Periphyton community structure and dynamics in a subarctic lake

Periphyton species composition was analysed at 20 stations around an island in a large (1239 km2) oligotrophic lake in subarctic Quebec (Lac à l'Eau Claire; latitude 56°10′N, longitude 74°30′W) to describe the mature communities colonizing the upper littoral region and to evaluate periphyton abundance and distribution relative to the physical environment. Four major communities could be clearly distinguished in the field by their macroscopic features, specifically colour (black, brown, and green) and growth form (filamentous or encrusted), as well as by their standing stock (cover and areal Chl a concentration) and photosynthetic characteristics. (1) Black crust—This community was dominated by the cyanobacterium Gloeocapsa, with highest percent cover in shallow waters (≤ 0.25 m) protected from wave action by offshore boulder barriers. Photosynthesis under full sunlight was low per unit biomass (0.7 μg C∙(μg Chl a)−1∙h−1). (2) Brown film—This community was dominated by Calothrix, with Gloeocapsa and Phormidium as subdominants. Maximum abundance was at 0.5 m, with photosynthetic rates that were similar to the black community. (3) Green crust—This community was dominated by the mucilaginous chlorophyte Gloeocystis, with Oscillatoria as subdominant, and colonized shallow depths (≤ 0.25 m) in the shaded underlayer of rocks. It had slow, light-limited photosynthetic rates (0.1 μg C∙(μg Chl a)−1∙h−1). (4) Green filaments—This community was dominated by Ulothrix zonata, with associated chlorophytes and diatoms, and was a rapidly growing assemblage characterized by the highest diversity, species richness, and productivity per unit biomass (3.5 μg C∙(μg Chl a)−1∙h−1). It occurred on gravel beds at depths < 0.5 m and was restricted to the well-illuminated south-facing shores of the island. Communities 1, 2, and 3 had similar maximum standing stocks throughout the period of sampling (mean of 1.3 μg Chl a∙cm−2), whereas the Ulothrix community rose from 1.9 μg Chl a∙cm−2 in late July to 5.5 μg Chl a∙cm−2 by mid-August. The overall rich biodiversity of the Lac à l'Eau Claire periphyton (> 200 taxa recorded) may reflect the diversity of microenvironments and intermediate disturbance in the upper littoral zone. Key words: cyanobacteria, chlorophytes, diatoms, epilithon, periphyton, photosynthesis.

Effect of the intracellular inorganic carbon pool on chlorophyll a fluorescence quenching and O2 photoreduction in air-grown cells of the cyanobacterium Synechococcus UTEX 625

Simultaneous measurements were made of O2 exchange, inorganic carbon (Ci) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of Ci to cells at the CO2 compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2 fixation inhibitor, iodoacetamide. The magnitude of quenching was related to electron flow to terminal electron acceptors such as CO2 and O2. When photosynthetic CO2 fixation was allowed, the rate of electron transport, as expressed by (F*m – F)/F*m, was highly correlated with the onset of photosynthesis. When CO2 fixation was inhibited by the addition of iodoacetamide, the observed fluorescence quenching was consistent with the enhanced rate of O2 photoreduction that occurred when Ci was added. There was a close correlation (r = 0.98) between the magnitude of O2-dependent fluorescence quenching and the amount of O2 photoreduction. The degree of stimulation of electron flow to O2 photoreduction was dependent on the inorganic carbon concentration. The K1/2 (Ci) for extracellular Ci was 1.36 ± 0.13 μM (mean ± SD, n = 3) and K1/2 (Ci) for the intracellular Ci pool was 1.4 ± 0.18 mM (mean ± SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also stimulated by the addition of Ci, whereas the addition of Ci had no effect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. The results suggest that Ci stimulates electron flow in photosystem I. Key words: cyanobacteria, O2 photoreduction, fluorescence, Ci concentrating mechanism, inorganic carbon pool, linear electron transport, kinetic study.

Oxygen photoreduction and its effect on CO2 accumulation and assimilation in air-grown cells of Synechococcus UTEX 625

Mass spectrometric measurements of 16O2, 18O2, and 13CO2 were used to measure the rates of gross O2 evolution, O2 uptake, and CO2 assimilation in relation to light intensity, temperature, pH, and O2 concentration by air-grown cells of the cyanobacterium Synechococcus UTEX 625. CO2 fixation and O2 photoreduction increased with increased light intensity and, although CO2 fixation was saturated at 250 μmol ∙ m−2 ∙ s−1, O2 photoreduction was not saturated until about 550 μmol ∙ m−2 ∙ s−1. At high light intensity addition of inorganic carbon to the cells stimulated O2 photoreduction 2-fold when CO2, fixation was allowed and 5-fold when CO2, fixation was inhibited with iodoacetamide. The ability of O2, to act as an acceptor of photosynthetically generated reducing power was dependent upon the O2 concentration, and the substrate concentration required for half maximum rate (K½(O2)) was 53.2 ± 4.2 μM (mean ± SD, n = 3). The Q10 for oxygen photoreduction was about 2. A certain amount (10%) of O2 appeared to be required for maximum photosynthesis, as photosynthesis was inhibited under anaerobic conditions, especially at high light intensity. The point of inhibition is unknown but it seemed unlikely to be on CO2 transport or the concentration of intracellular dissolved inorganic carbon (Ci), as the rate of initial CO2 transport was enhanced and the intracellular Q1 pool increased in size under anaerobic conditions. Key words: cyanobacteria, photosynthesis, Ci concentrating mechanism, inorganic carbon pool, O2 photoreduction, electron transport, temperature.

Cyanobiont diversity within and among cycads of one field site

Limited diversity was found among cyanobionts from a cultivated population of cycads at a field site in Florida. All isolates were classified as Nostoc but were different from the one Nostoc species found in the soil. These cyanobacteria were root endophytes of several plants of Zamia integrifolia and one of Dioon. The isolates were similar morphologically and in their reactions to four fluorescein isothiocyanate conjugated lectins. Electrophoretic protein profiles and zymograms distinguished one cyanobiont and the soil Nostoc. A tenacious Anabaena epiphyte was also discovered inhabiting the surfaces of root nodules. Key words: cyanobacteria, cycad, Nostoc, symbiosis.

Active HCO3− transport in cyanobacteria

Cyanobacteria possess systems for the active transport of both CO2 and HCO3−. While the active CO2 transport system seems to be present in cells grown on all levels of CO2 or dissolved inorganic carbon, the bicarbonate transport systems are only present in cells grown on low levels of CO2 or dissolved inorganic carbon (air levels or lower). Active bicarbonate transport can be shown to occur when the rate of photosynthesis exceeds that which could be sustained by the production of CO2 from the dehydration of bicarbonate or when CO2 transport is inhibited with carbon oxysulfide or hydrogen sulfide. Two systems for active bicarbonate transport have been identified: one is dependent on the presence of millimolar concentrations of sodium, and the other is independent of the sodium requirement. Cells grown with air bubbling normally possess the first whereas cells grown in standing culture normally possess the second. The sodium-dependent bicarbonate transport can be inhibited by omitting sodium from the reaction medium or competitively with lithium when sodium is present. Monensin and amiloride also inhibit sodium-dependent bicarbonate transport. It does not appear to be inhibited by ethoxyzolamide. The inhibition of sodium-independent bicarbonate transport is not yet established. Bicarbonate transport appears to have no effect on CO2 transport and CO2 transport appears to have no effect on bicarbonate transport. Hence, the transport systems seems to be independent. Although a number of mechanisms have been proposed for bicarbonate transport, the experimental data are not sufficient to clearly distinguish between them. Key words: cyanobacteria, active CO2 transport, active HCO3− transport, photosynthesis, sodium.

A model for inorganic carbon fluxes and photosynthesis in cyanobacterial carboxysomes

A barrier to CO2 diffusion within the cyanobacterial cell has been regarded as essential for the inorganic carbon concentrating mechanism. We present here an extension of our earlier quantitative model demonstrating that it may be unnecessary to postulate any barrier other than the ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) molecules themselves. It is proposed that carbonic anhydrase is located in the interior of the carboxysome and that the CO2 generated is largely fixed as it diffuses outwards past Rubisco sites located along the diffusion path. Equations have been developed, by combining a mass balance equation with Fick's Law and the Michaelis-Menten equation (representing CO2 fixation), estimate the value that must be assigned to the diffusion coefficient for CO2 within the carboxysome if the CO2 concentration is to be reduced to near zero at the carboxysome outer surface. A solution has been obtained for two limiting cases, that where CO2 concentration is nearly saturating and that where it is at the Km(CO2) value or below. These two estimates predict that the permeability constant for the Rubisco zone in the carboxysome would have to be 10−2–10−3 cm∙s−1, a value that we suggest is reasonable for three-dimensional diffusion through a densely packed protein layer. The concentration gradient in the inward direction, for substrates penetrating the carboxysomes from the cytoplasm, is shown to be relatively flat, owing to the concentrating effect experienced by solutes passing from the periphery to the center of a sphere. Key words: cyanobacteria, carboxysomes, inorganic carbon fluxes, photosynthesis, model.

Effects of nutritional status on cyanobacterial buoyancy, blooms, and dominance, with special reference to inorganic carbon

Evidence from correlative studies suggesting that nutrient deficiency is responsible for cyanobacterial surface blooms is contrasted with evidence from lake and laboratory experiments suggesting that the ability of cyanobacterial populations to monopolize light in the surface layers of stratified water columns depends on the availability of nutrients required for gas-vesicle synthesis as well as for growth. A resolution of the paradoxical roles of inorganic carbon in cyanobacterial buoyancy regulation is advanced and is used to explain the success of different types of cyanobacteria in environments that differ greatly in bicarbonate content. Key words: cyanobacteria, gas vesicles, buoyancy, blooms, dominance, bicarbonate.

The effects of inorganic carbon and oxygen on fluorescence in the cyanobacterium Synechococcus UTEX 625

The active transport of inorganic carbon and the accumulation of the internal pool caused quenching of chlorophyll a fluorescence both when CO2 fixation was allowed or when CO2 fixation was inhibited. Upon the addition of inorganic carbon in the presence of 240 μM oxygen the rate of change in fluorescence (or quenching) was correlated (r = 0.98) with the rate of active CO2 uptake, and the extent of quenching was correlated (r = 0.99) with the size of the internal inorganic carbon pool. Fluorescence was quenched by the fixation of inorganic carbon in the absence of oxygen but the reoxidation of QA following a flash of light was slow. In the presence of inorganic carbon, with or without the inhibition of CO2 fixation, oxygen quenched fluorescence. If CO2 fixation was inhibited, the degree of quenching depended upon the oxygen concentration with a K1/2 (O2) of about 42 μM. Below 60 μM oxygen there was a further reduction of QA following a flash of light and the reoxidation of QA was slow. Rapid reoxidation of QA following a flash of light required about 240 μM oxygen. From the response to added 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, the quenching by oxygen was photochemical quenching and nonphotochemical quenching did not seem to be present. For reasons that are unknown, however, only about 80% of the quenching could be reversed with high intensity flashes of light. The photoreduction of oxygen was regulated by the presence of inorganic carbon, although fixation of CO2 was not required. The mechanism of this regulation is not known but it may be due to bicarbonate relief of electron transfer between QA and QB. Some results on measuring Fo, F′o, Fm, and F′m, in Synechococcus UTEX 625 are presented. Key words: cyanobacteria, fluorescence, oxygen photoreduction, active inorganic carbon transport.

Active CO2 transport in cyanobacteria

Cyanobacteria appear to possess an active transport system for molecular CO2. This system, first discovered by Badger and Andrews in 1982 (1982. Plant Physiol. 70: 517–523), is without reported precedence in the bacterial, animal, or plant literature. The transport system operates so efficiently that in dense cell suspensions the extracellular CO2 concentration is pulled far below the equilibrium value. This CO2 drawdown is not due to CO2 fixation but can be accounted for by a transport system that recognizes molecular CO2 and causes it to be transported into the cell. The fact that operation of the system causes a massive disequilibration of the extracellular CO2–HCO3− system means that there must be an expenditure of metabolic energy. The CO2 is actually moved against a considerable CO2 concentration gradient. In this review we discuss methods that can be used to monitor CO2 transport in cyanobacteria. We present evidence that CO2 transport is an active process. It is emphasized that little is known about the concomitant ion fluxes that must occur to ensure charge and pH regulation during CO2 transport. Key words: cyanobacteria, active CO2 transport, metabolic inhibitors, transport models.

Selection and analysis of mutants of the CO2-concentrating mechanism in cyanobacteria

The selection and analysis of mutants of the CO2-concentrating mechanism in cyanobacteria have greatly advanced our understanding of the physiological and genetic components of this mechanism. This paper reviews the processes for the creation of mutants and the properties of mutants that have been produced so far. In addition to this, consideration is given to developing new mutant selection protocols, based on our current knowledge of the operation of the CO2-concentrating mechanism. As a result, new screens are suggested for the isolation of transport mutants that are defective in either HCO3− or CO2 transport activity, and putative carboxysomal mutants that have altered carbonic anhydrase activities. The procedures for physiological analysis of mutants are also reviewed, with a conclusion that there is a great need for the further development of in vitro assays, particularly for the inorganic carbon transport process and carboxysome function. Particular consideration is given to the in vitro carboxysome assay, and it is concluded that many of the properties of a carboxysomal Rubisco may be difficult to resolve owing to increases in carboxysomal leakiness during isolation and the higher ratio of [CO2] to [HCO3−] experienced during assays compared with the in vitro situation. Finally, consideration is given to the genetic analysis of cyanobacterial mutants and the progress that will need to be made in the future, discovering what are the functions of the proteins coded for by the genes that are isolated. Key words: cyanobacteria, mutants, photosynthesis, carboxysome, CO2-concentrating mechanism.