BTXs removals by modified clay during mitigation of Karenia brevis bloom: Insights from adsorption and transformation.

Harmful algal blooms (HABs), especially those caused by toxic dinoflagellates, are spreading in marine ecosystems worldwide. Notably, the prevalence of Karenia brevis blooms and potent brevetoxins (BTXs) pose a serious risk to public health and marine ecosystems. Therefore, developing an environmentally friendly method to effectively control HABs and associated BTXs has been the focus of increasing attention. As a promising method, modified clay (MC) application could effectively control HABs. However, the environmental fate of BTXs during MC treatment has not been fully investigated. For the first time, this study revealed the effect and mechanism of BTX removal by MC from the perspective of adsorption and transformation. The results indicated that polyaluminium chloride-modified clay (PAC-MC, a typical kind of MC) performed well in the adsorption of BTX2 due to the elevated surface potential and more binding sites. The adsorption process was a spontaneous endothermic process that conformed to pseudo-second-order adsorption kinetics (k2 = 6.8 × 10-4, PAC-MC = 0.20 g L-1) and the Freundlich isotherm (Kf = 55.30, 20 °C). In addition, detailed product analysis using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated that PAC-MC treatment effectively removed the BTX2 and BTX3, especially those in the particulate forms. Surprisingly, PAC-MC could promote the transformation of BTX2 to derivatives, including OR-BTX2, OR-BTX3, and OR-BTX-B5, which were proven to have lower cytotoxicity.

Uncovering nutrient regeneration, transformation pattern, and its contribution to harmful algal blooms in mariculture waters.

The prevalence of harmful algal blooms (HABs), especially in mariculture waters, has become a concern for environmental and human health worldwide. Notably, the frequent occurrence of HABs relies upon a substantial supply of available nutrients, which are influenced by nutrient recycling. However, nutrient regeneration, transformation pattern, and their contribution to HABs in mariculture waters remain largely unknown. In this study, by combining field investigation and incubation experiments from June to September 2020, the temporal variations in nutrients and algal composition were revealed. In addition, the nutrient regeneration and assimilation rates in the water column during two continuous algal blooms were measured. The results indicated that organic nutrients, which were the dominant components, strongly stimulated nutrient regeneration. High regeneration rates were observed, with dissolved inorganic nitrogen (DIN) and phosphorous (DIP) regeneration rates ranging from 0.25 to 2.64 µmol/L·h and 0.01 to 0.09 µmol/L·h, respectively. Compared to the direct uptake of organic nutrients, the rapid regeneration of inorganic nutrients played a vital role in sustaining continuous algal blooms, as regenerated DIN contributed 100 % while regenerated DIP contributed 72-100 % of the algal assimilation demand. Furthermore, the redundancy analysis and inverse solution equations indicated that different N transformation patterns and utilization strategies occurred during Heterosigma and Nannochloris blooms. The shorter N recycling pathway and faster NH4+ supply rates provided favorable conditions for the dominance of Nannochloris over Heterosigma, which had a preference for the uptake of NO3-. In conclusion, we propose that nutrient regeneration is a key maintenance mechanism underlying the maintenance of continuous algal blooms, and different N transformation patterns and utilization strategies regulate algal communities in mariculture waters.

Novel insights into toxin changes associated with the growth of Alexandrium pacificum: Revealing active toxin-secretion ability and toxin cell quota variation.

Paralytic shellfish toxins (PSTs) are widely distributed globally and are produced by Alexandrium pacificum in marine system. However, the characteristics of toxins producing and secreting associated with growth phases are still unclear, especially whether A. pacificum has the ability to actively secrete PSTs is controversial. In this study, variation characteristics of intracellular and extracellular PSTs contents associated with A. pacificum growth phases were investigated thoroughly. The results showed that intracellular and extracellular PSTs contents increased sharply during the exponential phase. But during the stationary phase, the intracellular PSTs content increased by only 26 %, and the extracellular PSTs content did not increase significantly. Since the increase in extracellular PSTs content mainly occurred at the exponential phase, when most cells were living, we speculated that active PSTs secretion of living cells might be an important production pathway of extracellular toxins besides leakage from dead cells. Furthermore, toxin cell quota variation associated with the growth phase was analysed. In the exponential phase, the toxin cell quota first increased and then decreased, with a maximum of 19.02 ± 1.80 fmol/cell at 6 d. However, after entering the stationary phase, this value slowly increased again, suggesting that vigilance should be raised for the plateau of Alexandrium blooms. In addition, cells in the exponential phase mainly produced O-sulfated components such as GTX1&4, cells in the stationary phase mainly produced O-sulfate-free components such as GTX5. In this study, the toxigenic rules of A. pacificum were comprehensively uncovered, which provided theoretical guidance for the prevention and mitigation of A. pacificum blooms.

Uncovering the regulation effect of modified clay on toxin production in Alexandrium pacificum: From physiological insights.

As a common dinoflagellate, Alexandrium pacificum can produce paralytic shellfish toxins (PSTs). It can be removed from water by Polyaluminium chloride modified clay (PAC-MC), but it is unclear whether PAC-MC can prevent PSTs content and toxicity from increasing and whether PAC-MC can stimulate PSTs biosynthesis by A. pacificum. Effect of PAC-MC on PSTs and the physiological mechanism were analysed here. The results showed total PSTs content and toxicity decreased respectively by 34.10 % and 48.59 % in 0.2 g/L PAC-MC group at 12 days compared with control group. And the restriction of total PSTs by PAC-MC was mainly achieved via inhibition of algal cell proliferation, by affecting A. pacificum physiological processes and changing phycosphere microbial community. Meanwhile, single-cell PSTs toxicity did not increase significantly throughout the experiment. Moreover, A. pacificum treated with PAC-MC tended to synthesize sulfated PSTs such as C1&2. Mechanistic analysis showed that PAC-MC induced upregulation of sulfotransferase sxtN (related to PSTs sulfation), and functional prediction of bacterial community also showed significant enrichment of "sulfur relay system" after PAC-MC treatment, which might also promote PSTs sulfation. The results will provide theoretical guidance for the application of PAC-MC to field control of toxic Alexandrium blooms.

Nitrate dynamics and source apportionment on the East China Sea shelf revealed by nitrate stable isotopes and a Bayesian mixing model.

The excess input of nitrate is one of the primary factors triggering nearshore eutrophication. To estimate the source apportionment of nitrate on the East China Sea (ECS) shelf, the nitrogen and oxygen stable isotopes in nitrate (δ15N-NO3- and δ18O-NO3-) collected in winter and late spring 2016 were analyzed alongside essential physical, chemical and biological parameters. The temporal and spatial distributions and characteristic values of nitrate-bearing water masses were presented. Accordingly, the end-member mixing model and Rayleigh model were applied to systematically analyze biogeochemical processes. The biogeochemical processes of nitrate were weak in winter, except in the southern ECS, where assimilation and nitrification probably occurred. In contrast, the biogeochemical processes were intensive in spring. The stable isotopic fractionations of N and O were unified in the whole area, and the ratio between δ18O-NO3- and δ15N-NO3- was 1.81 ± 0.04, which indicated significant assimilation accompanying nitrification in spring. Furthermore, a Bayesian stable isotope mixing model was used to reveal the source contributions of nitrate on the ECS shelf for the first time, demonstrating that the Changjiang Diluted Water and Kuroshio Subsurface Water were always sustained and provided steady nitrate sources for the whole ECS. The nitrate inputs from the Yellow Sea to the northern ECS increased from approximately 30 % in spring to nearly 70 % in winter, while that from the Taiwan Strait Warm Water to the southern ECS decreased from approximately 40 % in spring to zero in winter. Moreover, although the nitrate contributions from nitrification were significantly weak in the middle and northern ECS during winter, they were important over the entire ECS during spring. This study qualitatively and quantitatively improves the understanding of seasonal nitrate control from various sources, and these findings are important for nutrient management and policy making to mitigate nearshore eutrophication.

Hypoxia formation in the East China Sea by decomposed organic matter in the Kuroshio Subsurface Water.

Although internal decomposition of organic matter (OM) in the Kuroshio Subsurface Water (KSSW) is a crucial factor for hypoxia formation in the East China Sea (ECS), the seasonal source and contributions of this OM remain debated. This study applied datasets collected in June and October 2015 to discuss these issues qualitatively and quantitatively. According to the variations in several parameters along the KSSW route, N2 fixation signals related to decomposed OM were apparent in the southern ECS during June, while terrestrial input signals were revealed in the northern ECS during June and most of the ECS during October. The terrestrial input contributed 47% of the decomposed OM near the historic hypoxic area in June, indicating that the terrestrial and marine sources contributed almost equally to the development of ECS hypoxia. These results provide vital information for understanding the mechanism of hypoxia formation driven by eutrophication and oceanic circulation.

Heterogeneity of the sediment oxygen demand and its contribution to the hypoxia off the Changjiang estuary and its adjacent waters.

The severe hypoxia off the Changjiang estuary (CE) has a dual-core structure, and the two hypoxic zones exhibit behavioural, physical and biochemical differences. Currently, few studies have revealed straightforward differences regarding the key biochemical processes between these two hypoxic zones. In this study, the phytoplankton sinking rate (PSR) and sediment oxygen demand (SOD) were measured by field experiments and compared between the two hypoxic regions. PSR and SOD ranged from 0.75-3.34 m day-1 and 5.67-16.19 mmol m-2 day-1, respectively. Interestingly, PSR and SOD were higher in the southern region than in the northern region, implying stronger pelagic-benthic biogeochemical coupling in the southern region. SOD accounted for approximately 44% and 51% of DO net consumption in the northern and southern regions, respectively, from July to August. The southern hypoxic region appeared to exhibit intense DO consumption and fast DO supplementation, while the northern hypoxic region seemed to exhibit slow DO consumption and supplementation.

Main factors dominating the development, formation and dissipation of hypoxia off the Changjiang Estuary (CE) and its adjacent waters, China.

Hypoxia off the Changjiang Estuary (CE) and its adjacent waters is purported to be the most severe in China, attracting considerable concern from both the scientific community and the general public. Currently, continuous observations of dissolved oxygen (DO) levels covering hypoxia from its appearance to disappearance are lacking. In this study, twelve consecutive monthly cruises (from February 2015 to January 2016) were conducted. The consecutive spatiotemporal variations in hypoxia throughout the annual cycle were elucidated in detail, and the responses of annual variations in hypoxia to the different influential factors were explored. Overall, hypoxia experienced a consecutive process of expanding from south to north, then disappearing from north to south. The annual variations in hypoxia were mainly contingent on stratification variations. Among different stages, there was significant heterogeneity in the dominant factors. Specifically, low-DO waters initially appeared from the intrusion of nearshore Kuroshio branch current (NKBC), as NKBC intrusion provided a low-DO background and triggered stratification. Thereafter, stratification was enhanced and gradually expanded northward, which promoted the extension of low-DO areas. The formation of hypoxia was regionally selective, and more intense organic matter decomposition at local regions facilitated the occurrence and discontinuous distribution of hypoxia. Hypoxic zones were observed at the Changjiang bank and Zhejiang coastal region from August (most extensively at 14,800 km2) to October. Thereafter, increased vertical mixing facilitated the dissipation of hypoxia from north to south.

Distribution and key influential factors of dissolved oxygen off the Changjiang River Estuary (CRE) and its adjacent waters in China.

Based on two multidisciplinary investigations conducted in summer and winter 2015, the distribution of dissolved oxygen (DO) and the associated seasonal variations off the Changjiang River Estuary (CRE) were studied. The DO content was high in winter, ranging from 6.81-10.29mg/L, and the distribution was mainly controlled by temperature and salinity. The DO concentration was 1.92-9.67mg/L in summer, and a hypoxic zone (DO<3mg/L) covered 14,800km2, which was mainly controlled by stratification and organic matter decomposition. The hypoxic zone exhibited a "dual-core" structure and the differences in the biochemical and physical processes between the southern and northern regions were compared: the northern region exhibited stronger pycnocline intensity; while larger biomass and higher TOC as well as TN contents were observed in the southern region. Hypoxia in the northern region might be mainly dominated by stratification, while that in the southern region was mainly associated with organic matter decomposition.