Physiological characteristics of picophytoplankton, isolated from Lake Kinneret: responses to light and temperature

Two different phylogenetic groups of picophytoplankton, namelypicocyanobacteria and picoeukaryotes, are represented in LakeKinneret. Three species were isolated from the lake and identifiedas the picoeukaryote Mychonastes homosphaera and two picocyanobacteria,Synechococcus sp. A and B. Picocyanobacterial and M. homosphaeracultures grew well at light intensities up to 330 and 700 µmolphotons m^-2 s^-1, respectively, but poorly below 10 µmolphotons m^-2 s^-1. Picocyanobacterial and M. homosphaera culturesphotoacclimated to low light by increasing their chlorophyllper cell through increase in photosynthetic unit (PSU) sizeand PSU numbers, respectively. Growth rates of SynechococcusA and B were higher at temperatures characteristic of summer–autumnin the epilimnion, when maximum abundances of picocyanobacteriaoccur. Growth rates of M. homosphaera were higher at 14°C,corresponding to lake water temperatures during their occurrencein winter–spring. Temperature is a dominant factor influencingthe seasonal dynamics of both picocyanobacteria and picoeukaryotesin Lake Kinneret, while the vertical distribution is controlledby acclimation to different light conditions. Differences intemperature tolerance and photoacclimation suggest that SynechococcusA belongs to picocyanobacteria found in summer below surfacewaters, while Synechococcus B represents picocyanobacteria foundthroughout the year at all depths. Photoacclimation to highlight as shown in M. homosphaera cultures, may account for therelatively high abundance of picoeukaryotes in surface watersin Lake Kinneret.     

Mercury biotransformations and their potential for remediation of mercury contamination

Bacterially mediated ionic mercury reduction to volatile Hg⁰ was shown to play an important role in the geochemical cycling of mercury in a contaminated freshwater pond. This process, and the degradation of methylmercury, could be stimulated to reduce the concentration of methylmercury that is available for accumulation by biota. A study testing the utility of this approach is described.Abbreviations Hg^R inorganic mercury resistance - Org-Hg organomercury - Org-Hg^R organomercury resistance - SRB sulfate reducing bacteria - Methyl-B12 methylcobalamine     

Phytoplankton and zooplankton population dynamics and production of a recently formed African reservoir

The study provides a 2.5 year record of Rhenosterkop Dam (KwaNdebele, South Africa) plankton population dynamics and production in relation to physical and chemical changes which occurred during the trophic depression and stabilization phases of the reservoir. The mean volume of the reservoir was 4% of full storage capacity. Water temperatures ranged from 14 °C to 27 °C. Due to inorganic suspensoids, the euphotic zone averaged 2.6 m. An anaerobic zone developed each summer. The nitrogen, soluble reactive phosphorus (SRP) and silica concentrations did not displaya seasonal pattern, but the latter two nutrients declined over the study. The dominant phytoplankton group was the cryptophytes while the zooplankton population was dominated by crustaceans. Chlorophyll a concentrations ranged from 1.1 to 27 mg m^–3 and were positively correlated to silica and SRP concentrations and inversely with NH₄-N concentrations. Primary production ranged from 22.6 to 375 mgC m^–2 h^–1; changes in A_max were positively correlated to silica and SRP concentrations. Total zooplankton dry weight biomass varied from <0.5 to >4 mg l^–1. Annual zooplankton (secondary) production was 8 to 15 gC m^–3 a^–1; both primary and secondary production were greatest in the first 12 months of study and remained at low levels for the remainder, similar to the trends for silica and SRP. The data indicate that the reservoir shifted from eutrophic to mesotrophic during the study, typical of events in new reservoirs, and that changes in the plankton populations were largely the result of changing nutrient concentrations.     

The influence of temperature and light on the upper limit of Microcystis aeruginosa production in a hypertrophic reservoir

Primary production was measured for 7 years, using the in situ¹⁴C-method in hypertrophic Hartbeespoort Dam, South Africa,to examine the influence of light and water temperature on theupper limit of Microcystis aeruginosa production. Water temperaturesvaried from 11 to >25°C and chlorophyll concentrationsreached 6500 mg m^–3. The maximum volumetric rate of production(A_max) was 12->8800 mg C m^–3 h^–1 with areal productions(A) of 69->3300 mg C m^–2 h^–1 for euphotic zonedepths of <0.5–8.4 m. The intrinsic parameters of phytoplanktonproduction (, A_max/B, I_k) indicated that the phytoplankton populationwas adapted to high light levels. Both A_max/B and I_k were correlatedwith temperature. Under optimal conditions, , the theoreticalupper limit of A, was calculated to be 2.8 g Cm^–2 h^–1,while the measured rate was 2.5 g Cm^–2 h^–1. Measuredareal rates exceeding were overestimated due to methodologicalproblems when working with Microcystis scums. Light and watertemperature interacted to yield high production rates: watertemperature through its direct effect on photosynthetic ratesand indirectly in the formation of diurnal mixed layers; lightindirectly through water temperature and directly through itsattenuation and induction of light-adapted physiology in Microcystis.     

Insights into the Molecular Mechanisms of CO2-Mediated Regulation of Stomatal Movements

Plants must continually balance the influx of CO₂ for photosynthesis against the loss of water vapor through stomatal pores in their leaves. This balance can be achieved by controlling the aperture of the stomatal pores in response to several environmental stimuli. Elevation in atmospheric [CO₂] induces stomatal closure and further impacts leaf temperatures, plant growth and water-use efficiency, and global crop productivity. Here, we review recent advances in understanding CO₂-perception mechanisms and CO₂-mediated signal transduction in the regulation of stomatal movements, and we explore how these mechanisms are integrated with other signaling pathways in guard cells.     

Resisting annihilation: relationships between functional trait dissimilarity,assemblage competitive power and allelopathy

Allelopathic species can alter biodiversity. Using simulated assemblages that are characterised by neutrality, lumpy coexistence and intransitivity, we explore relationships between within‐assemblage competitive dissimilarities and resistance to allelopathic species. An emergent behaviour from our models is that assemblages are more resistant to allelopathy when members strongly compete exploitatively (high competitive power). We found that neutral assemblages were the most vulnerable to allelopathic species, followed by lumpy and then by intransitive assemblages. We find support for our modeling in real‐world time‐series data from eight lakes of varied morphometry and trophic state. Our analysis of this data shows that a lake's history of allelopathic phytoplankton species biovolume density and dominance is related to the number of species clusters occurring in the plankton assemblages of those lakes, an emergent trend similar to that of our modeling. We suggest that an assemblage's competitive power determines its allelopathy resistance.