pseudopodial structures, (2) linear or ringlike structures showing intense fluorescence, and (3) small but strongly labelled granular structures that often exhibit very rapid dynamics (Fig. 1; Movie S1)
Fluorescence labelling has revealed three considerably different SiR-actin-labelled patterns, which include (1) weak but non-uniform staining following all membranous surfaces of pseudopodial structures (Figs. 1, 2); (2) linear or ring-like structures showing intense fluorescence (Fig. 1); and (3) small, strongly labelled granular structures that often exhibit very rapid dynamics (Figs. 2–3, 5–6, S4–S5, and Movies S2–S3)
Summary
Since foraminifera were firstly recognised by science in the beginning of the 19th century, thanks to the work of d’Orbigny (Lipps et al, 2011), they have been the subject of extensive study. Most foraminifera species create shells (tests) that have great potential for preservation in the fossil record and are primarily important in Earth science disciplines. To properly understand fossils, it is essential to take into account the physiology of the living organisms.
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