Multi-factor kinetic modelling of microalgal biomass cultivation for optimised lipid production.

This paper presents a new quadruple-factor kinetic model of microalgal cultivation considering carbon and nitrogen concentration, light intensity and temperature, developed in conjunction with laboratory-scale experiments using the well-studied chlorophyte microalgal species Chlamydomonas reinhardtii. Multi-parameter quantification was exploited to assess the predictive capabilities of the model. The validated model was utilized in an optimization study to determine the optimal light intensity and temperature for achieving maximum lipid productivity while using optimal acetate and nitrogen concentrations (2.1906 g L-1 acetate and 0.0742 g L-1 nitrogen) computed in a recent publication. It was found that the optimal lipid productivity increased by 50.9% compared to the base case, and by 13.6% compared to the previously computed optimal case. Optimization results were successfully validated experimentally. Such comprehensive modelling approaches can be exploited for robust design, scale-up and optimization of microalgal oil production, reducing operating costs and bringing this important technology closer to industrialization.

CAX-ing a wide net: Cation/H(+) transporters in metal remediation and abiotic stress signalling.

Cation/proton exchangers (CAXs) are a class of secondary energised ion transporter that are being implicated in an increasing range of cellular and physiological functions. CAXs are primarily Ca(2+) efflux transporters that mediate the sequestration of Ca(2+) from the cytosol, usually into the vacuole. Some CAX isoforms have broad substrate specificity, providing the ability to transport trace metal ions such as Mn(2+) and Cd(2+) , as well as Ca(2+) . In recent years, genomic analyses have begun to uncover the expansion of CAXs within the green lineage and their presence within non-plant species. Although there appears to be significant conservation in tertiary structure of CAX proteins, there is diversity in function of CAXs between species and individual isoforms. For example, in halophytic plants, CAXs have been recruited to play a role in salt tolerance, while in metal hyperaccumulator plants CAXs are implicated in cadmium transport and tolerance. CAX proteins are involved in various abiotic stress response pathways, in some cases as a modulator of cytosolic Ca(2+) signalling, but in some situations there is evidence of CAXs acting as a pH regulator. The metal transport and abiotic stress tolerance functions of CAXs make them attractive targets for biotechnology, whether to provide mineral nutrient biofortification or toxic metal bioremediation. The study of non-plant CAXs may also provide insight into both conserved and novel transport mechanisms and functions.

Regulation of CAX1, an Arabidopsis Ca(2+)/H+ antiporter. Identification of an N-terminal autoinhibitory domain.

Regulation of Ca(2+) transport determines the duration of a Ca(2+) signal, and hence, the nature of the biological response. Ca(2+)/H+ antiporters such as CAX1 (cation exchanger 1), play a key role in determining cytosolic Ca(2+) levels. Analysis of a full-length CAX1 clone suggested that the CAX1 open reading frame contains an additional 36 amino acids at the N terminus that were not found in the original clone identified by suppression of yeast (Saccharomyces cerevisiae) vacuolar Ca(2+) transport mutants. The long CAX1 (lCAX1) could not suppress the yeast Ca(2+) transport defects despite localization to the yeast vacuole. Calmodulin could not stimulate lCAX1 Ca(2+)/H+ transport in yeast; however, minor alterations in the 36-amino acid region restored Ca(2+)/H+ transport. Sequence analysis suggests that a 36-amino acid N-terminal regulatory domain may be present in all Arabidopsis CAX-like genes. Together, these results suggest a structural feature involved in regulation of Ca(2+)/H+ antiport.

Structural determinants of Ca2+ transport in the Arabidopsis H+/Ca2+ antiporter CAX1.

Ca(2+) levels in plants, fungi, and bacteria are controlled in part by H(+)/Ca(2+) exchangers; however, the relationship between primary sequence and biological activity of these transporters has not been reported. The Arabidopsis H(+)/cation exchangers, CAX1 and CAX2, were identified by their ability to suppress yeast mutants defective in vacuolar Ca(2+) transport. CAX1 has a much higher capacity for Ca(2+) transport than CAX2. An Arabidopsis thaliana homolog of CAX1, CAX3, is 77% identical (93% similar) and, when expressed in yeast, localized to the vacuole but did not suppress yeast mutants defective in vacuolar Ca(2+) transport. Chimeric constructs and site-directed mutagenesis showed that CAX3 could suppress yeast vacuolar Ca(2+) transport mutants if a nine-amino acid region of CAX1 was inserted into CAX3 (CAX3-9). Biochemical analysis in yeast showed CAX3-9 had 36% of the H(+)/Ca(2+) exchange activity as compared with CAX1; however, CAX3-9 and CAX1 appear to differ in their transport of other ions. Exchanging the nine-amino acid region of CAX1 into CAX2 doubled yeast vacuolar Ca(2+) transport but did not appear to alter the transport of other ions. This nine-amino acid region is highly variable among the plant CAX-like transporters. These findings suggest that this region is involved in CAX-mediated Ca(2+) specificity.

Emerging mechanisms for heavy metal transport in plants.

Heavy metal ions such as Cu(2+), Zn(2+), Mn(2+), Fe(2+), Ni(2+) and Co(2+) are essential micronutrients for plant metabolism but when present in excess, these, and non-essential metals such as Cd(2+), Hg(2+) and Pb(2+), can become extremely toxic. Thus mechanisms must exist to satisfy the requirements of cellular metabolism but also to protect cells from toxic effects. The mechanisms deployed in the acquisition of essential heavy metal micronutrients have not been clearly defined although a number of genes have now been identified which encode potential transporters. This review concentrates on three classes of membrane transporters that have been implicated in the transport of heavy metals in a variety of organisms and could serve such a role in plants: the heavy metal (CPx-type) ATPases, the natural resistance-associated macrophage protein (Nramp) family and members of the cation diffusion facilitator (CDF) family. We aim to give an overview of the main features of these transporters in plants in terms of structure, function and regulation drawing on information from studies in a wide variety of organisms.

Two additional type IIA Ca(2+)-ATPases are expressed in Arabidopsis thaliana: evidence that type IIA sub-groups exist.

High affinity Ca(2+)-ATPases play a central role in calcium homeostasis by catalysing the active efflux of calcium from the cytoplasm. This study reports the identification of two additional type IIA (SERCA-type) Ca(2+)-ATPases from Arabidopsis (AtECA2 and AtECA3), and describes the detailed sequence analysis of these genes in comparison with AtECA1 and other plant and animal Ca(2+)-ATPases. Southern analysis suggests that each of these genes is present as a single copy and also that there may be a small family of moderately related genes that encode type IIA Ca(2+)-ATPases in Arabidopsis. Evidence is also provided from RT-PCR that these genes are expressed in Arabidopsis. Hydropathy analysis predicts that the topology of the Arabidopsis type IIA proteins is similar to the animal SERCA proteins. Sequence and phylogenetic analyses suggest that the type IIA Ca(2+)-ATPases can be further divided into sub-groups.