SiR-actin-labelled granules in foraminifera: patterns, dynamics, and hypotheses
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Published:2020-02-25
Issue:4
Volume:17
Page:995-1011
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ISSN:1726-4189
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Container-title:Biogeosciences
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language:en
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Short-container-title:Biogeosciences
Author:
Goleń JanORCID, Tyszka Jarosław, Bickmeyer Ulf, Bijma JelleORCID
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.
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
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