Preservation and computer-aided microscopic analysis of planktonic Protozoa and algae

Samples of microplankton and larger nanoplankton (5 to 200 m) are preserved with a combination of Lugol's solution and DaFano's fixative. Organisms are then settled on a gelatin-coated slide, dried and embedded in 40 percent glycerin. Counting and sizing is performed under a microscope using a drawing tube, which facilitates measuring the organisms with a microcomputer-interfaced caliper. An interactive computer program, written in BASIC, allows for estimating the volumes of cells in up to 40 shape/species categories. The program then saves data on a disk, retrieves them, and calculates the results either for individual species (abundance, biomass, and mean cell volume) or as a pooled size spectrum of all organisms measured.     

Forest resistance to drought in a humid tropical dipterocarp forest in the Western Ghats of India

Droughts are linked to tree die-offs in the biodiverse humid tropics. We assessed drought response of a Dipterocarp Forest and found a marginal decrease in tree survival, indicating drought resistance. Understory and emergent species were sensitive to drought. Urgent focus is needed to understand drought impacts and plant physiological responses in South Asian forests.     

神经肌肉电刺激下小腿肌肉的电学特性研究

目的 采用电阻抗成像（electrical impedance tomography,EIT）方法研究神经肌肉电刺激（neuromuscular electrical stimulation,NMES）下小腿肌肉的电学特性,旨在将EIT作为一种长期监测方法,从而可视化NMES训练对人类小腿肌肉的训练效果。方法 16名实验对象被随机分配到对照组（control group,CG,n=8）和最佳电压强度的NMES训练组（optimal voltage intensity training group,OG,n=8）。对照组保持正常生活方式并不进行NMES和其他的肌肉训练;NMES训练组中使用商业NMES设备对实验对象右小腿进行23 min的NMES训练,每周3次,为期5周。应用EIT测量在每周一训练开始前的电导率分布。并且采用生物电阻抗分析（bioelectrical impedance analysis,BIA）方法测量右腿细胞外含水量与身体总含水量的比率（ECW/TBW） βrl,及身体总含水量（TBW）τ。为了量化NMES在肌肉训练过程中的作用,使用配对样本t检验分析EIT重建图像的...     

A robust approach to estimate relative phytoplankton cell abundances from metagenomes

Phytoplankton account for >45% of global primary production, and have an enormous impact on aquatic food webs and on the entire Earth System. Their members are found among prokaryotes (cyanobacteria) and multiple eukaryotic lineages containing chloroplasts. Genetic surveys of phytoplankton communities generally consist of PCR amplification of bacterial (16S), nuclear (18S) and/or chloroplastic (16S) rRNA marker genes from DNA extracted from environmental samples. However, our appreciation of phytoplankton abundance or biomass is limited by PCR-amplification biases, rRNA gene copy number variations across taxa, and the fact that rRNA genes do not provide insights into metabolic traits such as photosynthesis. Here, we targeted the photosynthetic gene psbO from metagenomes to circumvent these limitations: the method is PCR-free, and the gene is universally and exclusively present in photosynthetic prokaryotes and eukaryotes, mainly in one copy per genome. We applied and validated this new strategy with the size-fractionated marine samples collected by Tara Oceans, and showed improved correlations with flow cytometry and microscopy than when based on rRNA genes. Furthermore, we revealed unexpected features of the ecology of these ecosystems, such as the high abundance of picocyanobacterial aggregates and symbionts in the ocean, and the decrease in relative abundance of phototrophs towards the larger size classes of marine dinoflagellates. To facilitate the incorporation of psbO in molecular-based surveys, we compiled a curated database of >18,000 unique sequences. Overall, psbO appears to be a promising new gene marker for molecular-based evaluations of entire phytoplankton communities.     

PIGMENT TURNOVER IN THE MARINE DIATOM THALASSIOSIRA WEISSFLOGII. I. THE 14CO2-LABELING KINETICS OF CHLOROPHYLL a1

The turnover of chlorophyll a (chl a) was investigated in the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle using a new method based on the incorporation of ¹⁴C into chl a. The alga was maintained in its exponential growth phase under continuous light; ¹⁴C was supplied as bicarbonate. The time course of label accumulation into the tetrapyrrole ring and the phytol side chain was determined for time periods equivalent to 1–2 cell doublings. The labeling kinetics of the tetrapyrrole ring and the phytol side chain were described satisfactorily by a simple precursor-pigment model with two free parameters, the precursor turnover rate and the pigment turnover rate, both having dimensions of time^?1. The model was fit to the experimental data to determine the values of these two free parameters. The turnover rates of the tetrapyrrole ring and the phytol side chain were not significantly different, ranging from 0.01 to 0.1 per day. These rates are equivalent to turnover times ranging from days to weeks. Growth rate-normalized turnover rates did not vary with irradiance (7.5–825 μE · m^?2· s^?1). The precursor turnover rates of the tetrapyrrole ring and the phytol side chain differed by an order of magnitude. These results indicate that chl a is not degraded significantly in cultures of T. weissflogii grown under continuous light. Neither irradiance nor growth rate affected growth rate-normalized chlorophyll turnover rates. Our results are inconsistent with the hypothesis that steady-state cellular concentrations of chl a are maintained by a dynamic equilibrium between rates of synthesis and degradation.     

HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY OF PHYTOPLANKTON PIGMENTS USING A POLYMERIC REVERSED-PHASE C18 COLUMN1

A high-performance liquid chromatographic (HPLC) method is described that allows improved resolution of several chemotaxonomically significant phytoplankton pigments. The protocol, which employs two pumps and a modified Mantoura and Llewellyn (1983) solvent system, can be easily adapted for many HPLC systems currently in use. The most unique aspect of the method is the use of a polymeric C₁₈ reversed phase HPLC column (VYDAC 201TP). In comparison to the monomeric C₁₈ columns typically used in the characterization of phytoplankton pigments, polymeric C₁₈ columns offer superior selectivity for structurally similar compounds. The protocol was evaluated for the ability to resolve most of the phytoplankton pigments of diagnostic importance using algal cultures from nine classes. Pigment pairs that were resolved by the method include a) lutein and zeaxanthin, b) neoxanthin and 19′-hexanoyloxyfucoxanthin, and c) α-carotene and β-carotene, and partial resolution of chlorophyll c₁ and chlorophyll c₂.     

EFFECTS OF ENVIRONMENTAL VARIATION ON SINKING RATES OF MARINE PHYTOPLANKTON1

The effects of environmental variables, particularly irradiance, on the sinking rates of phytoplankton were investigated using cultures of Chaetoceros gracilis Schütt and C. flexuosum Mangin in laboratory experiments; these data were compared with results from assemblages in the open ocean and marginal ice zone of the Greenland Sea. In culture experiments both the irradiance under which the diatom was grown and culture growth rate were positively correlated with sinking rates. Sinking rates (ψ) in the Greenland Sea were smallest when determined from chlorophyll (mean ψ_chl= 0.14 m · d^?1) and biogenic silica (ψ_si= 0.14 m · d^?1) and greatest when determined from particulate carbon (ψ_c= 0.55 m · d^?1) and nitrogen (ψ_N= 0.64 m · d^?1). Field measurements indicated that variations in sinking may be associated with changes in irradiance and nitrate concentrations. Because these factors do not directly affect water density, they must be inducing physiological changes in the cell which affect buoyancy. Although a direct response to a single environmental variable was not always evident, sinking rates were positively correlated with growth rates in the marginal ice zone, further indicating a connection to physiological processes. Estimats of carbon flux at stations with vertically mixed euphotic zones indicated that approximately 30% of the daily primary production sank from the euphotic zone in the form of small particulates. Calculated carbon flux tended to increase with primary productivity.     

INFLUENCE OF CELL SHAPE AND SIZE ON ALGAL COMPETITIVE ABILITY1

Allometric relations between physiological processes and cell volume and surface area are combined with the variable-internal-stores model of growth to predict the ability of hypothetical phytoplankton to compete for phosphorus at equilibrium. The analysis shows that for spherical cells, smaller cells are better competitors than large ones. For cells that are very elongated in shape, however, large cells are often better competitors than small ones. The cells predicted to be the best competitors compare favorably in size and shape with the species observed to dominate in phosphorus-limited chemostats at equilibrium.     

PHOTOSYNTHETIC CHARACTERISTICS OF THE COCCOLITHOPHORID EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) EXPOSED TO ELEVATED CONCENTRATIONS OF DISSOLVED INORGANIC CARBON1

Light-saturated photosynthesis (P_max) of Emiliania huxleyi (Lohmann) Hay et Mohler is known to be carbonlimited at natural concentrations of dissolved inorganic carbon (DIC). In the present study, light-limited and light-saturated photosynthetic rates of E. huxleyi were studied at three concentrations of DIC (2.4, 7.4, and 12.4 mM) for high-calcite (C_in/C_tot=0.48) and low-calcite (C_in/C_tot=0.08) cells of the same strain. The photosynthetic efficiency (α) and the maximum quantum yield (θ_max)A increased by more than a factor of 2 from the lowest to the highest DIC level. P_max a, and θ_max were always higher for the high-calcite than for the low-calcite cells at identical DIC levels. This may indicate that the calcifcation process acts as an extra supplier of CO₂ for photosynthesis making the CO₂ shortage at natural DIC levels a little smaller for high-calcite than for low-calcite E. huxleyi. A dependency of θ_max on DIC has not previously been shown for marine phytoplankton. θ_max is a key parameter in recent biooptical models of phytoplankton productivity, and the results from the present study are therefore important for modeling the productivity of E. huxleyi.