This paper discusses whole-plant responses to salinity in order to answer the question of what process limits growth of non-halophytes in saline soils. Leaf growth is more sensitive to salinity than root growth, so we focus on the process or processes that might limit leaf expansion. Effects of short-term exposure (days) are considered separately from long-term exposure (weeks to years). The answer in the short term is probably the water status of the root and we suggest that a message from the root is regulating leaf expansion. The answer to what limits growth in the long term may be the maximum salt concentration tolerated by the fully expanded leaves of the shoot; if the rate of leaf death approaches the rate of new leaf expansion, the photosynthetic area will eventually become too low to support continued growth.

Whole-plant responses to salinity

Sodium transport in plant cells.

INTRODUCTION 21 General Scope 21 Definition of Turgor Pressure Regulation and Osmotic Adaptation 23 GENERAL RESPONSES OF THE ALGAE AND RANGE OF TOLERANCE _ 23 Growth Rates 23 Photosynthesis and Respiration........ 25 Effect on the Fine Structure 26 PROCESSES OF OSMOTIC ACCLIMATION 27 Sensing of Turgor Pressure: The Detector Mechanisms 28 Changing the Cellular Concentrations of Osmolytes: The EIfector Processes and Their Regulation 31 MARINE ALGAE FROM AN UNUSUAL HABITAT: SEA ICE ALGAE 42

Salinity Tolerance of Eukaryotic Marine Algae

The green lineage (Viridiplantae) comprises the green algae and their descendants the land plants, and is one of the major groups of oxygenic photosynthetic eukaryotes. Current hypotheses posit the early divergence of two discrete clades from an ancestral green flagellate. One clade, the Chlorophyta, comprises the early diverging prasinophytes, which gave rise to the core chlorophytes. The other clade, the Streptophyta, includes the charophyte green algae from which the land plants evolved. Multi-marker and genome scale phylogenetic studies have greatly improved our understanding of broad-scale relationships of the green lineage, yet many questions persist, including the branching orders of the prasinophyte lineages, the relationships among core chlorophyte clades (Chlorodendrophyceae, Ulvophyceae, Trebouxiophyceae and Chlorophyceae), and the relationships among the streptophytes. Current phylogenetic hypotheses provide an evolutionary framework for molecular evolutionary studies and comparative genomics. This review summarizes our current understanding of organelle genome evolution in the green algae, genomic insights into the ecology of oceanic picoplanktonic prasinophytes, molecular mechanisms underlying the evolution of complexity in volvocine green algae, and the evolution of genetic codes and the translational apparatus in green seaweeds. Finally, we discuss molecular evolution in the streptophyte lineage, emphasizing the genetic facilitation of land plant origins.

/pdf/phylogeny-and-molecular-evolution-of-the-green-algae-4244e09mtu.pdf

Phylogeny and Molecular Evolution of the Green Algae

CO2 emissions arising from the burning of fossil fuels have altered seawater chemistry far more rapidly than the Earth has previously experienced, and the rate and extent of this change are expected to affect shallow water marine organisms. The increased CO2 diffuses from the atmos- phere into ocean surface waters, resulting in increased partial pressure of CO2, and reduced (CO3 2- ) and pH. The CO2-driven ocean acidification leads to a decrease in calcium carbonate (CaCO3) satu- ration state in the ocean surface waters and has potential impacts on calcifiers. The present study focuses on the effects of ocean acidification on early developmental and reproductive stages of calci- fiers, both of which are believed to be the most vulnerable stages to environmental change within a life cycle. Laboratory experiments revealed that ocean acidification has negative impacts on the fer- tilization, cleavage, larva, settlement and reproductive stages of several marine calcifiers, including echinoderm, bivalve, coral and crustacean species. There appear to be significant ontogenetic impacts and species-specific differences in tolerance to the high CO2 levels. The conclusion is that future changes in ocean acidity will potentially impact the population size and dynamics, as well as the community structure of calcifiers, and will therefore have negative impacts on marine eco- systems. Further studies are needed to evaluate the potential impacts on non-calcifiers, as well as the synergistic impacts of ocean acidification and climate change. Studies should also focus on the adaptive capability of marine organisms, which will be crucial to the ability to forecast how marine organisms and ecosystems will respond to the world's oceans as they warm and acidify.

/pdf/effects-of-co2-driven-ocean-acidification-on-the-early-3y8n4y7gxd.pdf

Effects of CO2-driven ocean acidification on the early developmental stages of invertebrates

Diffusion of (14)C-labeled CO(2) was measured through lipid bilayer membranes composed of egg lecithin and cholesterol (1:1 mol ratio) dissolved in n-decane. The results indicate that CO(2), but not HCO(3-), crosses the membrane and that different steps in the transport process are rate limiting under different conditions. In one series of experiments we studied one-way fluxes between identical solutions at constant pCO(2) but differing [HCO(3-)] and pH. In the absence of carbonic anhydrase (CA) the diffusion of CO(2) through the aqueous unstirred layers is rate limiting because the uncatalyzed hydration-dehydration of CO(2) is too slow to permit the high [HCO(3-)] to facilitate tracer diffusion through the unstirred layers. Addition of CA (ca. 1 mg/ml) to both bathing solutions causes a 10-100-fold stimulation of the CO(2) flux, which is proportional to [HCO(3-)] over the pH range 7-8. In the presence of CA the hydration- dehydration reaction is so fast that CO(2) transport across the entire system is rate limited by diffusion of HCO(3-) through unstirred layers. However, in the presence of CA when the ratio [HCO(3-) + CO(3=)]:[CO(2)] more than 1,000 (pH 9-10) the CO(2) flux reaches a maximum value. Under these conditions the diffusion of CO(2) through the membrane becomes rate limiting, which allows us to estimate a permeability coefficient of the membrane to CO(2) of 0.35 cm s(-1). In a second series of experiments we studied the effects of CA and buffer concentration on the net flux of CO(2). CA stimulates the net CO(2) flux in well buffered, but no in unbuffered, solutions. The buffer provides a proton source on the upstream side of the membrane and proton sink on the downstream side, thus allowing HCO(3-) to facilitate the net transport of CO(2) through the unstirred layers.

https://rupress.org/jgp/article-pdf/69/6/779/1246409/779.pdf

Diffusion of carbon dioxide through lipid bilayer membranes: effects of carbonic anhydrase, bicarbonate, and unstirred layers.

Osmotic acclimation and turgor pressure regulation in algae

Investigating uptake of water-dispersible CdSe/ZnS quantum dot nanoparticles by Arabidopsis thaliana plants.

Above a critical external pH (about 10.5), theChara membrane acquires new propertes. In this state the membrane potential is close to the equilibrium potentials for H+ and OH−, hyperpolarizing as external pH increases with a slope of −59 mV/pH unit. The membrane conductance increases by an average factor of 2.4 above the critical pH. These changes are explained by an increase in permeability to OH− (or H+). The establishment of a OH− (or H+ permeable membrane at high pH suggests that the large fluxes of OH− (or H+ which occur in the alkaline band in photosynthesizing cells are passive.

The Chara plasmalemma at high pH. Electrical measurements show rapid specific passive uniport of H + or OH −

Seventeen species of thalloid or giant-celled marine algae representing the three major classes, Chlorophyceae, Rhodophyceae and Phaeophyceae maintain turgor pressure constant over the range of 470-1860 mosmol/kg external osmotic concentration. This is achieved by changing internal concentrations of Na+, K+ and Cl-. Three different groups can be distinguished: (1) species with high Na+ ; (2) those with high K+; and (3) algae with both cations in about equal amounts. In general, a Cl- pump seems to be involved in turgor regulation. There is no correlation between ion composition and taxonomic class. The concentration of low-molecular-weight organic compounds also changes during regulation. As a rule, these compounds are the main photosynthetic products and are probably located in the cytoplasm.

Mary A. Bisson

Papers

Diffusion of carbon dioxide through lipid bilayer membranes: effects of carbonic anhydrase, bicarbonate, and unstirred layers.

Osmotic acclimation and turgor pressure regulation in algae

Investigating uptake of water-dispersible CdSe/ZnS quantum dot nanoparticles by Arabidopsis thaliana plants.

The Chara plasmalemma at high pH. Electrical measurements show rapid specific passive uniport of H + or OH −

Regulation of Turgor Pressure in Marine Algae: Ions and Low-molecular-weight Organic Compounds