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

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.

Permanent ectoplasmic structures in deep-sea <i>Cibicides</i> and <i>Cibicidoides</i> taxa – long-term observations at in situ pressure

“…Only for a few shallow-water benthic foraminifera, information on ectoplasmic extensions to interact with the environment has been published (Bowser and Travis, 2002;Travis et al, 2002). Hereby, the typical ectoplasmic extensions described are pseudopodia characterized by their forceful and rapid extension enabled by actin filaments and extremely dynamic microtubule systems (Bowser et al, 1988;Goleń et al, 2020;Travis and Bowser, 1986;Travis et al, 2002). Anastomosing, i.e.…”

Permanent ectoplasmic structures in deep-sea Cibicides/oides taxa – long-term observations at in situ pressure

“…Only for a few shallow-water benthic foraminifera, information on ectoplasmic extensions to interact with the environment has been published so far (Bowser, 2002;. Hereby, the typical ectoplasmic extensions described are pseudopodia characterised by their forceful and rapid extension enabled by actin filaments and extremely dynamic microtubule systems (Bowser et al, 1988;Goleń et al, 2020;Travis and Bowser, 1986;. Anastomosing, i.e.…”

“…Furthermore, a rapid bidirectional transport of both granules and surface-attached particles has been described for the pseudopodia of shallow-water foraminifera. Giving tribute to the granular appearance, the term 'granuloreticulopodia' is widely used for this pseudopodial network and separates it from the globular and lamellar pseudopodia involved in chamber formation (Goleń et al, 2020;Tyszka et al, 2019).…”

“…The rate at which cells can form projections, like pseudopodia, and transport granules and adhering particles is, in part, limited by the rate at which the cell assembles new or reorganises existing actin filaments (Bowser et al, 1988;Goleń et al, 2020;Travis and Bowser, 1986;Tyszka et al, 2019). This ATP consuming process is obviously much faster in shallowwater foraminifera than in deep-water Cibicides/Cibicidoides-taxa.…”

“…emerged from the static ectoplasmic structures, due to the stiffness of 'roots', 'trees', and 'twigs', they rather resemble flagella than pseudopodia. Yet, future transmission electron analyses or confocal microscope investigations at atmospheric pressure (Goleń et al, 2020;Tyszka et al, 2019) are needed to understand the cellular structure of these lasting ectoplasmic extensions.…”