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A model is presented on the production and development of microcystins (MCYSTs), or cyclic heptapeptide toxins, from nitrogen-limited Microcystis aeruginosa cyanobacterium cultures. The model includes categories for MCYST/dry weight ratio and MCYST/protein ratio; fast-growing N-limited M. aeruginosa cells are found to be smaller and have a lower mass than slow-growing cells.

We immersed the root systems of three emergent aquatic monocots (Cyperus involucratus Rottb., Eleocharis sphocelata R. Br. (both Cyperaceae), and Juncus ingens N. A. Wakef. (Juncecease) in nutrient agar gels with pH indicators and pH microclectrodes to study H[sup1] exchange by root systems. For all three species, the pH changed at the tips of the adventitious roots and around young laterals. Older, lignified surfaces had little effect on pH. The pH changes around the laterals were affected by the form of nitrogen in the nutrient gel. When NO[suba, sup-] was supplied as the N source, there was a small pH increase, whereas NH[SUB4] enhanced H[sup1] release, causing the pH adjacent to the laterals to decrease below 4.5. In contrast, root tips acidified the media, irrespective of the nutrient composition. H[sup1] exchange was quantified from titrimetric assays of H[sup1] exchange in nutrient solutions. There was a net H[sup1] efflux from the roots of all three species into complete nutrient solutions with NH[sub4, sup1] as the N source, ranging from 246 μmol H[sup1] h[sup1] g[sup1] d, wt for C, involucratus, to 32 μmol H[sup1] h[sup-1] g[sup-1] d. wt for J, ingens. The H[sup1] efflux into similar nutrient solutions with NO[suba] as the N source ranged from -89 μmol H[sup1] h[sup-1] g[sup-1] d. wt for C, involucratus to -0.8 μmol H[sup1] h[sup01] d, wt for J. ingens. By assuming that H[sup1] exchange was mainly located in the laterals, rates of H[sup1] excretion as high as 542 μmol H[sup1] h[sup-1] g[sup-1] d. wt can be calculated for C. involucratus in NH[sub4, sup1] solutions. Rates of H[sup1] excretion by C, involucratus were scarcely affected by solutions of most other ions. The NH[sub4]-induced H[sup1] release was dramatically reduced by H[sup1] pump inhibitors, attaining only 18% of control rates with 50 μmM diethylstilbestrol and 38% of control rates with 5 μM dicyclohexylearbodümide. [ABSTRACT FROM AUTHOR]

The ability of diffusive gas transport and pressurized, convective flow to satisfy internal oxygen demands was examined for an aquatic sedge, Eleocharis sphacelata R. Br.. Resistances to convention and diffusion through the plant were quantified form anatomical studies of the airspace dimensions, and these were used in mathematical models to calculate the fluxes required to satisfy oxygen demands measured in the tissue. The greatest resistance to diffusion in the underwater tissue was the submerged culm between the waterline and sediment surface (1560 Ms m-3 per m culm length). Resistances of the nodal intercalary meristem (52 Ms m-3) and rhizome internode (34 Ms m-3) were minor. In contrast resistances to convection were low in the culms (38 MPa s m-3 per m culm length), and higher in the nodal meristems (93 MPa s m-3). The rhizome internodes had large cortical canals with a low convective resistance (0.75 MPa s m-3), and a parallel spongy pith with a very high resistance (518 MPa s m-3) that is probably short-circuited by convection. The resistance of the submerged culm mean s that diffusion is inadequate to satisfy oxygen demands in plants growing in > 10 cm of water, and that convective oxygen influx as low as 2.8 × 10-8 mol s-1 per culm (equivalent to a gas flow rate of 3 μl s -1 per culm) could satisfy the entire oxygen demand of the underwater issue; this value is well below actual rates. At this flow rate, the spongy pith in the rhizome would also remain aerobic; it has a low resistance to diffusion (73 Ms m-3) and could receive sufficient oxygen by diffusion from the node. The data agree well with previous empirical measurements of convection in this species and show that diffusion and convection are both important processes for it aeration. [ABSTRACT FROM AUTHOR]

Abstract: We studied the physiological acclimation of growth, photosynthesis and CO2-concentrating mechanism (CCM) in Cylindrospermopsis raciborskii exposed to low (present day; L-CO2) and high (1300ppm; H-CO2) pCO2. Results showed that under H-CO2 the cell specific division rate (μ c ) was higher and the CO2- and light-saturated photosynthetic rates (V max and P max) doubled. The cells’ photosynthetic affinity for CO2 (K0.5CO2) was halved compared to L-CO2 cultures. However, no significant differences were found in dark respiration rates (R d ), pigment composition and light harvesting efficiency (α). In H-CO2 cells, non-photochemical quenching (NPQ), associated with state transitions of the electron transport chain (ETC), was negligible. Simultaneously, a reorganisation of PSII features including antenna connectivity (JconPSIIα), heterogeneity (PSIIα/β) and effective absorption cross sectional area (σPSIIα/β) was observed. In relation to different activities of the CCM, our findings suggest that for cells grown under H-CO2: (1) there is down-regulation of CCM activity; (2) the ability of cells to use the harvested light energy is altered; (3) the occurrence of state transitions is likely to be associated with changes of electron flow (cyclic vs linear) through the ETC; (4) changes in PSII characteristics are important in regulating state transitions. [Copyright &y& Elsevier]

Abstract: The potentially toxic cyanobacterium Cylindrospermopsis raciborskii (Wołoszyńska) Seenya et Subba Raju, originally described as a tropical-subtropical species, is increasingly found in temperate regions and its range is expanding. Climate change is hypothesised to be a factor in this expansion. We studied the effects of dissolved inorganic carbon (DIC) and pH on growth and photosynthesis of this species. We prepared six treatments in a continuous culture (turbidostat) grown at high light, two at low light, and eight in batch cultures grown under low light, by manipulating pH, and CO2 to assess the effect, if any, of these parameters on the growth rate, inorganic carbon acquisition and photosynthetic parameters of C. raciborskii. When the turbidostats were grown at 100 μmol photons (PAR) m−2 s−1, concentration and pH had a positive effect on growth rate; the specific growth rate in 6 mM , for example, was twice what it was in 0.6 mM (0.84 ± 0.10 and 0.44 ± 0.04 d−1 respectively). Growth was lower in turbidostats grown at 20 μmol photons (PAR) m−2 s−1. Isotope disequilibrium experiments showed that the contribution of to DIC acquisition is proportionately greater at the higher light. The maximum relative electron transport rate (rETRmax) was significantly higher at the higher light, while the slope of the linear portion of the rETRmax versus irradiance curve (α) was unchanged. In low light batch cultures, increasing concentration and pH had a negative effect on growth, while CO2 concentration had a small, positive effect. This species of cyanobacterium has an efficient CCM and under ideal growing conditions gets most of its carbon from . It may, therefore, be at a competitive disadvantage in a high CO2 world. [Copyright &y& Elsevier]

Cylindrospermopsin (CYN) and 7-deoxy-cylindrospermopsin (dCYN) are potent hepatotoxic alkaloids produced by numerous species of cyanobacteria, including the freshwater Cylindrospermopsis raciborskii. C. raciborskii is an invasive cyanobacterium, and the study of how environmental parameters drive CYN production has received significant interest from water managers and health authorities. Light and CO2 affect cell growth and physiology in photoautotrophs, and these are potential regulators of cyanotoxin biosynthesis. In this study, we investigated how light and CO2 affect CYN and dCYN pool size as well as the expression of the key genes, cyrA and cyrK, involved in CYN biosynthesis in a toxic C. raciborskii strain. For cells growing at different light intensities (10 and 100 μmol photonsm-2 s-1), we observed that the rate of CYN pool size production (μCYN) was coupled to the cell division rate (μc) during batch culture. This indicated that CYN pool size under our experimental conditions is constant and cell quotas of CYN (QCYN) and dCYN (QdCYN) are fixed. Moreover, a lack of correlation between expression of cyrA and total CYN cell quotas (QCYNs) suggests that the CYN biosynthesis is regulated posttranscriptionally. Under elevated CO2 (1,300 ppm), we observed minor effects on QCYN and no effects on expression of cyrA and cyrK. We conclude that the CYN pool size is constitutive and not affected by light and CO2 conditions. Thus, C. raciborskii bloom toxicity is determined by the absolute abundance of C. raciborskii cells within the water column and the relative abundance of toxic and nontoxic strains. [ABSTRACT FROM AUTHOR]