Geographic pattern of phytoplankton community and their drivers in lakes of middle and lower reaches of Yangtze River floodplain, China.

Disentangling the relative contributions of deterministic and stochastic processes was critical to compressive understanding of underlying mechanism governing geographic pattern and assembly of phytoplankton community, while it was seldom performed in connected lakes under human pressure. Here, we investigated phytoplankton community pattern in relation to environmental and spatial factors over 81 lakes located in the middle and lower reaches of Yangtze River (MLYR) floodplain, where many lakes suffered from eutrophication and cyanobacterial blooms. A majority of MLYR lakes had higher phytoplankton abundance surpassing 107 cells/L and were dominated by common bloom-forming cyanobacterial genera, including Pseudanabaena, Microcystis, Merismopedia, Dolichospermum, Limnothrix, and Raphidiopsis. Phytoplankton community exhibited a striking geographical pattern both for taxonomic and functional compositions, while functional groups were less sensitive, and dissimilarity in communities displayed no significant increases with increasing geographical distance. Further, species richness explained much higher percentage of community variations than species turnover, indicating a reduced effect of environmental filtering of phytoplankton species with tolerance to similar environments in connected MLYR lakes. Both deterministic and stochastic processes governed assembly and biogeographic of phytoplankton community. Variation partition analysis showed that spatial factors exhibited greater influence on phytoplankton community compared to environmental variables. The stronger influence of spatial factors was further demonstrated by Mantel test and neutral community model. These findings indicate that deterministic and stochastic processes exhibited similar biogeographic patterns for phytoplankton community in MLYR lakes, but stochastic process was overwhelmingly dominated. Moreover, a large proportion of unexplained variation implies that complex interactions exist to shape assembly mechanism of phytoplankton community in MLYR lakes.

Hydrologic and nutrient-driven regime shifts of cyanobacterial and eukaryotic algal communities in a large shallow lake: Evidence from empirical state indicator and ecological network analyses.

The detection and prediction of lake ecosystem responses to environmental changes are pressing scientific challenge of major global relevance. Specifically, an understanding of lake ecosystem stability over long-term scales is urgently needed to identify impending ecosystem regime shifts induced by human activities and improve lake ecosystem protection. This study investigated regime shifts in cyanobacterial and eukaryotic algal communities in a large shallow lake over a century in response to nutrient enrichment and hydrologic regulation using evidence from empirical state indicators and ecological network analyses of sedimentary-inferred communities. The diversity and structure of cyanobacterial and eukaryotic algal communities were investigated from sedimentary DNA records and used, for the first time, as state variables of the lake ecosystem to detect lake stability. Two regime shifts were inferred in the 1970s and 2000s based on temporal analysis of empirical indicators. Co-occurrence network analysis based on taxonomic abundance distributions and presence/absence patterns also supported the two regime shifts based on architectural features of the ecological networks. Moreover, the associations of cyanobacterial and eukaryotic algal taxa were observed to be non-random across time. The abrupt driver-mediated regime shift in the 1970s is characterized by the disappearance of submerged vegetation, significantly increased relative abundances of Microcystis and Chlorophyta taxa, and was primarily caused by sluice construction. The critical transition observed in the 2000s was manifested by the occurrence of serious cyanobacterial blooms and was triggered by increased nutrient loading with the development of urbanization and agricultural intensification. This study reveals the important roles of hydrologic regulation and nutrient loading in the temporal successional dynamics of a shallow lake ecosystem, providing new insights into regime shifts of lake ecosystems that can help inform future efforts to predict important lake ecosystem state changes.

Climate and Nutrient-Driven Regime Shifts of Cyanobacterial Communities in Low-Latitude Plateau Lakes.

Cyanobacterial blooms that form in response to climate warming and nutrient enrichment in freshwater lakes have become a global environmental challenge. Historical legacy effects and the mechanisms underlying cyanobacterial community succession are not well understood, especially for plateau lakes that are important global freshwater resources. This study investigated the temporal dynamics of cyanobacterial communities over centuries in response to nutrient enrichment and climate warming in low-latitude plateau lakes using high-throughput DNA sequencing of sedimentary DNA combined with traditional paleolimnological analyses. Our results confirmed that nutrients and climate warming drive shifts in cyanobacterial communities over time. Cyanobacterial community turnover was pronounced with regime shifts toward new ecological states, occurring after exceeding a tipping point of aquatic total phosphorus (TP). The inferred species interactions, niche differentiation, and identity of keystone taxa significantly changed after crossing the aquatic TP ecological threshold, as demonstrated by network analysis of cyanobacterial taxa. Further, the contribution of aquatic TP to cyanobacterial community dynamics was greater than that of air temperature when lakes were in an oligotrophic state. In contrast, as the aquatic TP threshold was exceeded, the contribution to community dynamics by air temperature increased and potentially surpassed that of aquatic TP. Overall, these results provide new evidence for how past nutrient levels in lacustrine ecosystems influence contemporary cyanobacterial community responses to global warming in low-latitude plateau lakes.

Identification of cable bacteria and its biogeochemical impact on sulfur in freshwater sediments from the Wenyu River.

Cable bacteria are filamentous sulfur-oxidizing microorganisms that couple the reduction of oxygen or nitrate in surface sediments with the oxidation of free sulfide in deeper sediments by transferring electrons across centimeter scale distances. The distribution and activities of cable bacteria in freshwater sediments are still poorly understood, especially the impact of cable bacteria on sulfur cycling. The goal of this study was to investigate electrogenic sulfide oxidation associated with cable bacteria in laboratory microcosm incubations of freshwater sediments using microsensor technology, 16S full-length rRNA sequencing, and fluorescence in situ hybridization (FISH) microscopy. Their activity was characterized by a pH maximum of 8.56 in the oxic zone and the formation of a 13.7 ± 0.6 mm wide suboxic zone after 25 days of incubation. Full-length 16S rRNA gene sequences related to cable bacteria were recovered from the sediments and exhibited 93.3%-99.4% nucleotide (nt) similarities with those from other reported freshwater cable bacteria, indicating that new species of cable bacteria were present in the sediments. FISH analysis indicated that cable bacteria density increased with time, reaching a maximum of 95.48 m cm-2 on day 50. The cells grew downwards to 40 mm but were mainly concentrated on the top 0-20 mm of sediment. The cable bacteria continuously consumed H2S in deeper layers and oxidized sulfide into sulfate in the 0-20 mm surface layers, thereby affecting the sulfur cycling within sediments. These findings provide new evidence for the existence of higher diversity of cable bacteria in freshwater sediments than previously known.

Factors related to aggravated Cylindrospermopsis (cyanobacteria) bloom following sediment dredging in an eutrophic shallow lake.

In recent years, Cylindrospermopsis raciborskii blooms have been widely found worldwide. Topics dealing with the mitigation of C. raciborskii bloom is of great importance for toxins produced could threaten public health. The paper first investigated C. raciborskii dynamics over three years following sediment dredging in a shallow eutrophic Lake Dongqian (China). Based on rpoC1 gene copies, C. raciborskii bloom formed with average density of 1.30 × 106 cells/L on July 2009. One year later after sediment dredging, C. raciborskii cell density decreased below 1.17 × 105 cells/L or under detected limits during summer days on 2010. While two years later, the C. raciborskii bloom period was returned with markedly increased cell density reaching up to 4.15 × 107 cells/L on October 2011, and the maximum peak density was shown at 20.3 °C that was much lower than reported optimal growth temperature. Inferred from Spearman correlation analysis, linear regression showed C. raciborskii density was significant and positive with pH and SD, whereas they were significant and negative with TP and DO. Multiple regression analysis further demonstrated that TN, TP, SRP, pH and DO provided the best model and explained 53.1% of the variance in C. raciborskii dynamics. The approaches managing nutrients reduction might not control C. raciborskii bloom as extremely low TN (avg. 0.18 mg/L) and TP concentrations (avg. 0.05 mg/L) resulted in the highest C. raciborskii cell density after sediment dredging.

Phytoplankton response to climate changes and anthropogenic activities recorded by sedimentary pigments in a shallow eutrophied lake.

Studies that address the potential effects of climate and anthropogenic activities on lacustrine phytoplankton succession are scarce in the shallow lakes. In the present work, the succession of phytoplankton community inferred from sedimentary pigments has been investigated; the impacts of climate and anthropogenic activities on the succession have been evaluated by the generalized additive models (GAMs) in a shallow eutrophied lake, Lake Chaohu, located eastern China. The results show that phytoplankton succession can be divided into two periods: pre-1960s and post 1960s. The mean values of ßß­Car and Chl a increased after the 1960s at both sites sampled, from 0.013 to 0.359, and 0.013 to 1.382 µg g-1, respectively (site C4), and from 0.015 to 0.530, and 0.010 to 0.921 µg g-1, respectively (site C14), reflecting significant increases of primary productivity since the 1960s. The percentage of diatoms and dinoflagellates preserved in sediments decreased from ~90% to ~15% since the 1960s, while cyanobacteria and green algae increased from ~5% to ~35%, respectively, reflecting the shift of the lake phytoplankton community. This succession was related to construction of the Chaohu Dam in 1963, increasing discharges of anthropogenic N and P into the Lake, and a generally warming environment as reflected by increasing average air temperatures. The results of GAMs showed aquatic total phosphorus (TP) concentration is the dominant contributor to phytoplankton community change, explaining 42.74%, 40.27%, 40.77% and 72.28% of the variance of total algae, cyanobacteria, green algae, and diatoms and dinoflagellates, respectively. The positive impacts of increasing TP concentrations on abundances of total algae, cyanobacteria, and green algae were observed during periods of relatively high TP concentrations since the mid-1970s. The positive responses of total algae, cyanobacteria, green algae and diatoms and dinoflagellates to increasing average air temperatures were observed since the mid-1990s, showing that a generally warmer environment facilitated algae proliferation.

Variations of Growth and Toxin Yield in Cylindrospermopsis raciborskii under Different Phosphorus Concentrations.

The bloom-forming cyanobacteria, Cylindrospermopsis raciborskii, is a producer of the cytotoxic cylindrospermopsin (CYN). In this study, the growth, toxin yield, and expression of CYN biosynthesis genes of C. raciborskii were examined under varying phosphorus (P) concentrations. The results show the cell number at 0.00 and 0.01 mg·L-1 P was significantly lower than that at higher P concentrations (≥0.5 mg·L-1). The chlorophyll a content, filament length, heterocyst, and akinete numbers at P ≤ 0.05 mg·L-1 were also significantly reduced. The intracellular and extracellular CYN concentrations and the extracellular proportions increased during the culture period, and larger values were observed at higher P concentrations. Total CYN content reached 45.34-63.83 fg·cell-1 and extracellular CYN proportion reached 11.49%-20.44% at the stationary growth phase. A significantly positive correlation was observed between CYN production and cell growth rate. Three cyr genes were expressed constantly even at P-deficient conditions. The transcription of cyr genes at P-replete conditions or after P supplementation increased from 1.18-fold to 8.33-fold. In conclusion, C. raciborskii may rapidly reorganize metabolic processes as an adaptive response to environmental P fluctuations. CYN production and cyr gene expression were constitutive metabolic processes in toxic C. raciborskii.

An overview of diversity, occurrence, genetics and toxin production of bloom-forming Dolichospermum (Anabaena) species.

The new genus name Dolichospermum, for most of the planktonic former members of the genus Anabaena, is one of the most ubiquitous bloom-forming cyanobacterial genera. Its dominance and persistence have increased in recent years, due to eutrophication from anthropogenic activities and global climate change. Blooms of Dolichospermum species, with their production of secondary metabolites that commonly include toxins, present a worldwide threat to environmental and public health. In this review, recent advances of the genus Dolichospermum are summarized, including taxonomy, genetics, bloom occurrence, and production of toxin and taste-and-odor compounds. The recent and continuing acquisition of genome sequences is ushering in new methods for monitoring and understanding the factors regulating bloom dynamics.

Molecular separation of two long taxonomically debated cyanobacterial genera Cylindrospermopsis and Raphidiopsis (Nostocales) based on the ITS-L phylogeny.

The cyanobacterial genera Raphidiopsis and Cylindrospermopsis morphologically resembled each other and are difficult to distinguish, especially when heterocytes do not form in Cylindrospermopsis species. The phylogeny based on multiple gene loci sequences in previous studies cannot divide the strains of these two genera into separate clusters. In the present study, four gene loci 16S rRNA, ITS-L, cpcBA-IGS and rpoC1, sequenced from many Chinese strains (44 Cylindrospermopsis strains and 16 Raphidiopsis strains), were analyzed to infer the phylogenetic relationship between the two genera. According to the 16S rRNA, cpcBA-IGS and rpoC1 gene phylogeny, Cylindrospermopsis and Raphidiopsis strains are intermixed and divide into two or three groups. By contrast, the ITS-L analysis implied that they respectively form two separate and unique clusters. A fragment of 7bp (RAGAAACT) in ITS-L was conserved for Raphidiopsis strains, which could be distinguished from Cylindrospermopsis strains. A pair of primers targeting the ITS-L fragment specific to Raphidiopsis strains were designed and further verified.