Electrophysiology of Turgor Regulation in Marine Siphonous Green Algae M.A. Bisson 1 , M.J. Beilby 2 , V.A. Shepherd 2 1 Department of Biological Sciences, Cooke Hall 109, University at Buffalo, Buffalo, NY 14260, USA 2 Department of Biophysics, School of Physics, University of New South Wales, Sydney, NSW Australia Received: 19 December 2005/Revised: 1 April 2006 Abstract. Wereviewelectrophysiologicalmeasuresof turgor regulation in some siphonous green algae, primarily the giant-celled marine algae, Valonia, and Ventricaria, with particular comparison to the well studied charophyte algae Chara and Lamprothamni- um. The siphonous green algae have a less negative plasmamembranepotential,andareunlikelytohave a proton-based chemiosmotic transport system, dominatedbyactiveelectrogenicK + uptake.Wealso make note of the unusual cellular structure of the siphonous green algae. Hypertonic stress, due to in- creased external osmotic pressure, is accompanied by positive-going potential difference (PD), increase in conductance, and slow turgor-regulation. The rela- tionship between these is not yet resolved, but may involve changes in K + conductance (G K ) or active K + transport at both membranes. Hypotonic turgor regulation, in response to decrease external osmotic pressure, is 3 times faster than hypertonic turgor regulation.Itisaccompaniedbyanegative-going PD, although conductance also increases. The conduc- tance increase and the magnitude of the PD change are strongly correlated with the magnitude of hypo- tonic stress. Key words: Turgor— 1 Electrophysiologyofplants— Cell signaling — Transport physiology — Ion and watertransportinplants—CellPhysiology—Green algae Introduction In this review we will describe electrophysiological measures of turgor regulation in some siphonous green algae, primarily the giant-celled marine alga, Valonia, and Ventricaria (Fig. 1a), its close relative (Olsen & West, 1988). We will compare the behavior oftheseandothergiant-celledchlorophytealgaewith thatoftheCharophytes Chara and Lamprothamnium, and discuss the differences with respect to algal phylogeny. A cell must be able to detect turgor changes in order to regulate its turgor in response to environ- mental challenges. The mechanism by which a cell measures its turgor is not known and numerous hypotheses have been proposed (Coster & Zimmer- mann, 1976; Gutknecht et al., 1978; Pickard & Ding, 1993; Bisson & Kirst, 1995; Heidecker et al., 1999; Shepherd et al., 2002; Kacperska, 2004). These hypotheses generally invoke physical differences that occur as a result of pressure on or differential pres- sureacrossthemembrane,suchasproximitybetween elementsofthemembraneandcellwall,curvatureor compressionofthemembrane,andtensionwithinthe membrane. Communication of these changes to the protoplast may occur by alteration in the activity of membrane proteins, particularly ion transporters, and hence may have electrophysiological conse- quences. For instance, stretch-activated or -inacti- vated channels within the membrane may change membrane conductance (G) and electrical potential difference across the membrane (PD). In order to affect turgor, these changes need to result in an alteration of cytoplasmic osmotic pressure that re- verses the movement of water and restores the opti- mal turgor. Insofar as these processes involve the transport of charged species, they have electrophysi- ological consequences. Advantages and Limitations of Studying Giant-celled Algae Osmotic relations of the intact cell are controlled by the vacuole, which is often regarded as a simple salt solution enclosed by a tonoplast. Solute fluxes and Correspondence to: Mary A. Bisson; email: bisson@buffalo.edu J. Membrane Biol. j, 1–15 (2006) DOI: 10.1007/s00232-006-0860-1 Topical Review