Diatom-based dissolved inorganic nitrogen reconstruction in the Changjiang River estuary and its adjacent areas

A five-component weighted average partial least squares(WA-PLS)calibration model was developed by analysing diatom assemblages in 34 surface sediment samples(collected in 2015)from the Changjiang River estuary(CRE)and its adjacent areas to infer dissolved inorganic nitrogen(DIN)concentrations.Eighteen additional sets of surface sediment diatoms and corresponding upper water DIN data(collected in 2012)were used to evaluate the accuracy of the model,and the relationship between observed and diatom-inferred DIN(DI-DIN)values(R2=0.85)illustrated the strong performance of the transfer function,indicating that precise reconstructions of former DIN are possible.The diatom-DIN transfer function was applied to the diatom record from a sediment core DH8-2(1962‒2012)collected in the Fujian-Zhejiang area south of the CRE.The reconstruction based on the DI-DIN model showed a significant DIN increase from 1962-2012,reflecting the influence of human activities on the very large increase in eutrophication.Three distinct periods can be seen from the changes in DIN and diatom taxa.In the 1962-1972 period,the DIN content was relatively low,with an average of 5.94μmol/L,and more than 80%of the diatom species identified were benthic taxa.In the 1972-2004 period,as the impact of human activities intensified,large nutrient inputs caused the DIN content to increase,with an average of 8.25μmol/L.The nutrient inputs also caused a significant change in the nutrient components and a distinct increase in small planktonic taxa.In the 2004-2012 period,the DIN content continued to rise,fluctuating at approximately 10μmol/L.A continuous increase in the frequency of planktonic taxa(up to 65.48%)indicated that eutrophication was further intensified,which was confirmed by the transformation from diatom-induced red tide to dinoflagellate-induced red tide during this period.

Clay surface modification and its coagulation of red tide organisms

Surface modification of montmorillonite by means of Mg2+ insertion reaction has been studied and a positively charged montmorillonite has been prepared. The effects of preparation temperature and Mg2+ concentration on the positive charge property of the clay and on the clay coagulating Heterosigma akashiwo have been studied. The results showed that the modified clay enhanced the coagulation and the used amount decreased to 1/5—1/10 of the original. The removal rates of Heterosigma akashiwo were correlated positively with positive charge on the clay in accordance with theoretical model.

Effects of ammonium and nitrate on encystment and growth of Scrippsiella trochoidea

To avoid unfavorable environmental conditions, Scrippsiella trochoidea is capable of forming a resting cyst in the process of growth. In the present study,we investigated the effects of nitrate and ammonium on the growth and encystment of S. trochoidea. We incubated S.trochoidea in modified f/2 media without nitrogen and silicate in flasks. The flasks were divided into two groups.Nitrate was added as a nitrogen source in the first group,and ammonium was added in the second group. The concentrations of the nitrogen compounds were 0, 10, 30, 60,and 90 lmol/L. The results indicate that NO3--N favors cell growth, and the cultures with a higher concentration of NO3--N were ineffective at forming cysts. In contrast,NH4--N promoted cell growth and cyst formation. At similar concentrations as NO3--N, NH4--N had a toxic effect on cell growth and increased the cyst formation ratio.Thus, the NH4--N concentration is an important factor for controlling encystment. We believe that the impact of NH4--N in inducing cyst formation may be a useful feedback mechanism in ecological systems.

Profiles of and variations in aluminum species in PAC-MC used for the removal of blooming microalgae

Polyaluminum chloride modified clay(PAC-MC)is a safe and efficient red tide control agent that has been studied and applied worldwide.Although it is well known that the distribution of hydrolytic aluminum species in PAC affects its flocculation,little is known about the influence of particulars aluminum species on the microalgae removal efficiency of PAC-MC;this lack of knowledge creates a bottleneck in the development of more efficient MCs based on aluminum salts.The ferron method was used in this study to quantitatively analyze the distributions of and variations in different hydrolytic aluminum species during the process of microalgae removal by PAC-MC.The results showed that Ala,which made up 5%–20%of the total aluminum,and Alp,which made up 15%–55%of the total aluminum,significantly affected microalgae removal,with Pearson’s correlation coefficients of 0.83 and 0.89,respectively.Most of the aluminum in the PAC-MC sank rapidly into the sediments,but the rate and velocity of settlement were affected by the dose of modified clay.The optimal dose of PAC-MC for precipitating microalgae was determined based on its aluminum profile.These results provide guidance for the precise application of PAC-MC in the control of harmful algal blooms.

Regeneration and utilization of nutrients during collapse of a Mesodinium rubrum red tide and its influence on phytoplankton species composition

High-biomass red tides occur frequently in some semi-enclosed bays of Hong Kong where ambient nutrients are not high enough to support such a high phytoplankton biomass. These high-biomass red tides release massive inorganic nutrients into local waters during their collapse. We hypothesized that the massive inorganic nutrients released from the collapse of red tides would fuel growth of other phytoplankton species. This could influence phytoplankton species composition. We tested the hypothesis using a red tide event caused by Mesodinium rubrum(M. rubrum) in a semi-enclosed bay, Port Shelter. The red tide patch had a cell density as high as 5.0×10~5 cells L^(-1), and high chlorophyll a(63.71 μg L^(-1)). Ambient inorganic nutrients(nitrate:NO_3^-, ammonium: NH_4^+, phosphate: PO_4^(3-), silicate: SiO_4^(3-)) were low both in the red tide patch and the non-red-tide patch(clear waters outside the red tide patch). Nutrient addition experiments were conducted by adding all the inorganic nutrients to water samples from the two patches followed by incubation for 9 days. The results showed that the addition of inorganic nutrients did not sustain high M. rubrum cell density, which collapsed after day 1, and did not drive M. rubrum in the non-red-tide patch sample to the same high-cell density in the red tide patch sample. This confirmed that nutrients were not the driving factor for the formation of this red tide event, or for its collapse. The death of M. rubrum after day 1 released high concentrations of NO_3^-,PO_4^(3-), SiO_4^(3-), NH_4^+, and urea. Bacterial abundance and heterotrophic activity increased, reaching the highest on day 3 or 4, and decreased as cell density of M. rubrum declined. The released nutrients stimulated growth of diatoms, such as Chaetoceros affinis var. circinalis, Thalassiothrix frauenfeldii, and Nitzschia sp., particularly with additions of SiO_4^(3-) treatments, and other species. These results demonstrated that initiation of M. rubrum red tides in the bay was not directly driven by nutrients.However, the massive inorganic nutrients released from the collapse of the red tide could induce a second bloom in low-ambient nutrient water, influencing phytoplankton species composition.

Programmed cell death induced by modified clay in controlling Prorocentrum donghaiense bloom

Modified clay(MC),an effective material used for the emergency elimination of algal blooms,can rapidly reduce the biomass of harmful algal blooms(HABs)via flocculation.After that,MC can still control bloom population through indirect effects such as oxidative stress,whichwas initially proposed to be related to programmedcell death(PCD)at molecular level.To further study theMC induced cell death in residual bloom organisms,especially identifying PCD process,we studied the physiological state of the residual Prorocentrum donghaiense.The experimental results showed that flocculation changed the physiological state of the residual cells,as evidenced by growth inhibition and increased reactive oxygen species production.Moreover,this research provides biochemical and ultrastructural evidence showing that MC induces PCD in P.donghaiense.Nuclear changes were observed,and increased caspase-like activity,externalization of phosphatidylserine and DNA fragmentation were detected in MC-treated groups and quantified.And the mitochondrial apoptosis pathway was activated in both MC-treated groups.Besides,the features of MC-induced PCD in a unicellular organism were summarized and its concentration dependent manner was proved.All our preliminary results elucidate the mechanism through which MC can further control HABs by inducing PCD and suggest a promising application of PCD in bloom control.