Photosynthetic characteristics and physiological plasticity of an Aphanizomenon flos-aquae (Cyanobacteria, Nostocaceae) winter bloom in a deep oligo-mesotrophic lake (Lake Stechlin, Germany)

In winter of 2009/2010, Aphanizomenon flos-aquae bloomed in the ice and snow covered oligo-mesotrophic Lake Stechlin, Germany. The photosynthesis of the natural population was measured at eight temperatures in the range of 2–35°C, at nine different irradiance levels in the range of 0–1,320 μmol m⁻² s⁻¹ PAR at each applied temperature. The photoadaptation parameter (I _k) and the maximum photosynthetic rate (P _max) correlated positively with the temperature between 2 and 30°C, and there was a remarkable drop in both parameters at 35°C. The low I _k at low temperatures enabled the active photosynthesis of overwintering populations at low irradiance levels under ice and snow cover. The optimum of the photosynthesis was above 20°C at irradiances above 150 μmol m⁻² s⁻¹. At lower irradiance levels (7.5–30 μmol m⁻² s⁻¹), the photosynthesis was the most intensive in the temperature range of 2–5°C. The interaction between light and temperature allowed the proliferation of A. flos-aquae in Lake Stechlin resulting in winter water bloom in this oligo-mesotrophic lake. The applied 2°C is the lowest experimental temperature ever in the photosynthesis/growth studies of A. flos-aquae, and the results of the P–I and P–T measurements provide novel information about the tolerance and physiological plasticity of this species.     

Comparative studies on phosphate utilization of two bloom-forming Microcystis spp. (cyanobacteria) isolated from Lake Taihu (China)

Microcystis is a common freshwater bloom-dominating cyanobacterial genus. However, the crucial factors that affect the seasonal succession and extent of dominance of different species remain largely unknown. This study investigated inorganic phosphorus (P) uptake, growth, and P utilization of two dominant Microcystis species from Lake Taihu (China) at different P concentrations and temperatures. Compared with Microcystis wesenbergii, Microcystis flos-aquae had higher maximum uptake rate and luxury storage coefficient. However, M. flos-aquae also had a higher P demand for growth. Under P-rich conditions (64.5 to 174.5 μM), M. flos-aquae had a higher growth rate (0.16 to 0.21 d^?1) than M. wesenbergii (0.06 to 0.19 d^?1) at 22 to 30 °C. M. wesenbergii, with higher phosphate affinity, had a lower P demand to sustain its growth, and yielded a higher growth rate of 0.10 d^?1 at low P concentration (6.5 μM) at 34 °C, whereas M. flos-aquae had a lower growth rate of 0.03 d^?1. Therefore, M. flos-aquae was dominant in late spring under P-rich conditions, whereas M. wesenbergii prevailed in hot summer after M. flos-aquae decreased under P-limited conditions. These results agree well with their succession patterns in the field. Overall, succession and dominance of M. flos-aquae and M. wesenbergii in Lake Taihu are regulated by P concentration and water temperature.     

Plankton community dynamics during decay of a cyanobacteria bloom: a mesocosm experiment

The aim of the work was to study the effects of a decaying cyanobacteria bloom on nutrient dynamics, plankton community development and production rates of bacteria and primary producers. It was hypothesised that the system would turn more heterotrophic following the decay of the bloom. A 10-day outdoors mesocosm experiment was performed in early June in a brackish-water environment. Non-toxic filamentous cyanobacteria Aphanizomenon flos-aquae were added to the treatment, whereas the control lacked cyanobacteria. A. flos-aquae decayed rapidly, and was absent from the units by day 2. Significantly higher bacteria abundances, lower nanoflagellate densities and higher ciliate abundances were found, suggesting a bottom-up regulated process in the treatments bags. N:P ratios were low (6?C12), suggesting N-limitation. Bacteria correlated negatively with numbers of heterotrophic nanoflagellates (HNF), suggesting grazing on bacteria by HNF. Primary production correlated positively with irradiance, chlorophyll a and inorganic nutrients in all units. The rapidly decaying A. flos-aquae biomass imposed a significant bottom-up regulation in the treatment mesocosms, and the system turned from autotrophic into more heterotrophic with time. The rapid decay also caused some similarities and parallel changes between the treatment and the control.     

Effects of nitrogen and phosphorus availability on the decomposition of aquatic plants

The responses of decomposition to nitrogen (N) and phosphorus (P) supply were investigated in three leaf species: Eichhornia crassipes, Vallisneria natans, and Potamogeton maackianus. Decomposition was fastest in E. crassipes (0.047–0.099 day⁻¹), intermediate in V. natans (about 0.030 day⁻¹), and slowest in P. maackianus (about 0.010 day⁻¹). Increase in P-availability increased the decomposition rate of E. crassipes by 68–87%, whereas the impact of N-availability alone was insignificant. Both N- and P-availability in waters had no significant impact on the decomposition rates of V. natans and P. maackianus (P > 0.05). The effects of P-availability on the N and P content levels of the three species were significant (P < 0.01), except for the impact on N content of V. natans (P = 0.526). In contrast, environment N-availability was insignificant. These results indicate that the responses of decomposition to nutrient availability depend on plant species and nutrient type. P-availability has stronger effects on litter nutrient dynamics than N-availability.     

Dual Roles of Cadaverine-Producing Pseudomonas sp. on Microcystis spp. in Hyper-Eutrophic Water

A bacterium isolated from Lake Taihu was identified as Pseudomonas sp. A3CT, which performed different effects on Microcystis spp. Growth of Microcystis flos-aquae and Microcystis aeruginosa was assessed in co-culture with A3CT to determine the stimulatory or inhibitory effects on these toxic, bloom-forming Microcystis strains. Results demonstrated that the impacts of A3CT were species specific. A3CT promoted the growth of M. aeruginosa but inhibited growth of M. flos-aquae. To investigate the cause of this phenomenon, the chemical composition of A3CT exudates and the impact of exposure to A3CT exudates on the two Microcystis species were determined. Results suggested that the observed differential growth responses of the two microalgae to A3CT exposure might be related to two components in A3CT exudates NH₄ ⁺ and cadaverine. Growth stimulation of M. aeruginosa by A3CT was significantly related to NH₄ ⁺ concentration. Cadaverine possibly acted as a growth inhibitor of M. flos-aquae. The different effects of cadaverine on growth of the two Microcystis strains suggested that A3CT might play a role in intrageneric succession patterns observed during Microcystis blooms in Lake Taihu.     

Changing patterns of carbon allocation in lake phytoplankton: an FTIR analysis

Fourier transform infra-red (FTIR) spectroscopy was used to assess carbon allocation in selected phytoplankton (Pediastrum duplex, Ceratium hirundinella, Anabaena flos-aquae) in a eutrophic lake. In contrast to bulk-analysis techniques, FTIR spectroscopy can provide information on carbon allocation at the species level within natural mixed populations. Carbon allocation was determined using lipid/protein (L/P) and carbohydrate/protein (C/P) ratios and multivariate analysis of complete spectra and showed considerable intraspecific heterogeneity within samples dates, and clear seasonal changes in the spectra of Pediastrum and Anabaena, with both alga showing increased L/P and C/P ratios on selected sampling dates. Although increased ratios can indicate nutrient deficiency, the alga were not nutrient deficient during these periods, as indicated by continued population growth and high (>0.1%) internal phosphorus concentrations. FTIR spectra from Ceratium did not show any clear seasonal change. In all three algae, the transition from population growth to decline was not accompanied by any significant change in ratios, nor were low epilimnetic nutrients correlated with increased ratios, marking a significant contrast to laboratory batch cultures (previous studies) where low nutrients and the transition to stationary phase led to marked spectral changes.     

Susceptibility of Riparian Wetland Plants to Perfluorooctanoic Acid (PFOA) Accumulation

As plants have been shown to accumulate organic compounds from contaminated sediments, there is a potential for long-lasting ecological impact as a result of contaminant accumulation in riparian areas of wetlands, particularly the accumulation of non-biodegradable contaminants such as perfluorooctanoic acid (PFOA). In this study, commonly found riparian wetland plants including reeds, i.e., Xanthium strumarium, Phragmites australis, Schoenoplectus corymbosus, Ruppia maritime; Populus canescens, Polygonum salicifolium, Cyperus congestus; Persicaria amphibian, Ficus carica, Artemisia schmidtiana, Eichhornia crassipes, were studied to determine their susceptibility to PFOA accumulation from PFOA contaminated riparian sediment with a known PFOA concentration, using liquid chromatography/tandem mass spectrometry (LC/MS/MS). The bioconcentration factor (BCF) indicated that the plants affinity to PFOA accumulation was; E. crassipes, > P. salicifolium, > C. congestus, > P. x canescens, > P. amphibian, > F. carica, > A. schmidtiana, > X. strumarium, > P. australis, > R. maritime, > S. corymbosus. The concentration of PFOA in the plants and/or reeds was in the range 11.7 to 38 ng/g, with a BCF range of 0.05 to 0.37. The highest BCF was observed in sediment for which its core water had a high salinity, total organic carbon and a pH which was near neutral. As the studied plants had a higher affinity for PFOA, the resultant effect is that riparian plants such as E. crassipes, X. strumarium, and P. salicifolium, typified by a fibrous rooting system, which grow closer to the water edge, exacerbate the accumulation of PFOA in riparian wetlands.     

The copper-resistant bacterium ACU isolated from the rhizosphere of Eichhornia crassipes (Mart.) increased the endurance of Potamogeton crispus L. to copper toxicity

Aims: This study aimed to develop endurance to copper stress in Potamogeton crispus L. by inoculation with the anti-copper strain ACU – a novel Enterobacteriaceae bacterium isolated from the rhizosphere of Eichhornia crassipes with high copper-removal ability. Methods and Results: A spherical copper-resistant bacterium, namely ACU, was isolated from the rhizosphere of E. crassipes. It was demonstrated to have substantial copper-removing capability, even at copper concentrations as high as 69 mg l⁻¹. The 16S rRNA gene sequence of ACU suggested it to be a novel Enterobacteriaceae bacterium most closely related to Providencia sp. With increasing copper concentrations, the growth rate of ACU gradually decreased with a delay in the logarithmic growth phase. ACU demonstrated high copper-removal ability at the lag phase when cultivated in media with high copper concentrations. A 48-kDa extracellular copper-binding protein was detected in ACU. When P. crispus was inoculated with ACU, the growth ability of P. crispus significantly improved at all the tested copper concentrations, and the lethal time for 10 mg l⁻¹ was delayed. Further study revealed that while ACU cells were rarely detected in the culture solution, they were associated with the surface of P. crispus. These findings indicated that ACU grew by anchoring itself on the surface of P. crispus and could increase the ability of P. crispus to resist copper toxicity. Conclusion: To the best of our knowledge, the Enterobacteriaceae bacterium ACU is a novel nonpathogenic bacterium with high copper-removing ability from water. Significance and Impact of the Study: This study demonstrated that the Enterobacteriaceae bacterium ACU has potential applicability for use in copper removal and in the protection of aquatic plants in copper-polluted water.     

Comparative genomic analyses of transport proteins encoded within the red algae Chondrus crispus,Galdieria sulphuraria,and Cyanidioschyzon merolae11

Galdieria sulphuraria and Cyanidioschyzon merolae are thermo‐acidophilic unicellular red algal cousins capable of living in volcanic environments, although the former can additionally thrive in the presence of toxic heavy metals. Bioinformatic analyses of transport systems were carried out on their genomes, as well as that of the mesophilic multicellular red alga Chondrus crispus (Irish moss). We identified transport proteins related to the metabolic capabilities, physiological properties, and environmental adaptations of these organisms. Of note is the vast array of transporters encoded in G. sulphuraria capable of importing a variety of carbon sources, particularly sugars and amino acids, while C. merolae and C. crispus have relatively few such proteins. Chondrus crispus may prefer short chain acids to sugars and amino acids. In addition, the number of encoded proteins pertaining to heavy metal ion transport is highest in G. sulphuraria and lowest in C. crispus. All three organisms preferentially utilize secondary carriers over primary active transporters, suggesting that their primary source of energy derives from electron flow rather than substrate‐level phosphorylation. Surprisingly, the percentage of inorganic ion transporters encoded in C. merolae more closely resembles that of C. crispus than G. sulphuraria, but only C. crispus appears to signal via voltage‐gated cation channels and possess a Na⁺/K⁺‐ATPase and a Na⁺ exporting pyrophosphatase. The results presented in this report further our understanding of the metabolic potential and toxic compound resistances of these three organisms.     

Functional Ecology of Two Contrasting Freshwater Ciliated Protists in Relation to Temperature

We conducted microcosm experiments with two contrasting freshwater ciliates on functional traits (FTs) related to their growth rate (numerical response, NR) and ingestion rate (functional response, FR) over a range of ecologically relevant temperatures. Histiobalantium bodamicum and Vorticella natans are common planktonic ciliates but their abundance, swimming behavior, and temperature tolerance are different. In contrast to most sessile peritrich species, the motile V. natans is not strictly bacterivorous but also voraciously feeds upon small algae. We observed three main alterations in the shape of NR of both species with temperature, that is, change in the maximum growth rate, in the initial slope and in the threshold food level needed to sustain the population. Similarly, maximum ingestion rate, gross growth efficiency (GGE), and cell size varied with temperature and species. These findings caution against generalizing ciliate performance in relation to the ongoing global warming. Our results suggest that V. natans is the superior competitor to H. bodamicum in terms of temperature tolerance and bottom-up control. However, the abundance of V. natans is usually low compared to H. bodamicum and other common freshwater ciliates, suggesting that V. natans is more strongly top-down controlled via predation than H. bodamicum. The taxonomic position of V. natans has been debated. Therefore, to confirm species and genus affiliation of our study objects, we sequenced their small subunit ribosomal RNA (SSU rDNA) gene.     

Introduction of Nonindigenous Aquatic Vascular Plants in Southern New England: A Historical Perspective

Aquatic plants comprise few species worldwide, yet introductions of nonindigenous hydrophytes represent some of the most severe examples of biological invasions. Often innocuous in their indigenous regions, many aquatic plant species have caused extreme ecological and economic consequences when introduced into nonindigenous habitats. Typically, aquatic plant invasions are unnoticed or overlooked until they are perceived as problematic. By then, plants are virtually impossible to eradicate and negative ecological impacts caused by their spread into natural communities are irreparable. We present criteria to facilitate decisions whether a species should be characterized as nonindigenous or invasive. Historical data are used to clarify methods of introduction, avenues and means of dispersal, and extent of invasiveness of the following aquatic plants in southern New England: Acorus calamus, Butomus umbellatus, Cabomba caroliniana, Callitriche stagnalis, Egeria densa, Hydrilla verticillata, Limnobium spongia, Marsilea quadrifolia, Myriophyllum aquaticum, Myriophyllum heterophyllum, Myriophyllum spicatum, Najas minor, Najas guadalupensis, Nasturtium officinale, Nymphoides peltata, Potamogeton crispus, Trapa natans and Veronica beccabunga.     

Solid state fermentation of aquatic macrophytes for crude protein extraction

Aquatic macrophytes such as Elodea nuttalli, Vallisneria natans, Alterranthera philoxerides that are widely distributed in water environments of Lake Taihu basin were used as substrate of solid state fermentation to produce crude protein extraction. The effects of single-strain fermentation and mixed strains fermentation of aquatic macrophytes on the production of crude protein extraction and cellulase activity are analyzed, respectively. The experimental results showed that the crude protein content of products with mixed strains fermentation is higher than that with single-strain fermentation. The crude protein content of V. natans fermented by Aspergillus niger and Candida utilis is the highest among the aquatic macrophytes examined in this study. V. natans is used as the substrate to be fermented by C. utilis and A. niger; their ratio is 1:1 at 28 ± 1 °C for 72 h. The crude protein of fermented V. natans is as high as 49.54%, with 128.82% of its increase rate. The cellulose activity reaches a maximum of 4.21 μ/ml at 84 h of fementation of V. natans. Thus, the solid state fermentation of aquatic macrophytes to produce crude protein extraction is promising, which make aquatic macrophytes a potential resource and thus is beneficial to the long-term ecological restoration of eutrophic lakes.     

Chemical composition of leaves and structure of photosynthetic apparatus in aquatic higher plants

Chemical composition of leaves (the content of carbon, nitrogen, nonstructural carbohydrates, organic acids, mineral substances, and water) and the structure of photosynthetic apparatus (specific leaf weight, cell volume, and the number of cells per unit leaf area) were investigated for 18 species of aquatic plants featuring various degrees of contact with aqueous environment and sediment. The rooted hydrophytes with floating leaves were characterized by comparatively high content of carbon and nitrogen (437 and 37 mg/g dry wt, respectively) and by low concentration of nonstructural carbohydrates, mineral substances, and organic acids (161, 54, and 60 mg/g dry wt, respectively). Unlike rooted plants, the free-floating nonrooted hydrophytes had characteristically higher content of nonstructural polysaccharides and mineral substances (by a factor of 1.3 and 1.6, respectively), while the leaf nitrogen content was 1.4 times lower, and the proportion of soluble carbohydrates in the total content of nonstructural carbohydrates was rather low (9%). The chemical composition of leaves in submerged rooted hydrophytes was intermediate between those for rooted hydrophytes with floating leaves and for nonrooted free plants. We found reliable positive correlations between the volume of photosynthesizing cells and the leaf content of organic acids (r = 0.69), as well as between specific leaf weight, the number of photosynthesizing cells per unit leaf area, and carbon content (r = 0.67 and r = 0.62, respectively). The content of nitrogen and nonstructural carbohydrates in hydrophytes was unrelated to structural characteristics of photosynthetic apparatus and depended on the absence or presence of plant attachment to the sediment. It is concluded that the structural traits of photosynthetic apparatus and the leaf chemical composition in hydrophytes featuring different degrees of plant contact with water and sediment reflect the specificity of plant adaptation to complex conditions of their habitats.     

Chrysosporum ovalisporum is synonymous with the true-branching cyanobacterium Umezakia natans (Nostocales/Aphanizomenonaceae)

True branching is a facultative characteristic only known from two cyanobacteria in the Aphanizomenonaceae, Umezakia natans and Dolichospermum brachiatum. In both cases, its expression has been associated with environmental stress, and its practical use as a diacritical feature has been previously evaluated. In this study, we undertook further evaluation of the phylogeny of Umezakia natans and its relationship to Chrysosporum ovalisporum as a previous study suggested the two were potentially congeneric. We used combined morphological, phylogenetic, and phylogenomic approaches to determine their relatedness using new strains available from a broad geographic range. Phylogenetic analysis based on 16S rRNA gene sequences showed that Australian C. ovalisporum and Japanese U. natans strains clustered together with accessions of C. ovalisporum originating from Australia, Israel, and Spain, with high p-distance similarity values (99.5%–99.9%). Additionally, differences between the two species in the 16S–23S ITS region was low (0%–2.5%). The average nucleotide identity of the U. natans and C. ovalisporum strains was also high (ANI of > 99.5 and AF > 0.9) and supported a genus-level separation from Chrysosporum bergii (83 ANI between clusters). Furthermore, in culture, strains of both species grown in vitamin-free media showed facultative true branching, a feature not previously known in C. ovalisporum. Collectively, the results support unification of C. ovalisporum and U. natans according to the principle of priority as Umezakia ovalisporum.     

Validation and application of reference genes for quantitative gene expression analyses in various tissues of <Emphasis Type="Italic">Bupleurum chinense</Emphasis>

It is crucial to select stable references in gene expression analyses using quantitative real-time PCR (qRT-PCR). In this work, seven frequently used reference genes, 18S, Actin, EF1α, α-tubulin, β-tubulin, Cyclophilin and Cytoplasmic ribosomal protein L2 (L2), from Bupleurum chinense DC. were evaluated as the internal control in five tissues, roots, stems, leaves, flowers and fruits, before tissue specific gene expression assays. The results showed that β-tubulin was the most stable and reliable reference gene among the seven candidate genes in the measured tissues. The expression levels of four genes involved in saikosaponins (the pharmacological active compounds of B. chinense) biosynthesis, HMGR, IPPI, FPS and β-AS, were assayed with β-tubulin as the internal control in the five tissues. All the four genes were expressed in the five tissues with different profiles and HMGR in the order of roots > flowers, stems and leaves > fruits, IPPI of stems > leaves and fruits > roots and flowers, FPS of flowers > fruits > stems and roots > leaves and β-AS of roots > flowers, stems and fruits > leaves. The genes of FPS and β-AS were expressed predominantly in flowers and roots, respectively. This study may provide a suitable internal control for quantitative gene expression assays in various tissues and give insight into the tissue expression profiles of four saikosaponins biosynthesis-involved genes of medicinal B. chinense.     

Habitat conditions of the phytocoenoses dominated by Luronium natans (L.) Rafin in Poland

Phytosociological and habitat studies of the phytocoenoses of Luronium natans have been conducted. The present results were compared with data on L. dortmanna and I. lacustris. It is demonstrated that the community of L. natans differs from the other two communities with respect to habitat conditions despite the fact that they have been reported to occur jointly and alongside in Lobelia lakes. It appears that significant differences between the communities are found not only regarding their waters, but also their substrates. L. natans dominated phytocoenoses are confined to oligotrophic, extremely soft waters, markedly poor in Ca²⁺, but richer in Na⁺ and SO₄ ^2- than those of Lobelia and Isoëtes. Luronium natans develops best on acidic, highly hydrated substrates rich in organic matter, NO₃ ^– and total N. The results obtained indicate that L. natans and the phytocoenoses formed by it are characterized by their narrow ecological amplitude in Poland as opposed to those occurring in western Europe, which tolerate a relatively wide range of habitats. The present findings confirm the data from numerous works, which point to the weak competitive ability of the species compared with species typical of eutrophic waters.     

Phytoremediation efficiency of Eichhornia crassipes in fly ash pond

The present study was focused on field research to examine the phytoremediation potential of naturally grown Eichhornia crassipes in fly ash (FA) pond. Field results indicate the efficiency of E. crassipes for remediation of heavy metals from FA pond. The bioconcentration factor trend was Cr (3.75) > Cu (2.62) > Cd (1.05), and Cu (1.35) in root and stem, respectively. The survival and abundance growth of E. crassipes in the circumstance of heavy metal enriched FA pond is another highlight of the present research that reveals its toxitolerant characteristics. Thus, this lesson on phytoremediation proved that E. crassipes is a potential accumulator of Cu, Cr, and Cd from FA ponds and is a promising species for FA pond's remediation globally.     

Opportunities for Phytoremediation and Bioindication of Arsenic Contaminated Water Using a Submerged Aquatic Plant:Vallisneria natans (lour.) Hara.

The identification of plants with high arsenic hyperaccumulating efficiency from water is required to ensure the successful application of phytoremediation technology. Five dominant submerged plant species (Vallisneria natans (Lour.) Hara., Potamageton crispus L., Myriophyllum spicatum L., Ceratophyllum demersum L. and Hydrilla verticillata (L.f.) Royle) in China were used to determine their potential to remove As from contaminated water. V. natans had the highest accumulation of As among them. The characteristics of As accumulation, transformation and the effect of phosphate on As accumulation in V. natans were then further studied. The growth of V. natans was not inhibited even when the As concentration reached 2.0 mg L^?1. After 21 d of As treatment, the bioconcentration factor (BCF) reached 1300. The As concentration in the environment and exposure time are major factors controlling the As concentration in V. natans. After being absorbed, As(V) is efficiently reduced to As(III) in plants. The synthesis of non-enzymic antioxidants may play an important role under As stress and increase As detoxication. In addition, As(V) uptake by V. natans was negatively correlated with phosphate (P) uptake when P was sufficiently supplied. As(V) is probably taken up via P transporters in V. natans.     

Distinguishable root plaque on root surface of Potamogeton crispus grown in two sediments with different nutrient status

The properties of plaques were different on the root surface of Potamogeton crispus planted in sediments from two different shallow lakes. Lake Tangxunhu sediment, with low pH, contained low organic matter, whereas Lake Yuehu sediment, with high pH, had high calcium deposits mixed with high organic matter. The contents of mineral elements in sediment of Lake Tangxunhu was lower than that of Lake Yuehu, except for iron (Fe) content, but the contents of mineral elements extracted by sodium dithionite–sodium citrate–sodium bicarbonate (DCB) from root plaques were higher in Lake Tangxunhu than those in Lake Yuehu, except for Fe. These element distributions on P. crispus root plaques were characterized by scanning electron microscope combined with energy-dispersive X-ray spectrometer and were consistent with the contents of mineral elements in sediment. The root plaque of P. crispus planted in Lake Tangxunhu sediment mainly contained silicon (Si) and Fe, and the content of Si was greater than Fe, which may be contributed to the formation of poly-silicic-ferric in the natural conditions. However, the root plaque of P. crispus planted in the sediment with higher calcium content of Lake Yuehu was rich in Fe, Si, phosphorus (P), and calcium (Ca). Due to oxygen secretion by plant roots, the root plaque has more Fe₃(PO₄)₂ and a certain amount of Ca₃(PO₄)₂. The ratio of magnesium (Mn) to Fe extracted by DCB from root plaque in Lake Tangxunhu sediment was 0.031 and 0.010 in Lake Yuehu sediment. In Lake Tangxunhu sediment, lower content of organic matter results in weak reducibility. Enhanced oxidation ability by oxygen secretion of P. crispus root could oxidize low-valent Fe and Mn into iron–manganese oxide, which leads to formation of iron–manganese plaque on the root surface. However, this case is different in Lake Yuehu sediment, where Fe and Mn can be reduced in high organic sediment and low-valent Mn can precipitate in the sediment in which pH is >8. Thus, low-valent Fe in Lake Yuehu sediment moves to the root surface of P. crispus, where it oxidizes into Fe oxide, i.e., Fe plaque.     

修复白洋淀镉污染水体的沉水植物筛选试验

为了筛选出适宜修复白洋淀镉(Cd)污染水体的沉水植物,该研究通过室内模拟试验,分析了四种沉水植物黑藻、狐尾藻、金鱼藻和菹草对Cd的耐受性及对底泥Cd的富集和迁移能力。结果表明:(1)通过毒性测试研究,Cd对黑藻、狐尾藻、金鱼藻及菹草的4 d-EC₅₀(半数抑制浓度)分别为0.51、0.81、0.03、0.12 mg·L^-1,狐尾藻对Cd的耐性最强,黑藻次之,金鱼藻对Cd的耐性最低; 四种沉水植物对Cd的最大富集量分别为27.89、15.28、22.54、32.74 g·kg^-1,菹草对Cd的富集能力最强,黑藻次之,狐尾藻对Cd的富集能力最低。(2)通过Cd污染底泥修复研究,黑藻、狐尾藻和菹草体内Cd富集量整体表现为根>叶片和茎(P<0.05); 地上部、根对Cd的富集能力分别表现为黑藻>菹草>狐尾藻,菹草>黑藻>狐尾藻; 三种沉水植物对Cd的迁移能力则表现为黑藻>狐尾藻>菹草。总之,黑藻对底泥中Cd富集和迁移能力均较强,且耐性较高,是最适合修复白洋淀Cd污染水体的沉水植物。