Journal Article

Secretion in Unicellular Marine Phytoplankton: Demonstration of Regulated Exocytosis in <i>Phaeocystis globosa</i>

Wei-Chun Chin, Mónica V. Orellana, Ivan Quesada and Pedro Verdugo

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 45, issue 5, pages 535-542
Published in print April 2004 | ISSN: 0032-0781
Published online April 2004 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pch062
Secretion in Unicellular Marine Phytoplankton: Demonstration of Regulated Exocytosis in Phaeocystis globosa

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  • Molecular and Cell Biology
  • Plant Sciences and Forestry

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Almost half of the global photosynthetic activity is carried out in the ocean. During blooms, Phaeocystis can fix CO2 at rates up to 40 g C m–2 month–1. Most of this carbon is released as polysaccharides. However, the cellular mechanism whereby this huge amount of organic material is exported into the seawater remains unknown. A vaguely defined process of “exudation” is believed responsible for the release of these biopolymers. Here we report the first demonstration that Phaeocystis globosa does not “exude”, but secretes microscopic gels. Secretion is stimulated by blue light (λ = 470±20 nm), and it is transduced by a characteristic intracellular Ca2+ signal that precedes degranulation. The polysaccharides that form the matrix of these gels remain in condensed phase while stored in secretory vesicles. Upon exocytosis, the exopolymer matrix undergoes a characteristic phase transition accompanied by extensive swelling resulting in the formation of microscopic hydrated gels. Owing to their tangled topology, once released into the seawater, the polymers that make these gels can reptate (axially diffuse), interpenetrate neighboring gels, and anneal them together forming massive mucilage accumulations that are characteristic of Phaeocystis blooms. These gel masses can supply a rich source of microbial substrates, disperse in the seawater, and/or eventually sediment to the ocean floor.

Keywords: Keywords: Exocytosis — Exopolymer — Granule — Light — Mucilage — Phaeocystis.; Abbreviations: ASW, artificial seawater; DOC, dissolved organic carbon; PBS, phosphate-buffered saline.

Journal Article.  6310 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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