title: Active water transport in unicellular algae: where, why, and how.
abstract: The occurrence of active water transport (net transport against a free energy gradient) in photosynthetic organisms has been debated for several decades. Here, active water transport is considered in terms of its roles, where it is found, and the mechanisms by which it could occur. First there is a brief consideration of the possibility of active water transport into plant xylem in the generation of root pressure and the refilling of embolized xylem elements, and from an unsaturated atmosphere into terrestrial organisms living in habitats with limited availability of liquid water. There is then a more detailed consideration of volume and osmotic regulation in wall-less freshwater unicells, and the possibility of generation of buoyancy in marine phytoplankton such as large-celled diatoms. Calculations show that active water transport is a plausible mechanism to assist cells in upwards vertical movements, requires less energy than synthesis of low-density organic solutes, and potentially on a par with excluding certain ions from the vacuole.
authors.affiliation: Division of Plant Sciences, University of Dundee at the James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK School of Plant Biology, University of Western Australia, M048, 35 Stirling Highway, Crawley, WA 6009, Australia j.a.raven@dundee.ac.uk.
authors.forename: John A
authors.initials: JA
authors.lastname: Raven
authors.affiliation: Plant Functional Biology and Climate Change Cluster (C3), University of Technology Sydney, PO Box 123, NSW 2007, Australia.
authors.forename: Martina A
authors.initials: MA
authors.lastname: Doblin
mesh_terms: D001272:Atmosphere; D001693:Biological Transport, Active; D056890:Eukaryota / Q000378:metabolism*; D010839:Phytoplankton / Q000378:metabolism; D018517:Plant Roots / Q000502:physiology; D011312:Pressure; D014867:Water / Q000378:metabolism*; D052584:Xylem / Q000502:physiology
chemical_list: D014867:Water
keywords: Active water transport; aquaporins; contractile vacuoles; diatoms; water vapour uptake; xylem refilling.