Nutrient transport in microalgae

Nitrogen source and availability regulate plastic population dynamics in the marine diatom Thalassiosira weissflogii

Variation in nitrogen (N) availability significantly influences population dynamics and the productivity of marine phytoplankton. As N availability in the ocean is conditioned by the N source, it is important to understand the capacity of phytoplankton organisms to adjust their physiology and dynamics under different N conditions. We investigated the growth dynamics of Thalassiosira weissflogii, a coastal diatom, in response to different N sources (Nitrate, NO3-; Ammonium, NH4+; urea, CH4N2O) and availabilities (45 and 5 μM). Our findings demonstrate that T. weissflogii can display plastic adjustments in population dynamics to different N sources. These responses evidenced a greater preference for NH4+ and urea than NO3-, particularly under high N availability. The relative growth rate (μ) is higher (1.18 ± 0.01) under NH4+-high treatment compared to NO3--high (1.01 ± 0.01). The carrying capacity (K) varied only among concentrations, indicating equal N utilization efficiency for biomass production. No effects of N source were detected under the low concentration, suggesting that the preference for NH₄⁺ and urea was diminished by limited nitrogen supply due to potential interactions. These results provide valuable insights into the physiological flexibility of T. weissflogii to varying N conditions, shedding light on the ecological success and resilience of this species in highly variable coastal environments.

Algae in Biomedicine

Algae, which live in marine or freshwater, are photosynthetic organisms. They vary greatly in size, morphology, and degree of complexity of their body structures. Algae are generally divided into two main groups, microalgae, which are small in size, and macroalgae, which are larger in size. These aquatic organisms have rich and valuable compounds including sterols, polysaccharides, pigments, fatty acids, proteins, enzymes, minerals, and vitamins that could be used in different application fields due to their bioactive functions. In recent years, algae and their components have attracted interest in biomedicine and health applications as their bioactive components could show antioxidant, anticancer, anti-inflammatory, antiviral, antiangiogenic, antidiabetic, antiobesity, immunostimulatory, vaccine adjuvant, and hypolipidemic activities. In this chapter, these activities and bioactive components underlying these properties are reviewed.

Feeding in mixoplankton enhances phototrophy increasing bloom-induced pH changes with ocean acidification

Plankton phototrophy consumes CO₂, increasing seawater pH, while heterotrophy does the converse. Elevation of pH (>8.5) during coastal blooms becomes increasingly deleterious for plankton. Mixoplankton, which can be important bloom-formers, engage in both photoautotrophy and phagoheterotrophy; in theory, this activity could create a relatively stable pH environment for plankton growth. Using a systems biology modelling approach, we explored whether different mixoplankton functional groups could modulate the environmental pH compared to the extreme activities of phototrophic phytoplankton and heterotrophic zooplankton. Activities by most mixoplankton groups do not stabilize seawater pH. Through access to additional nutrient streams from internal recycling with phagotrophy, mixoplankton phototrophy is enhanced, elevating pH; this is especially so for constitutive and plastidic specialist non-constitutive mixoplankton. Mixoplankton blooms can exceed the size of phytoplankton blooms; the synergisms of mixoplankton physiology, accessing nutrition via phagotrophy as well as from inorganic sources, enhance or augment primary production rather than depressing it. Ocean acidification will thus enable larger coastal mixoplankton blooms to form before basification becomes detrimental. The dynamics of such bloom developments will depend on whether the mixoplankton are consuming heterotrophs and/or phototrophs and how the plankton community succession evolves.

The effects of exogenous amino acids on production of microcystin variants in Microcystis aeruginosa

Dissolved free amino acids are a significant component of dissolved organic nitrogen (DON) in natural waterbodies. The effects of four amino acids (glutamic acid, phenylalanine, leucine, and arginine) on the growth and microcystins (MCs) production of Microcystis aeruginosa were studied in batch culture. The profiles of five MCs variants and the expression levels of target genes involved in MCs biosynthesis and nitrogen metabolism were measured. When amino acids were used as the sole nitrogen source instead of nitrate at different levels (0.5, 2.0 and 8.0 mg/L based on N) in BG-11 medium, algal cell growth and intracellular MCs quotas were inhibited slightly by the treatments with glutamic acid and arginine. The treatments with phenylalanine and leucine, on the other hand, had a strong inhibitory effect on algal cell growth and MCs production. Moreover, the concentrations of Chlorophyll a, phycocyanin and allophycocyanin in cells cultured in glutamic acid, leucine and phenylalanine were lower than those in the control group with nitrate as nitrogen source. The existence of leucine or phenylalanine can lead to a significant increase in the relative abundance of MCs variants structured with the corresponding amino acids. The expression of microcystin-producing gene mcyD was downregulated while the gene pipX associated with nitrogen metabolism was upregulated during the cultivation of M. aeruginosa with amino acids as sole nitrogen source. M. aeruginosa undergoes significant alterations due to exogenous amino acids and exhibits advanced strategies for MCs production.

Role of hydrodynamics in shaping chemical habitats and modulating the responses of coastal benthic systems to ocean global change

Marine coastal zones are highly productive, and dominated by engineer species (e.g. macrophytes, molluscs, corals) that modify the chemistry of their surrounding seawater via their metabolism, causing substantial fluctuations in oxygen, dissolved inorganic carbon, pH, and nutrients. The magnitude of these biologically driven chemical fluctuations is regulated by hydrodynamics, can exceed values predicted for the future open ocean, and creates chemical patchiness in subtidal areas at various spatial (µm to meters) and temporal (minutes to months) scales. Although the role of hydrodynamics is well explored for planktonic communities, its influence as a crucial driver of benthic organism and community functioning is poorly addressed, particularly in the context of ocean global change. Hydrodynamics can directly modulate organismal physiological activity or indirectly influence an organism's performance by modifying its habitat. This review addresses recent developments in (i) the influence of hydrodynamics on the biological activity of engineer species, (ii) the description of chemical habitats resulting from the interaction between hydrodynamics and biological activity, (iii) the role of these chemical habitat as refugia against ocean acidification and deoxygenation, and (iv) how species living in such chemical habitats may respond to ocean global change. Recommendations are provided to integrate the effect of hydrodynamics and environmental fluctuations in future research, to better predict the responses of coastal benthic ecosystems to ongoing ocean global change.

New insights in copper handling strategies in the green alga Chlamydomonas reinhardtii under low-iron condition

Copper (Cu) is a redox-active transition element critical to various metabolic processes. These functions are accomplished in tandem with Cu binding ligands, mainly proteins. The main goal of this work was to understand the mechanisms that govern the intracellular fate of Cu in the freshwater green alga, Chlamydomonas reinhardtii, and more specifically to understand the mechanisms underlying Cu detoxification by algal cells in low-Fe conditions. We show that Cu accumulation was up to 51-fold greater for algae exposed to Cu in low-Fe medium as compared to the replete-Fe growth medium. Using the stable isotope 65Cu as a tracer, we studied the subcellular distribution of Cu within the various cell compartments of C. reinhardtii. These data were coupled with metallomic and proteomic approaches to identify potential Cu-binding ligands in the heat-stable protein and peptide fractions of the cytosol. Cu was mostly found in the organelles (78%), and in the heat-stable proteins and peptides (21%) fractions. The organelle fraction appeared to also be the main target compartment of Cu accumulation in Fe-depleted cells. As Fe levels in the medium were shown to influence Cu homeostasis, we found that C. reinhardtii can cope with this additional stress by utilizing different Cu-binding ligands. Indeed, in addition to expected Cu-binding ligands such as glutathione and phytochelatins, 25 proteins were detected that may also play a role in the Cu detoxification processes in C. reinhardtii. Our results shed new light on the coping mechanisms of C. reinhardtii when exposed to environmental conditions that induce high rates of Cu accumulation.

Elevated pH Conditions Associated With Microcystis spp. Blooms Decrease Viability of the Cultured Diatom Fragilaria crotonensis and Natural Diatoms in Lake Erie

Cyanobacterial Harmful Algal Blooms (CyanoHABs) commonly increase water column pH to alkaline levels ≥9.2, and to as high as 11. This elevated pH has been suggested to confer a competitive advantage to cyanobacteria such as Microcystis aeruginosa. Yet, there is limited information regarding the restrictive effects bloom-induced pH levels may impose on this cyanobacterium’s competitors. Due to the pH-dependency of biosilicification processes, diatoms (which seasonally both precede and proceed Microcystis blooms in many fresh waters) may be unable to synthesize frustules at these pH levels. We assessed the effects of pH on the ecologically relevant diatom Fragilaria crotonensis in vitro, and on a Lake Erie diatom community in situ. In vitro assays revealed F. crotonensis monocultures exhibited lower growth rates and abundances when cultivated at a starting pH of 9.2 in comparison to pH 7.7. The suppressed growth trends in F. crotonensis were exacerbated when co-cultured with M. aeruginosa at pH conditions and cell densities that simulated a cyanobacteria bloom. Estimates demonstrated a significant decrease in silica (Si) deposition at alkaline pH in both in vitro F. crotonensis cultures and in situ Lake Erie diatom assemblages, after as little as 48 h of alkaline pH-exposure. These observations indicate elevated pH negatively affected growth rate and diatom silica deposition; in total providing a competitive disadvantage for diatoms. Our observations demonstrate pH likely plays a significant role in bloom succession, creating a potential to prolong summer Microcystis blooms and constrain diatom fall resurgence.

Enhanced removal of cesium by potassium-starved microalga, Desmodesmus armatus SCK, under photoheterotrophic condition with magnetic separation

This study investigated the feasibility of using photoheterotrophic microalga, Desmodesmus armatus SCK, for removal of cesium (Cs⁺) followed by recovery process using magnetic nanoparticles. The comparison of three microalgae results indicated that D. armatus SCK removed the most Cs⁺ at both 25 °C and 10 °C. The results also revealed that the use of microalga grown in potassium (K⁺)-starved condition improves the accumulation of Cs⁺. Heterotrophic mode with addition of volatile fatty acids (VFAs), especially acetic acids (HAc), also enhanced removal of Cs⁺ by K⁺-starved D. armatus SCK; maximum removal efficiency of Cs⁺ was almost 2-fold higher than that of cells grown without organic carbon source. The Cs⁺ taken up by this microalga was efficiently harvested using magnetic nanoparticles, polydiallyldimethylammonium (PDDA)-FeO₃. Finally, this strain eliminated more than 99% of radioactive ¹³⁷Cs from solutions of 10, 100, and 1000 Bq mL^-1. Therefore, use of K⁺-starved microalga, D. armatus SCK, with VFAs could be promising means to remove the Cs from the liquid wastes.

Dynamic CO2 and pH levels in coastal, estuarine, and inland waters: Theoretical and observed effects on harmful algal blooms

Rising concentrations of atmospheric CO₂ results in higher equilibrium concentrations of dissolved CO₂ in natural waters, with corresponding increases in hydrogen ion and bicarbonate concentrations and decreases in hydroxyl ion and carbonate concentrations. Superimposed on these climate change effects is the dynamic nature of carbon cycling in coastal zones, which can lead to seasonal and diel changes in pH and CO₂ concentrations that can exceed changes expected for open ocean ecosystems by the end of the century. Among harmful algae, i.e. some species and/or strains of Cyanobacteria, Dinophyceae, Prymnesiophyceae, Bacillariophyceae, and Ulvophyceae, the occurrence of a CO₂ concentrating mechanisms (CCMs) is the most frequent mechanism of inorganic carbon acquisition in natural waters in equilibrium with the present atmosphere (400 μmol CO₂  mol^-1 total gas), with varying phenotypic modification of the CCM. No data on CCMs are available for Raphidophyceae or the brown tide Pelagophyceae. Several HAB species and/or strains respond to increased CO₂ concentrations with increases in growth rate and/or cellular toxin content, however, others are unaffected. Beyond the effects of altered C concentrations and speciation on HABs, changes in pH in natural waters are likely to have profound effects on algal physiology. This review outlines the implications of changes in inorganic cycling for HABs in coastal zones, and reviews the knowns and unknowns with regard to how HABs can be expected to ocean acidification. We further point to the large regions of uncertainty with regard to this evolving field.

Identification and Expression Analyses of the Nitrate Transporter Gene (NRT2) Family Among Skeletonema species (Bacillariophyceae)

High-affinity nitrate transporters are considered to be the major transporter system for nitrate uptake in diatoms. In the diatom genus Skeletonema, three forms of genes encoding high-affinity nitrate transporters (NRT2) were newly identified from transcriptomes generated as part of the marine microbial eukaryote transcriptome sequencing project. To examine the expression of each form of NRT2 under different nitrogen environments, laboratory experiments were conducted under nitrate-sufficient, ammonium-sufficient, and nitrate-limited conditions using three ecologically important Skeletonema species: S. dohrnii, S. menzelii, and S. marinoi. Primers were developed for each NRT2 form and species and Q-RT-PCR was performed. For each NRT2 form, the three Skeletonema species had similar transcriptional patterns. The transcript levels of NRT2:1 were significantly elevated under nitrogen-limited conditions, but strongly repressed in the presence of ammonium. The transcript levels of NRT2:2 were also repressed by ammonium, but increased 5- to 10-fold under nitrate-sufficient and nitrogen-limited conditions. Finally, the transcript levels of NRT2:3 did not vary significantly under various nitrogen conditions, and behaved more like a constitutively expressed gene. Based on the observed transcript variation among NRT2 forms, we propose a revised model describing nitrate uptake kinetics regulated by multiple forms of nitrate transporter genes in response to various nitrogen conditions in Skeletonema. The differential NRT2 transcriptional responses among species suggest that species-specific adaptive strategies exist within this genus to cope with environmental changes.

Removal of radioactive cesium from an aqueous solution via bioaccumulation by microalgae and magnetic separation

We evaluated the potential sequestration of cesium (Cs⁺) by microalgae under heterotrophic growth conditions in an attempt to ultimately develop a system for treatment of radioactive wastewater. Thus, we examined the effects of initial Cs⁺ concentration (100–500 μM), pH (5–9), K⁺ and Na⁺ concentrations (0–20 mg/L), and different organic carbon sources (acetate, glycerol, glucose) on Cs⁺ removal. Our initial comparison of nine microalgae indicated that Desmodesmus armatus SCK had removed the most Cs⁺ under various environmental conditions. Addition of organic substrates significantly enhanced Cs⁺ uptake by D. armatus, even in the presence of a competitive cation (K⁺). We also applied magnetic nanoparticles coated with a cationic polymer (polyethylenimine) to separate ¹³⁷Cs-containing microalgal biomass under a magnetic field. Our technique of combining bioaccumulation and magnetic separation successfully removed more than 90% of the radioactive ¹³⁷Cs from an aqueous medium. These results clearly demonstrate that the method described here is a promising bioremediation technique for treatment of radioactive liquid waste.

Iron Modulation of Copper Uptake and Toxicity in a Green Alga ( Chlamydomonas reinhardtii)

Little attention has been paid to the role of essential trace elements on the toxicity of another element. In this work, we examined if low concentrations of essential elements (Co, Mn, Zn, and Fe) modified the response of a freshwater green alga ( Chlamydomonas reinhardtii) to copper. To do so, we followed cell growth over 72 h in exposure media where the essential element concentrations were manipulated. Among these elements, iron proved to have a strong impact on the cells' response to copper. The free Cu²⁺ concentrations required to inhibit cellular growth by 50% (EC50) over 72 h decreased from 2 nM in regular Fe medium (10^-17.6 M Fe³⁺) to 4 pM in low iron medium (10^-19.0 M Fe³⁺); a 500-fold increase in toxicity. Moreover, at low Cu²⁺ concentrations (10^-13.0 to 10^-10.5 M), Cu uptake increased under low iron conditions but remain relatively stable under regular iron conditions. These results show clearly that iron plays a protective role against copper uptake and toxicity to C. reinhardtii. In freshwaters, iron is always abundant but the expected free iron concentrations in surface waters can vary between 10^-14.0 to 10^-20.0 M, depending on pH (e.g., when pH increases from 6 to 8). We conclude that copper toxicity in natural waters can be modulated by iron and that, in some conditions, the Biotic Ligand Model may need to be further developed to account for the influence of iron.

Inorganic carbon and pH dependency of photosynthetic rates in Trichodesmium

Increasing atmospheric CO2 concentrations are leading to increases in dissolved CO2 and HCO3- concentrations and decreases in pH and CO32- in the world's oceans. There remain many uncertainties as to the magnitude of biological responses of key organisms to these chemical changes. In this study, we established the relationship between photosynthetic carbon fixation rates and pH, CO2, and HCO3- concentrations in the diazotroph, Trichodesmium erythraeum IMS101. Inorganic 14C-assimilation was measured in TRIS-buffered artificial seawater medium where the absolute and relative concentrations of CO2, pH, and HCO3- were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (<0 to ~5 mM) at constant pH, so that ratios of CO2 and HCO3- remained relatively constant. Second, we varied pH (~8.54 to 7.52) at constant TIC, so that CO2 increased whilst HCO3- declined. We found that 14C-assimilation could be described by the same function of CO2 for both approaches, but it showed different dependencies on HCO3- when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of the carbon-concentrating mechanism (CCM) of Trichodesmium showed that carboxylation rates are modulated by HCO3- and pH. The decrease in assimilation of inorganic carbon (Ci) at low CO2, when TIC was varied, was due to HCO3- uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO3- and CO2 leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions.

Effects of growth rate, cell size, motion, and elemental stoichiometry on nutrient transport kinetics

Nutrient acquisition is a critical determinant for the competitive advantage for auto- and osmohetero- trophs alike. Nutrient limited growth is commonly described on a whole cell basis through reference to a maximum growth rate (Gmax) and a half-saturation constant (KG). This empirical application of a Michaelis-Menten like description ignores the multiple underlying feedbacks between physiology contributing to growth, cell size, elemental stoichiometry and cell motion. Here we explore these relationships with reference to the kinetics of the nutrient transporter protein, the transporter rate density at the cell surface (TRD; potential transport rate per unit plasma-membrane area), and diffusion gradients. While the half saturation value for the limiting nutrient increases rapidly with cell size, significant mitigation is afforded by cell motion (swimming or sedimentation), and by decreasing the cellular carbon density. There is thus potential for high vacuolation and high sedimentation rates in diatoms to significantly decrease KG and increase species competitive advantage. Our results also suggest that Gmax for larger non-diatom protists may be constrained by rates of nutrient transport. For a given carbon density, cell size and TRD, the value of Gmax/KG remains constant. This implies that species or strains with a lower Gmax might coincidentally have a competitive advantage under nutrient limited conditions as they also express lower values of KG. The ability of cells to modulate the TRD according to their nutritional status, and hence change the instantaneous maximum transport rate, has a very marked effect upon transport and growth kinetics. Analyses and dynamic models that do not consider such modulation will inevitably fail to properly reflect competitive advantage in nutrient acquisition. This has important implications for the accurate representation and predictive capabilities of model applications, in particular in a changing environment.

Seawater pH, and not inorganic nitrogen source, affects pH at the blade surface of Macrocystis pyrifera: implications for responses of the giant kelp to future oceanic conditions

Ocean acidification (OA), the ongoing decline in seawater pH, is predicted to have wide-ranging effects on marine organisms and ecosystems. For seaweeds, the pH at the thallus surface, within the diffusion boundary layer (DBL), is one of the factors controlling their response to OA. Surface pH is controlled by both the pH of the bulk seawater and by the seaweeds' metabolism: photosynthesis and respiration increase and decrease pH within the DBL (pHDBL ), respectively. However, other metabolic processes, especially the uptake of inorganic nitrogen (Ni ; NO3- and NH4+ ) may also affect the pHDBL . Using Macrocystis pyrifera, we hypothesized that (1) NO3- uptake will increase the pHDBL , whereas NH4+ uptake will decrease it, (2) if NO3- is cotransported with H+ , increases in pHDBL would be greater under an OA treatment (pH = 7.65) than under an ambient treatment (pH = 8.00), and (3) decreases in pHDBL will be smaller at pH 7.65 than at pH 8.00, as higher external [H+ ] might affect the strength of the diffusion gradient. Overall, Ni source did not affect the pHDBL . However, increases in pHDBL were greater at pH 7.65 than at pH 8.00. CO2 uptake was higher at pH 7.65 than at pH 8.00, whereas HCO3- uptake was unaffected by pH. Photosynthesis and respiration control pHDBL rather than Ni uptake. We suggest that under future OA, Macrocystis pyrifera will metabolically modify its surface microenvironment such that the physiological processes of photosynthesis and Ni uptake will not be affected by a reduced pH.

Importance of controlling pH-depended dissolved inorganic carbon to prevent algal bloom outbreaks

This study investigated effects of pH-depended inorganic carbon (IC) species and pH on algal growth in the sewage simulation system, and fruitfully discussed the relationships among IC, pH and algal growth by the Monod kinetics. Results showed HCO3(-) significantly increased algal growth by 3.17-6.52 times than that of CO3(2-) and/or glucose when the value of pH was in the range of 8.0-9.5, and also the preferentially utilized indicated by the affinity coefficient (Kp) of HCO3(-), CO3(2-) and glucose (0.17, 15.14 and 31.22, respectively). Meanwhile, the same pH range facilitated HCO3(-) to become a dominated species (e.g., 48.80-93.19% of total IC). More importantly, good linear correlations pairwise existed among pH, IC species and algae growth. These results suggested pH plays a critical role in regulation of IC species and algae growth, which would be an efficient method to control the IC discharge from sewage effluents and weaken bloom outbreak.

Differential effects of ocean acidification on carbon acquisition in two bloom-forming dinoflagellate species

Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) with very low CO2 affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO2 partial pressure (pCO2) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom-forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO2 (i.e. 180-1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane-inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO2 range, but we observed a number of species-specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO2-dependent contribution of HCO3(-) uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO2-independent carbonic anhydrase activity. Comparing both species, a general trade-off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO2 .

Independent colimitation for carbon dioxide and inorganic phosphorus

Simultaneous limitation of plant growth by two or more nutrients is increasingly acknowledged as a common phenomenon in nature, but its cellular mechanisms are far from understood. We investigated the uptake kinetics of CO(2) and phosphorus of the algae Chlamydomonas acidophila in response to growth at limiting conditions of CO(2) and phosphorus. In addition, we fitted the data to four different Monod-type models: one assuming Liebigs Law of the minimum, one assuming that the affinity for the uptake of one nutrient is not influenced by the supply of the other (independent colimitation) and two where the uptake affinity for one nutrient depends on the supply of the other (dependent colimitation). In addition we asked whether the physiological response under colimitation differs from that under single nutrient limitation.We found no negative correlation between the affinities for uptake of the two nutrients, thereby rejecting a dependent colimitation. Kinetic data were supported by a better model fit assuming independent uptake of colimiting nutrients than when assuming Liebigs Law of the minimum or a dependent colimitation. Results show that cell nutrient homeostasis regulated nutrient acquisition which resulted in a trade-off in the maximum uptake rates of CO(2) and phosphorus, possibly driven by space limitation on the cell membrane for porters for the different nutrients. Hence, the response to colimitation deviated from that to a single nutrient limitation. In conclusion, responses to single nutrient limitation cannot be extrapolated to situations where multiple nutrients are limiting, which calls for colimitation experiments and models to properly predict growth responses to a changing natural environment. These deviations from single nutrient limitation response under colimiting conditions and independent colimitation may also hold for other nutrients in algae and in higher plants.

Mapping the Fundamental Niches of Two Freshwater Microalgae,Chlorella vulgaris(Trebouxiophyceae) andPeridinium cinctum(Dinophyceae), in 5-Dimensional Ion Space

The fundamental niche defined by five ions,NO3 −,PO4 3−, K+, Na+, andCl−, was mapped forChlorella vulgaris(Trebouxiophyceae) andPeridinium cinctum(Dinophyceae) growth rates and maximum cell densities in batch cultures. A five dimensional ion-mixture experimental design was projected across a total ion concentration gradient of 1 to 30 mM to delineate the ion-based, “potential” niche space, defined as the entiren-dimensional hypervolume demarcated by the feasible ranges of the independent factors under consideration. The growth rate-based, fundamental niche volumes overlapped for ca. 94% of the ion mixtures, although the regions of maximal growth rates and cell densities were different for each alga. BothC. vulgarisandP. cinctumexhibited similar positive responses to cations and negative responses to anions. It was determined that total ion concentration for these five ions, from 1 to 30 mM, did not directly affect either growth rate or maximal cell density for either alga, although it did play an interactive role with several ions. This study is the first that we are aware of to attempt the mapping of a multivariate, ion-based, fundamental niche volume. The implications of the experimental design utilized and the potential utility of this type of approach are discussed.

The evolution of inorganic carbon concentrating mechanisms in photosynthesis

Inorganic carbon concentrating mechanisms (CCMs) catalyse the accumulation of CO(2) around rubisco in all cyanobacteria, most algae and aquatic plants and in C(4) and crassulacean acid metabolism (CAM) vascular plants. CCMs are polyphyletic (more than one evolutionary origin) and involve active transport of HCO(3)(-), CO(2) and/or H(+), or an energized biochemical mechanism as in C(4) and CAM plants. While the CCM in almost all C(4) plants and many CAM plants is constitutive, many CCMs show acclimatory responses to variations in the supply of not only CO(2) but also photosynthetically active radiation, nitrogen, phosphorus and iron. The evolution of CCMs is generally considered in the context of decreased CO(2) availability, with only a secondary role for increasing O(2). However, the earliest CCMs may have evolved in oxygenic cyanobacteria before the atmosphere became oxygenated in stromatolites with diffusion barriers around the cells related to UV screening. This would decrease CO(2) availability to cells and increase the O(2) concentration within them, inhibiting rubisco and generating reactive oxygen species, including O(3).

Insights into the evolution of CCMs from comparisons with other resource acquisition and assimilation processes

Regarding inorganic carbon as 'just another' chemical resource used in the growth of aquatic photolithotrophs, we ask three questions and then attempt to answer them. (1) How common are catalysed chemical changes of the resource outside the cell, and accumulation of the resource inside the cell prior to assimilation, for the diverse chemical resources used? (2) Do acquisition and assimilation meet evolutionary optimality criteria with respect to the use of other resources? (3) Are there clues to the evolutionary origin of inorganic carbon concentrating mechanism (CCMs) in the mechanisms of acquisition of other resources and vice versa? Evidence considered includes molecular genetic similarities between CCM components and components of other resource acquisition mechanisms, and palaeogeochemical evidence on the timing of restrictions on the availability of the resources such that extracellular transformation of materials, and their accumulation within cells prior to assimilation, are needed. Provisional answers to the questions are as follows: (1) Many common chemical resources other than inorganic carbon are subject to extracellular chemical conversion and/or accumulation prior to assimilation, e.g. ammonium, nitrate, urea, amino acids, organic and inorganic phosphate and iron; (2) There is some evidence for optimality of CCMs and of less complex resource acquisition processes, exemplified by NH(4)(+) entry and assimilation, though many more data are needed and (3) There are molecular genetic similarities between CCM components and transporters for other solutes and components of respiratory NADH dehydrogenases that are consistent with their use in CCMs representing a derived evolutionary state. Palaeogeochemical evidence suggests that CCMs evolved later than did at least some of the extracellular chemical transformation and/or accumulation mechanisms for other resources.

The role of functional traits and trade-offs in structuring phytoplankton communities: scaling from cellular to ecosystem level

Trait-based approaches to community structure are increasingly used in terrestrial ecology. We show that such an approach, augmented by a mechanistic analysis of trade-offs among functional traits, can be successfully used to explain community composition of marine phytoplankton along environmental gradients. Our analysis of literature on major functional traits in phytoplankton, such as parameters of nutrient-dependent growth and uptake, reveals physiological trade-offs in species abilities to acquire and utilize resources. These trade-offs, arising from fundamental relations such as cellular scaling laws and enzyme kinetics, define contrasting ecological strategies of nutrient acquisition. Major groups of marine eukaryotic phytoplankton have adopted distinct strategies with associated traits. These diverse strategies of nutrient utilization can explain the distribution patterns of major functional groups and size classes along nutrient availability gradients.

Cyanobacterial heterocysts: terminal pores proposed as sites of gas exchange

In many filamentous cyanobacteria, oxygenic photosynthesis is restricted to vegetative cells, whereas N(2) fixation is confined to microoxic heterocysts. The heterocyst has an envelope that provides a barrier to gas exchange: N(2) and O(2) diffuse into heterocysts at similar rates, which ensures that concentrations of N(2) are high enough to saturate N(2) fixation while respiration maintains O(2) at concentrations low enough to prevent nitrogenase inactivation. I propose that the main gas-diffusion pathway is through the terminal pores that connect heterocysts with vegetative cells. Transmembrane proteins would make the narrow pores permeable enough and they might provide a means of regulating the rate of gas exchange, increasing it by day, when N(2) fixation is most active, and decreasing it at night, minimizing O(2) entry. Comparisons are made with stomata, which regulate gas exchange in plants.

An anaplerotic role for mitochondrial carbonic anhydrase in Chlamydomonas reinhardtii

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO(2) concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO(2) concentrations decreases when the external NH(4)(+) concentration decreases, to the point of being undetectable when NH(4)(+) supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO(2) by increasing the NH(4)(+) concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO(3)(-) for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.

The relationship between the dissolved inorganic carbon concentration and growth rate in marine phytoplankton

A range of marine phytoplankton was grown in closed systems in order to investigate the kinetics of dissolved inorganic carbon (DIC) use and the influence of the nitrogen source under conditions of constant pH. The kinetics of DIC use could be described by a rectangular hyperbolic curve, yielding estimations of KG(DIC) (the half saturation constant for carbon-specific growth, i.e. C mu) and mu max (the theoretical maximum C mu). All species attained a KG(DIC) within the range of 30-750 microM DIC. For most species, NH4+ use enabled growth with a lower KG(DIC) and/or, for two species, an increase in mu max. At DIC concentrations of > 1.6 mM, C mu was > 90% saturated for all species relative to the rate at the natural seawater DIC concentration of 2.0 mM. The results suggest that neither the rate nor the extent of primary productivity will be significantly limited by the DIC in the quasi-steady-state conditions associated with oligotrophic oceans. The method needs to be applied in the conditions associated with dynamic coastal (eutrophic) systems for clarification of a potential DIC rate limitation where cells may grow to higher densities and under variable pH and nitrogen supply.

Modelling the interactions between ammonium and nitrate uptake in marine phytoplankton

An empirically based mathematical model is presented which can simulate the major features of the interactions between ammonium and nitrate transport and assimilation in phytoplankton. The model (ammonium-nitrate interaction model), which is configured to simulate a generic microalga rather than a specified species, is constructed on simplified biochemical bases. A major requirement for parametrization is that the N:C ratio of the algae must be known and that transport and internal pool sizes need to be expressed per unit of cell C. The model uses the size of an internal pool of an early organic product of N assimilation (glutamine) to regulate rapid responses in ammonium-nitrate interactions. The synthesis of enzymes for the reduction of nitrate through to ammonium is induced by the size of the internal nitrate pool and repressed by the size of the glutamine pool. The assimilation of intracellular ammonium (into glutamine) is considered to be a constitutive process subjected to regulation by the size of the glutamine pool. Longer term responses have been linked to the nutrient history of the cell using the N:C cell quota. N assimilation in darkness is made a function of the amount of surplus C present and thus only occurs at low values of N:C. The model can simulate both qualitative and quantitative temporal shifts in the ammonium-nitrate interaction, while inclusion of a derivation of the standard quota model enables a concurrent simulation of cell growth and changes in nutrient status.

Do plant photoreceptors act at the membrane level?

All of the photoreceptors involved in the absorption and transduction of light energy in photosynthesis are integral (carotenoid, chlorophyll) or peripheral (phycobilin) membrane proteins. The informational photoreceptors (phytochrome) and the flavoprotein (carotenoprotein?) cryptochrome, could be integral (carotenoprotein, flavoprotein) or peripheral or soluble (phytochrome, flavoprotein) pigment-protein complexes. The primary activity of the informational photoreceptors is unlikely to involve energization of primary active transport: the solute fluxes produced in this way would not form a quantitatively significant link in the perception-transduction-response sequence. By contrast, regulation of mediated solute fluxes at the plasmalemma could effect a substantial amplification of the absorbed photon signal, i.e. a large change in moles of solute transported could result from the absorption of 1 mol of photons. Modulation of the passive influx (or active efflux) of protons or calcium ions at the plasmalemma are likely targets for regulation by photoreceptors. Calcium flux regulation is particularly attractive in view of the ubiquity of calmodulin activity in eukaryotes, although problems could arise in maintaining the uniqueness of phytochrome messages vis-à-vis cryptochrome messages. Temporal analysis of the relation between photoreceptor changes and electrical effects resulting from changes in ion fluxes cannot, in general, rule out the involvement of intermediates between the redox or conformational change in the photoreceptor and the observed change in ion flux. Although slow in terms of the potential rate of change on solute fluxes resulting from direct interaction of a photoreceptor and a solute porter, the observed rates of signal transduction are well in excess of any obvious ‘need’ on the part of the plant in terms of rates of response to environmental changes.

The permeability of heterocysts to the gases nitrogen and oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retina gas vacuoles for several days after differentiation. It is demonstrated that the rate of gas diffusion into a heterocyst that is near an overlying gas phase can be determined approximately from observations on the rate of gas pressure rise required to collapse 50% of its gas vacuoles. The mean permeability coefficient ( α ) of heterocysts O 2 and N 2 was found to be 0.3 s -1 . From this it was calculated that the average permeability ( k ) of the heterocyst surface layer is about 0.4 μm s -1 (within a factor of 2). This is probably within the range that could be provided by a few layers of the 26-C glycolipids in the heterocyst envelope. It is likely, but not proven, that the main route for gas diffusion is through the envelope rather than through the terminal pores of the heterocyst. From measurements of cell nitrogen content (2.7 pg). doubling time (3 days) and heterocyst: vegetative cell ratio (1:24) it was calculated that the average heterocyst fixed 5.9 x 10 -18 mol N 2 s -1 ; this must equal the diffusion rate of N 2 inside the average heterocyst that was 22% below the outside air-saturated concentration. the maximum N 2 fixation rate allowed by the estimated permeability coefficeint would be 2.7 x 10 -17 mol s -1 per heterocyst, slightly greater than the maximum calcualted N 2 fixation rate. The observed permeability coefficient is low enough for the oxygen concentration in the heterocyst to be maintained close to zero by the probable rate of respiration, providing an anaerobic environment for nitrogenase. The rate of O 2 diffusion will limit the N 2 -fixation rate in the dark by limiting the rate at which ATP is supplied by oxidative phosphorylation.

Transport of small lons and molecules through the plasma membrane of filamentous fungi

Less than 1% of the estimated number of fungal species have been investigated concerning the transport of low-molecular-weight nutrients and metabolites through the plasma membrane. This is surprising if one considers the importance of the processes at the plasma membrane for the cell: this membrane mediates between the cell and its environment. Concentrating on filamentous fungi, in this review emphasis is placed on relating results from biophysical chemistry, membrane transport, fungal physiology, and fungal ecology. Among the treated subjects are the consequences of the small dimension of hyphae, the habitat and membrane transport, the properties of the plasma membrane, the efflux of metabolites, and the regulation of membrane transport. Special attention is given to methodological problems occurring with filamentous fungi. A great part of the presented material relies on work with Neurospora crassa, because for this fungus the most complete picture of plasma membrane transport exists. Following the conviction that we need "concepts instead of experiments", we delineate the lively network of membrane transport systems rather than listing the properties of single transport systems.

Caesium accumulation by microorganisms: uptake mechanisms, cation competition, compartmentalization and toxicity

The continued release of caesium radioisotopes into the environment has led to a resurgence of interest in microbe-Cs interactions. Caesium exists almost exclusively as the monovalent cation Cs+ in the natural environment. Although Cs+ is a weak Lewis acid that exhibits a low tendency to form complexes with ligands, its chemical similarity to the biologically essential alkali cation K+ facilitates high levels of metabolism-dependent intracellular accumulation. Microbial Cs+ (K+) uptake is generally mediated by monovalent cation transport systems located on the plasma membrane. These differ widely in specificity for alkali cations and consequently microorganisms display large differences in their ability to accumulate Cs+; Cs+ appears to have an equal or greater affinity than K+ for transport in certain microorganisms. Microbial Cs+ accumulation is markedly influenced by the presence of external cations, e.g. K+, Na+, NH4+ and H+, and is generally accompanied by an approximate stoichiometric exchange for intracellular K+. However, stimulation of growth of K(+)-starved microbial cultures by Cs+ is limited and it has been proposed that it is not the presence of Cs+ in cells that is growth inhibitory but rather the resulting loss of K+. Increased microbial tolerance to Cs+ may result from sequestration of Cs+ in vacuoles or changes in the activity and/or specificity of transport systems mediating Cs+ uptake. The precise intracellular target(s) for Cs(+)-induced toxicity has yet to be clearly defined, although certain internal structures, e.g. ribosomes, become unstable in the presence of Cs+ and Cs+ is known to substitute poorly for K+ in the activation of many K(+)-requiring enzymes.

Caesium accumulation and interactions with other monovalent cations in the cyanobacterium Synechocystis PCC 6803

Growth of Synechocystis PCC 6803 in BG-11 medium supplemented with 1 mM-CsCl resulted in intracellular accumulation of Cs+ to a final level of approximately 510 nmol (109 cells)-1 after incubation for 10 d. The doubling time was increased by 64% and the final cell yield was decreased by 70% during growth in the presence of Cs+ as compared to growth in control BG-11 medium. When the total monovalent cation concentration of the medium was doubled by adding either K+ or Na+, levels of accumulated Cs+ were decreased by approximately 50% to 220 and 270 nmol (109 cells)-1, respectively, after 28 d with little inhibition of growth being apparent. Short-term experiments revealed that extracellular K+ and Na+ inhibited Cs+ accumulation to a similar extent, with 90% inhibition of Cs+ accumulation occurring at the highest concentrations used (50 mM-K+ or Na+; 1 mM-Cs+). In all experiments, Cs+ accumulation resulted in a reduction in intracellular K+, except when cells were grown in K+-depleted medium, although a stoichiometric relationship was not apparent, the amount of Cs+ accumulated generally being greater than the amount of K+ released. Cs+ accumulation had no discernible effect on intracellular Na+. When K+, Na+, Rb+, Li+ or Tl+ were supplied at equimolar (1 mM) concentrations to Cs+, only Tl+ significantly reduced Cs+ accumulation. However, an approximately 50% inhibition of Cs+ accumulation resulted when concentrations of K+, Na+, Rb+ or Li+ were increased to 10 mM, which suggests that Cs+ may have a higher affinity for the monovalent cation transport system than K+, Rb+ and TI+ also caused a decrease in intracellular K+, whereas Na+ and Li+ stimulated K+ uptake. Cs+ accumulation was dependent on the external Cs+ concentration and showed a linear relationship to external Cs+ concentrations≤2 mM over 12 h incubation. However, prolonged incubation in external Cs+ concentrations≥ 0·8 mM resulted in Cs+ release from the cells and after 48 h, similar amounts of Cs+ and K+ were present in cells incubated at these higher concentrations. Cs+ accumulation was energy- and pH-dependent. Incubation in the light at 4 °C, or in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), or at 22 °C in the dark resulted in decreased Cs+ accumulation and decreased K+ release from the cells. Increased amounts of Cs+ were accumulated as the pH of the external medium was increased, with maximal accumulation [approximately 1330 nmol Cs+ (109 cells)-1 after 24 h incubation] occurring at pH 10, the highest pH value used. It is suggested that an important mechanism of Cs+ toxicity in Synechocystis PCC 6803 arises through replacement of cellular K+ by Cs+. The possible role of primary producers such as cyanobacteria in the mobilization of this radionuclide in aquatic habitats is discussed.

H+ extrusion and organic-acid synthesis in N2 -fixing symbioses involving vascular plants

An analysis of published data suggests that the N₂ -fixing symbiotic vascular plants extrude more H⁺ per unit N fixed than would be expected from data on the same genotypes growing on NH₄ ⁺ if the plants had the same chemical composition when grown on the two N sources. The H⁺ /N ratio with urea as the N source is similar to that with N₂ . The higher H⁺ /N ratio and higher organic acid/N ratio with N₂ or urea as N source implies higher whole-plant energy and water costs per unit of biomass and, ultimately, inclusive fitness, produced. The rhizosphere acidification resulting from H⁺ extrusion may serve to change rhizosphere pH to some 'optimal' value, and to increase the availability of such limiting resources as P, Mo and Fe which are especially needed in diazotrophy. Data in the literature are consistent with these possibilities in the few cases examined. Within the plant, data on xylem and phloem sap composition in conjunction with shoot composition, of diazotrophically-growing legumes suggest that shoot acid-base homoiostasis can be maintained via the import of appropriate solutes in the xylem and the export of appropriate solutes in the phloem. Acid-base regulation of the nodules in the absence of any H⁺ exchange with their environment can also probably be explained in terms of the solutes supplied in the phloem and exported in the xylem. This conclusion is based on data in the literature on the composition of stem phloem sap and of xylem sap exuding from detached nodules of diazotrophic vascular plants. These considerations do not exclude the possibility of net H⁺ efflux from nodules fixing N₂ in contact with an aqueous medium. The limited data available are consistent with extrusion of some of the H⁺ generated in nodules as an alternative to their neutralization by metabolism of organic anions entering in the phloem. Such H⁺ extrusion by nodules could aid in their acquisition of Fe from the medium, albeit not always at a phase in the life or the nodule when there is a net requirement for Fe.

Transport and assimilation of inorganic carbon by Lichina pygmaea under emersed and submersed conditions

Photosynthetic O₂ evolution by the upper littoral lichen, Lichina pygmaea (Lightf.) C.Ag., under light-saturated conditions at 5 °C is saturated by the 2 mol m^-3 inorganic C found in seawater at pH 8.0. Photosynthesis is not reduced when pH is increased to pH 9.4, and is slightly reduced at pH 10.0, when submersed in seawater with 2 mol m^-3 inorganic C. The rate of photosynthesis at pH 10 greatly exceeds the rate of uncatalysed conversion of HCO₃ ^- . It is concluded that HCO₃ ^- is used in photosynthesis. Since extracellular carbonic anhydrase is present, it is possible that CO₂ enters the photobiont (Calothrix) cells even during HCO₃ use. pH drift experiments support the notion of HCO₃ ^- use. Emersed photosynthesis at 5 °C is more than half-saturated by 35 Pa (normal atmospheric) CO₂ ; the light- and CO₂ -saturated emersed photosynthetic rate is not significantly different from the light and inorganic C-saturated photosynthetic rate when submersed. Inorganic C diffusion from the thallus surface to the photobiont needs, at least under some conditions, carbonic anhydrase activity which permits HCO₃ ^- fluxes to supplement CO₂ movement. The CO₂ compensation partial pressure at 5 °C is 0.83 Pa, i.e. at the low range of values found for terrestrial cyanobacterial lichens. Dark ¹⁴ C-inorganic C assimilation when submersed is a small fraction of the dark respiratory rate, consistent with the observed absence of diel CAM-like variation in intracellular titratable acidity. The high value (-11.5%) of δ¹³ C, the low CO₂ compensation partial pressure, and the relatively high affinity for inorganic C., are consistent with the operation of an inorganic C concentrating mechanism such as occurs in free-living cyanobacteria and probably occurs in terrestrial cyanobacterial lichens and in most intertidal algae.

Measurement of internal pH in the coccolithophoreEmiliania huxleyi using 2',7'-bis-(2-carboxyethyl)-5(and-6)carboxyfluorescein acetoxymethylester and digital imaging microscopy

Internal pH (pHi) was determined inEmiliania huxleyi (Lohmann) using the probe 2',7'-bis-(2-carboxyethyl)-5(and-6)carboxyfluoresceinacetoxymethylester (BCEF-AM) and digital imaging microscopy. The probe BCECF-AM was taken up and hydrolysed to the free acid by the cells. A linear relationship was established between pHi and the 490/450 fluorescence ratio of BCECF-AM over the pH range 6.0 to 8.0 using the ionophore nigericin. Two distinct pH domains were identified within the cell, the cytoplasmic domain (approx. pH 7.0) and the chloroplast domain (approx. pH 8.0). The average pHi was 7.29 (±0.11) for cells in the presence of 2 mM HCO 3 (-) . In the absence of HCO 3 (-) the pHi was decreased by 0.8 pH unit. The importance of these changes in pHi is considered in relation to inorganic-carbon uptake.

Regulation of carbonic-anhydrase activity, inorganic-carbon uptake and photosynthetic biomass yield inChlamydomonas reinhardtii

The regulation of carbonic anhydrase by environmental conditions was determined forChlamydomonas reinhardtii. The depression of carbonic anhydrase in air-grown cells was pH-dependent. Growth of cells on air at acid pH, corresponding to 10 μm CO2 in solution, resulted in complete repression of carbonic-anhydrase activity. At pH 6.9, increasing the CO2 concentration to 0.15% (v/v) in the gas phase, corresponding to 11 μM in solution, was sufficient to completely repress carbonic-anhydrase activity. Photosynthesis and intracellular inorganic carbon were measured in air-grown and high-CO2-grown cells using a silicone-oil centrifugation technique. With carbonic anhydrase repressed cells limited inorganic-carbon accumulation resulted from non-specific binding of CO2. With air-grown cells, inorganic-carbon uptake at acid pH, i.e. 5.5, was linear up to 0.5 mM external inorganic-carbon concentration whereas at alkaline pH, i.e. 7.5, the accumulation ratio decreased with increase in external inorganic-carbon concentration. It is suggested that in air-grown cells at acid pH, CO2 is the inorganic carbon species that crosses the plasmalemma. The conversion of CO2 to HCO 3 (-) by carbonic anhydrase in the cytosol results in inorganic-carbon accumulation and maintains the diffusion gradient for carbon dioxide across the cell boundary. However, this mechanism will not account for energy-dependent accumulation of inorganic carbon when there is little difference in pH between the exterior and cytosol.

Transport and accumulation of nickel ions in the cyanobacterium Anabaena cylindrica

The uptake of nickel ions by the cyanobacterium Anabaena cylindrica was studied. Nickel transport was dependent on the membrane potential of the cells and the rate of uptake was decreased in the dark or by the addition of inhibitors, including uncouplers and electron transport inhibitors, which decreased or abolished the membrane potential of cells. The transport process obeyed hyperbolic kinetics, with a high affinity (apparent Km = 17 +/- 11 (SEM) nM) and low turnover number (maximum velocity = 22.3 +/- 5.4 (SEM) pmol h-1 mg dry wt-1 of cells or flux rate of 3.1 nmol h-1 m-2 of plasma membrane surface area). The process was also apparently specific for Ni2+, the rate being unaffected by the presence of a range of other metal ions in large excess. Equilibrium experiments showed that, over a range of nickel ion concentrations, the cells concentrated Ni2+ by a factor of 2700 +/- 240 (SEM)-fold, corresponding to a chemical diffusion potential for Ni2+ of 101 mV. It was concluded that the cells transport nickel ions by a carrier-facilitated transport process with the concentration factor for the ions being determined by the cell membrane potential according to the Nernst equation.

TANSLEY REVIEW No. 2: REGULATION OF PH AND GENERATION OF OSMOLARITY IN VASCULAR PLANTS: A COST-BENEFIT ANALYSIS IN RELATION TO EFFICIENCY OF USE OF ENERGY, NITROGEN AND WATER

The benefits which this paper addresses are those of maintaining the intracellular acid-base balance during growth, and of generating osmolarity related to regulation of turgor in environments of low water potential. These benefits may incur costs in terms of the quantity of potentially growth-limiting resources (photons, water, nitrogen) which are needed to produce unit quantity of 'baseline' plant biomass. The direction (excess H⁺ or excess OH^- ) and magnitude of acid-base perturbation during growth depends on the nature of the N-source (NH₄ ⁺ , N₂ or NO₃ ^- ), so that the costing of pH homoiostasis involves consideration of the costs of overall N-assimilation for comparison with the other costs of growth of a terrestrial C₃ plant. Photon costs for the various biochemical and transport processes involved in overall growth, N-assimilation, pH regulation and osmolarity generation are computed using known stoichiometries of coupled reactions. Water costs are deduced from the C-requirements for the various processes (including C lost in associated decarboxylations) by assuming a constant value of water lost in transpiration per unit net C fixed in an illuminated shoot. Nitrogen costs are deduced from the N-content of the plants or compounds under consideration. The computed costs for N-assimilation and the generation of osmolarity are referred to the costs of 'baseline' plant synthesis using the cheapest mechanisms (NH₄ ⁺ as source for N-assimilation; inorganic ions as the basis for osmolarity generation) so that the increment of cost related to assimilation of N₂ or NO₃ ^- , or of osmolarity generation using an organic compatible solute, can be presented. Photon costs of growth with N₂ fixation and the processes associated with regulation of pH are (granted the assumptions made as to stoichiometries and plant composition) 9 % higher than are those of growth with NH₄ ⁺ as N˜ source. The predicted cost of growth with NO₃ ^- as N source depends on the location of NO₃ ^- reduction and the mechanism of OH^- disposal, and ranges from 5 to 12% more than that for growth with NH₄ ⁺ as N source. H₂ O (transpiration) costs follow a similar pattern, with growth on N₂ as N source costing 12% more, and growth on NO₃ ^- costing to 1-2 to 167 % more, than growth with NH₄ ⁺ as N source. The extra costs in photons of using compatible solutes (sorbitol, proline or glycine betaine) to generate an osmolarity of 500 osmol m^-3 in all of the non-apoplastic water of the plant add 21·5 to 26·1 % to the total costs of growth, while use of compatible solutes to generate osmolarity in 'N' phases (i.e. cytosol, plastid stroma, mitochondrial matrix) alone would add 5·2 to 6·2% The costs of growth in terms of transpirational water are increased 7·9 to 98 % by the use of compatible solutes for osmolarity generation in the 'N' phases only. The increments for the N-containing solutes are higher when NO₃ ^- is the N-source rather than NH₄ ⁺ . The N-cost of growth with N-containing compatible solutes generating 500 osmol m^-3 in 'N' phases increases the N cost of growth by 33%. These predicted costs are under-estimates of 'real' costs which take into account the occurrence of alternate oxidase activity under some growth conditions and the production of additional organic acid anions with N₂ as opposed to NH₄ ⁺ as N source. Nevertheless, the predicted minimum costs of attaining the benefits of pH regulation and of turgor generation are of use in suggesting where selectively significant (i.e. low requirement for a scarce resource) alternative mechanisms may occur. Examples include a possible photon saving by using NH₄ ⁺ rather than N₂ or NO₃ ^- where all three are available; a possible water saving by use of photoreduction of NO₃ ^- in leaves in arid environments; and a possible N saving by use of non-N-containing compatible solutes (polyols) in environments of low water potential. Proof of these suggestions involves comparisons of inclusive fitness of genotypes possessing the trait under consideration with that of genotypes lacking the trait. CONTENTS Summary 26 I. Introduction 27 II. pH Regulation and Osmolarity Generation 27 III. Photon Costs of Various Syntheses Related to pH Regulation and Osmolarity Generation 31 IV. Conclusions on Energy Costs of pH Regulation During Nitrogen Assimilation and Growth 56 V. Conclusions on Energy Costs of Osmolarity Generation 60 VI. Water Costs of pH Regulation and Nitrogen Assimilation 61 VII. Water Costs of Osmolarity Generation 67 VIII. Nitrogen Costs of Osmolarity Generation 69 IX. Conclusions 70 Acknowledgements 72 References 73.