Five Karenia species along the Chinese coast: With the description of a new species, Karenia hui sp. nov. (Kareniaceae, Dinophyta).

Dinoflagellates within the genus Karenia are well known for their potential to cause harmful algal blooms and induce detrimental ecological consequences. In this study, five Karenia species, Karenia longicanalis, Karenia papilionacea, Karenia mikimotoi, Karenia selliformis, and a new species, Karenia hui sp. nov., were isolated from Chinese coastal waters. The new species exhibits the typical characteristics of the genus Karenia, including a linear apical groove and butanoyl-oxyfucoxanthin as the major accessory pigment. It is distinguished from the other Karenia species by a wide-open sulcal intrusion onto the epicone, a conical epicone with an apical crest formed by the rim of the apical groove, and a hunchbacked hypocone. It is most closely related to Karenia cristata, with a genetic divergence of 3.16 % (22 bp out of 883 bp of LSU rDNA). Acute toxicity tests indicated that the five Karenia species from China are all toxic to marine medaka Oryzias melastigma. Karenia selliformis and K. hui were very toxic to O. melastigma, resulting in 100 % mortality within 4 h and 24 h, respectively. Further analysis by high-performance liquid chromatography revealed that four species, K. selliformis, K. longicanalis, K. papilionacea and K. mikimotoi were capable of producing Gymnodimine-A (GYM-A). The highest GYM-A content was in K. selliformis (strain HK-43), in which the value was 889 fg/cell. No GYM-A was detected in the new species K. hui, however and its toxin remains unknown. Below we provide a comprehensive report of the morphology, phylogeny, pigment composition, and toxicity profiles of Karenia species along the Chinese coast. These findings contribute new insights for monitoring of Karenia species, with important toxicological and ecological implications.

From micro to mesoscale: Understanding the influence of macroalgal communities on Ostreopsis Schmidt blooms.

Harmful Algal Blooms (HABs) are increasing in temperate areas, and the growth rates of benthic harmful dinoflagellates may be favoured in the context of global climate change. Benthic dinoflagellates, including species belonging to the Ostreopsis Schmidt genus, are known to develop on the surface of macroalgae and different macroalgal morphotypes and communities could host higher or lower cell abundances. The physical structure of the macroalgal substrate at the small scale (cm, microhabitat scale) and the structural complexity of the macroalgal community at the medium scale (few m, mesohabitat scale) could play a relevant role in bloom facilitation: the hypothesis that Ostreopsis species could be associated with macroalgal turfs and shrubs, structurally less complex communities than canopy-forming macroalgae, is especially under discussion and, if confirmed, could link bloom occurrence to regime shifts in temperate ecosystems. The present study, performed in two locations of the Ligurian Sea (Rochambeau, France and Vernazzola, Italy) aimed at understanding marine vegetation's role at the micro and mesohabitat scales in controlling the distribution and abundance of Ostreopsis. The abundance of the microalgal cells was quantified at different spatial scales, from cm to a few m, on different macroalgal species and communities, including artificial substrates, to tease apart the micro and mesohabitat effects. The results obtained show a high spatio-temporal variability, potentially hiding habitat-related patterns. The substrate's preferences diminish when cell abundances are very high, as in the case of Rochambeau, while in presence of moderate cell abundances as in Vernazzola or the first phases of blooms, it is possible to appreciate differences in abundances among substrates (in our study, Dictyota fasciola (Roth) Lamouroux supporting higher abundances). Our results open new research topics such as the study of blooms at a larger scale (macrohabitat) and testing different sampling methods to standardise the cells' abundances independently on the substrate.

Basin scale variability of Ostreopsis spp. blooms provides evidence of effectiveness of an integrated sampling approach.

Ostreopsis spp. blooms have been occurring in the last two decades in the Mediterranean Sea in association with a variety of biotic and abiotic substrata (macroalgae, seagrasses, benthic invertebrates, sand, pebbles and rocks). Cells proliferate attached to the surfaces through mucilaginous trichocysts, which lump together microalgal cells, and can also be found in the plankton and on floating aggregates: such tychoplanktonic behavior makes the quantitative assessment of blooms more difficult than planktonic or benthic ones. Different techniques have been so far applied for quantifying cell abundances of benthic microalgae for research, monitoring and risk assessment purposes. In this context, the Benthic Dinoflagellates Integrator (BEDI), a non-destructive quantification method for benthic dinoflagellate abundances, was developed and tested within the EU ENPI-CBCMED project M3-HABs. This device allows mechanical detachment of cells without collecting the benthic substrate, providing an integrated assessment of both epiphytic and planktonic cells, i.e. of the number of cells potentially made available in the water volume from "resuspension" which could have harmful effects on other organisms (including humans). The present study confirms the effectiveness of the BEDI sampling device across different environments across the Mediterranean Sea and constitutes the first large-scale study of Ostreopsis spp. blooms magnitude in function of different macro- and meso­habitat features across the basin.

Metabarcoding reveals a high diversity and complex eukaryotic microalgal community in coastal waters of the northern Beibu Gulf, China.

Beibu Gulf is an important semi-enclosed bay located in the northwestern South China Sea, and is famous for its high bio-productivity and rich bio-diversity. The fast development along the Beibu Gulf Economical Rim has brought pressure to the environment, and algal blooms occurred frequently in the gulf. In this study, surface water samples and micro-plankton samples (20-200 µm) were collected in the northern Beibu Gulf coast. Diversity and distribution of eukaryotic planktonic microalgae were analyzed by both metabarcoding and microscopic analyses. Metabarcoding revealed much higher diversity and species richness of microalgae than morphological observation, especially for dinoflagellates. Metabarcoding detected 144 microalgal genera in 8 phyla, while microscopy only detected 40 genera in 2 phyla. The two methods revealed different microalgal community structures. Dinoflagellates dominated in microalgal community based on metabarcoding due to their high copies of 18 s rRNA gene, and diatoms dominated under microscopy. Altogether 48 algal bloom and/or toxic species were detected in this study, 34 species by metabarcoding and 19 species by microscopy. Our result suggested a high potential risk of HABs in the Beibu Gulf. Microalgal community in the surface water samples demonstrated significantly higher OTU/species richness, alpha diversity, and abundance than those in the micro-plankton samples, although more HAB taxa were detected by microscopic observations in the micro-plankton samples. Furthermore, nano-sized taxa, such as those in chlorophytes, haptophytes, and chrysophyceans, occurred more abundantly in the surface water samples. This study provided a comprehensive morphological and molecular description of microalgal community in the northern Beibu Gulf.

Temporal Dynamics and Influential Factors of Taste and Odor Compounds in the Eastern Drinking Water Source of Chaohu Lake, China: A Comparative Analysis of Global Freshwaters.

The escalating proliferation of cyanobacteria poses significant taste and odor (T/O) challenges, impacting freshwater ecosystems, public health, and water treatment costs. We examined monthly variations in four T/O compounds from September 2011 to August 2012 in Chaohu Lake's eastern drinking water source (DECL). More importantly, we compared the reported T/O occurrence and the related factors in freshwater bodies worldwide. The assessment of T/O issues indicated a severe and widespread problem, with many cases surpassing odor threshold values. Remarkably, China reported the highest frequency and severity of odor-related problems. A temporal analysis revealed variations in odor occurrences within the same water body across different years, emphasizing the need to consider high values in all seasons for water safety. Globally, T/O issues were widespread, demanding attention to variations within the same water body and across different layers. Algae were crucial contributors to odor compounds, necessitating targeted interventions due to diverse odorant sources and properties. A correlation analysis alone lacked definitive answers, emphasizing the essential role of further validation, such as algae isolation. Nutrients are likely to have influenced the T/O, as GSM and MIB correlated positively with nitrate and ammonia nitrogen in DECL, resulting in proposed control recommendations. This study offers recommendations for freshwater ecosystem management and serves as a foundation for future research and management strategies to address T/O challenges.

Transoceanic ships as a source of alien dinoflagellate invasions of inland freshwater ecosystems.

Ships' ballast water and sediments have long been linked to the global transport and expansion of invasive species and thus have become a hot research topic and administrative challenge in the past decades. The relevant concerns, however, have been mainly about the ocean-to-ocean invasion and sampling practices have been almost exclusively conducted onboard. We examined and compared the dinoflagellate cysts assemblages in 49 sediment samples collected from ballast tanks of international and domestic routes ships, washing basins associated with a ship-repair yard, Jiangyin Port (PS), and the nearby area of Yangtze River (YR) during 2017-2018. A total of 43 dinoflagellates were fully identified to species level by metabarcoding, single-cyst PCR-based sequencing, cyst germination and phylogenetic analyses, including 12 species never reported from waters of China, 14 HABs-causing, 9 toxic, and 10 not strictly marine species. Our metabarcoding and single-cyst sequencing also detected many OTUs and cysts of dinoflagellates that could not be fully identified, indicating ballast tank sediments being a risky repository of currently unrecognizable invasive species. Particularly important, 10 brackish and fresh water species of dinoflagellate cysts (such as Tyrannodinium edax) were detected from the transoceanic ships, indicating these species may function as alien species potentially invading the inland rivers and adjacent lakes if these ships conduct deballast and other practices in fresh waterbodies. Significantly higher numbers of reads and OTUs of dinoflagellates in the ballast tanks and washing basins than that in PS and YR indicate a risk of releasing cysts by ships and the associated ship-repair yards to the surrounding waters. Phylogenetic analyses revealed high intra-species genetic diversity for multiple cyst species from different ballast tanks. Our work provides novel insights into the risk of bio-invasion to fresh waters conveyed in ship's ballast tank sediments and washing basins of shipyards.

Advances in the Early Warning of Shellfish Toxification by Dinophysis acuminata.

In Western Europe, the incidence of DST is likely the highest globally, posing a significant threat with prolonged bans on shellfish harvesting, mainly caused by species of the dinoflagellate genus Dinophysis. Using a time series from 2014 to 2020, our study aimed (i) to determine the concentration of D. acuminata in water at which shellfish toxin levels could surpass the regulatory limit (160 µg OA equiv kg-1) and (ii) to assess the predictability of toxic events for timely mitigation actions, especially concerning potential harvesting bans. The analysis considered factors such as (i) overdispersion in the data, (ii) distinct periods of presence and absence, (iii) the persistence of cells, and (iv) the temporal lag between cells in the water and toxins in shellfish. Four generalized additive models were tested, with the Tweedie (TW-GAM) model showing superior performance (>85%) and lower complexity. The results suggest existing thresholds currently employed (200 and 500 cells L-1) are well-suited for the Portuguese coast, supported by empirical evidence (54-79% accuracy). The developed algorithm allows for thresholds to be tailored on a case-by-case basis, offering flexibility for regional variations.

Does climate change increase the risk of marine toxins? Insights from changing seawater conditions.

Marine toxins produced by marine organisms threaten human health and impose a heavy public health burden on coastal countries. Lately, there has been an emergence of marine toxins in regions that were previously unaffected, and it is believed that climate change may be a significant factor. This paper systematically summarizes the impact of climate change on the risk of marine toxins in terms of changes in seawater conditions. From our findings, climate change can cause ocean warming, acidification, stratification, and sea-level rise. These climatic events can alter the surface temperature, salinity, pH, and nutrient conditions of seawater, which may promote the growth of various algae and bacteria, facilitating the production of marine toxins. On the other hand, climate change may expand the living ranges of marine organisms (such as algae, bacteria, and fish), thereby exacerbating the production and spread of marine toxins. In addition, the sources, distribution, and toxicity of ciguatoxin, tetrodotoxin, cyclic imines, and microcystin were described to improve public awareness of these emerging marine toxins. Looking ahead, developing interdisciplinary cooperation, strengthening monitoring of emerging marine toxins, and exploring more novel approaches are essential to better address the risks of marine toxins posed by climate change. Altogether, the interrelationships between climate, marine ecology, and marine toxins were analyzed in this study, providing a theoretical basis for preventing and managing future health risks from marine toxins.

An On-Farm Workflow for Predictive Management of Paralytic Shellfish Toxin-Producing Harmful Algal Blooms for the Aquaculture Industry.

Paralytic shellfish toxins (PSTs) produced by marine dinoflagellates significantly impact shellfish industries worldwide. Early detection on-farm and with minimal training would allow additional time for management decisions to minimize economic losses. Here, we describe and test a standardized workflow based on the detection of sxtA4, an initial gene in the biosynthesis of PSTs. The workflow is simple and inexpensive and does not require a specialized laboratory. It consists of (1) water collection and filtration using a custom gravity sampler, (2) buffer selection for sample preservation and cell lysis for DNA, and (3) an assay based on a region of sxtA, DinoDtec lyophilized quantitative polymerase chain reaction (qPCR) assay. Water samples spiked with Alexandrium catenella showed a cell recovery of >90% when compared to light microscopy counts. The performance of the lysis method (90.3% efficient), Longmire's buffer, and the DinoDtec qPCR assay (tested across a range of Alexandrium species (90.7-106.9% efficiency; r2 > 0.99)) was found to be specific, sensitive, and efficient. We tested the application of this workflow weekly from May 2016 to 30th October 2017 to compare the relationship between sxtA4 copies L-1 in seawater and PSTs in mussel tissue (Mytilus galloprovincialis) on-farm and spatially (across multiple sites), effectively demonstrating an ∼2 week early warning of two A. catenella HABs (r = 0.95). Our tool provides an early, accurate, and efficient method for the identification of PST risk in shellfish aquaculture.

What makes a cyanobacterial bloom disappear? A review of the abiotic and biotic cyanobacterial bloom loss factors.

Cyanobacterial blooms present substantial challenges to managers and threaten ecological and public health. Although the majority of cyanobacterial bloom research and management focuses on factors that control bloom initiation, duration, toxicity, and geographical extent, relatively little research focuses on the role of loss processes in blooms and how these processes are regulated. Here, we define a loss process in terms of population dynamics as any process that removes cells from a population, thereby decelerating or reducing the development and extent of blooms. We review abiotic (e.g., hydraulic flushing and oxidative stress/UV light) and biotic factors (e.g., allelopathic compounds, infections, grazing, and resting cells/programmed cell death) known to govern bloom loss. We found that the dominant loss processes depend on several system specific factors including cyanobacterial genera-specific traits, in situ physicochemical conditions, and the microbial, phytoplankton, and consumer community composition. We also address loss processes in the context of bloom management and discuss perspectives and challenges in predicting how a changing climate may directly and indirectly affect loss processes on blooms. A deeper understanding of bloom loss processes and their underlying mechanisms may help to mitigate the negative consequences of cyanobacterial blooms and improve current management strategies.

An Intelligent Early Warning System for Harmful Algal Blooms: Harnessing the Power of Big Data and Deep Learning.

Harmful algal blooms (HABs) pose a significant ecological threat and economic detriment to freshwater environments. In order to develop an intelligent early warning system for HABs, big data and deep learning models were harnessed in this study. Data collection was achieved utilizing the vertical aquatic monitoring system (VAMS). Subsequently, the analysis and stratification of the vertical aquatic layer were conducted employing the "DeepDPM-Spectral Clustering" method. This approach drastically reduced the number of predictive models and enhanced the adaptability of the system. The Bloomformer-2 model was developed to conduct both single-step and multistep predictions of Chl-a, integrating the " Alert Level Framework" issued by the World Health Organization to accomplish early warning for HABs. The case study conducted in Taihu Lake revealed that during the winter of 2018, the water column could be partitioned into four clusters (Groups W1-W4), while in the summer of 2019, the water column could be partitioned into five clusters (Groups S1-S5). Moreover, in a subsequent predictive task, Bloomformer-2 exhibited superiority in performance across all clusters for both the winter of 2018 and the summer of 2019 (MAE: 0.175-0.394, MSE: 0.042-0.305, and MAPE: 0.228-2.279 for single-step prediction; MAE: 0.184-0.505, MSE: 0.101-0.378, and MAPE: 0.243-4.011 for multistep prediction). The prediction for the 3 days indicated that Group W1 was in a Level I alert state at all times. Conversely, Group S1 was mainly under an Level I alert, with seven specific time points escalating to a Level II alert. Furthermore, the end-to-end architecture of this system, coupled with the automation of its various processes, minimized human intervention, endowing it with intelligent characteristics. This research highlights the transformative potential of integrating big data and artificial intelligence in environmental management and emphasizes the importance of model interpretability in machine learning applications.

Sporadic diurnal fluctuations of cyanobacterial populations in oligotrophic temperate systems can prevent accurate characterization of change and risk in aquatic systems.

Cyanobacteria increasingly threaten recreational water use and drinking water resources globally. They require dynamic monitoring to account for variability in their distribution arising from diel cycles associated with oscillatory vertical migration. While this has been discussed in marine and eutrophic freshwater contexts, reports of diurnal vertical migration of cyanobacteria in oligotrophic freshwater lakes are scant. Typical monitoring protocols do not reflect these dynamics and frequently focus only on surface water sampling approaches, and either ignore sampling time or recommend large midday timeframes (e.g., 10AM-3PM), thereby preventing accurate characterization of cyanobacterial community dynamics. To evaluate the impact of diurnal migrations and water column stratification on cyanobacterial abundance and composition, communities were characterized in a shallow well-mixed lake interconnected to a thermally stratified lake in the Turkey Lakes Watershed (Ontario, Canada) using amplicon sequencing of the 16S rRNA gene across a multi-time point sampling series in 2018 and 2022. This work showed that cyanobacteria are present in oligotrophic lakes and their community structure varies (i) diurnally, (ii) across the depth of the water column, (iii) interannually within the same lake and (iv) between different lakes that are closely interconnected within the same watershed. It underscored the need for integrating multi-timepoint, multi-depth discrete sampling guidance into lake and reservoir monitoring programs to describe cyanobacteria community dynamics and signal change to inform risk management associated with the potential for cyanotoxin production. Ignoring variability in cyanobacterial community dynamics (such as that reported herein) and reducing sample numbers can lead to a false sense of security and missed opportunities to identify and mitigate changes in trophic status and associated risks such as toxin or taste and odor production, especially in sensitive, oligotrophic systems.

Multispecies mass mortality in the Beagle Channel associated with paralytic shellfish toxins.

The Beagle Channel is a Subantarctic semi-estuarine environment at the southern tip of South America, where intoxication events associated with harmful algal blooms have been reported since 1886, including a world record in toxicity due to Alexandrium catenella in 1992. Toxic algae affect public health and ecosystem services, particularly mussel aquaculture and fisheries management. During the austral summer of 2022, an intense bloom of A. catenella (5 × 104 cells L-1) occurred in the Beagle Channel, leading to the second most toxic event in the area, with mussel toxicity reaching 197,266 µg STXeq kg-1. This event was synchronous with the mortality of marine organisms from different trophic levels and terrestrial fauna, i.e., two Fuegian red foxes and a southern caracara. Stomach content and liver samples from dead kelp gulls (Larus dominicanus), Magellanic penguins (Spheniscus magellanicus), papua penguins (Pygoscelis papua), and imperial cormorants (Leucocarbo atriceps), presented variable paralytic shellfish toxins (PST) levels (up to 3427 µg STXeq kg-1) as measured by high performance liquid chromatography (HPLC), suggesting that deaths were associated with high PST toxicity level. The different toxin profiles found in phytoplankton, zooplankton, squat lobsters (Grimothea gregaria), Fuegian sprat (Sprattus fuegensis), and seabirds evidenced possible toxin transformation along the food web and the possible transfer vectors. The unexpected detection of PST in terrestrial fauna (up to 2707 µg STXeq kg-1) suggested intoxication by scavenging on squat lobsters, which had high toxicity (26,663 µg STXeq kg-1). PST trace levels were also detected in a liver sample of a dead false killer whale (Pseudorca crassidens), an oceanic odontocete stranded on the coast during the bloom. Overall, our results denote the exceptional nature of the toxic, multispecies mortality event and that toxins may propagate to several levels of the food web in this Subantarctic environment.

Panoptic elucidation of algicidal mechanism of Raoultella sp. S1 against the Microcystis aeruginosa by TMT quantitative proteomics.

Harmful algal blooms (HABs) due to eutrophication are becoming a serious ecological disaster worldwide, threatening human health and the optimal balance of aquatic ecosystems. The traditional approaches to eradicate HABs yield several drawbacks in practical application, while microbial algicidal technology is garnering mounting recognition due to its high efficiency, eco-friendliness, and low cost. In our previous study, we isolated a bacterium strain Raoultella sp. S1 from eutrophic water with high efficiency of algicidal properties. This study further investigated the flocculation and inactivation efficiency of S1 on Microcystis aeruginosa at different eutrophic stages by customizing the algal cell densities. The supernatant extract of S1 strain exhibited remarkable flocculation and inactivation effects against low (1 × 106 cell/mL)and medium (2.7 × 106 cell/mL)concentrations of algal cells, but unexceptional for higher densities. The results further revealed that algal cells at low and medium counts manifested a more apparent antioxidant defense response, while the photosynthetic efficiency and relative electron transport rate were considerably reduced within 24 h. TEM observations confirmed the disruption of thylakoid membranes and cell structure of algal cells by algicidal substances. Moreover, TMT proteomics revealed alterations in protein metabolic pathways of algal cells during the flocculation and lysis stages at the molecular biological level. This signified that the disruption of the photosynthetic system is the core algicidal mechanism of S1 supernatant. In contrast, the photosynthetic metabolic pathways in the HABs were significantly upregulated, increasing the energy supply for the NADPH dehydrogenation process and the upregulation of ATPases in oxidative phosphorylation. Insufficient energy provided by NADPH resulted in a dwindled electron transport rate, stagnation of carbon fixation in dark reactions, and blockage of light energy conversion into chemical energy. Nonetheless, carbohydrate metabolism (gluconeogenesis and glycolysis) proteins were down-regulated and hampered DNA replication and repair. This study aided in unveiling the bacterial management of eutrophication by Raoultella sp. S1 and further arrayed the proteomic mechanism of algal apoptosis.

Did algal toxin and Klebsiella infections cause the unexplained 2007 mass mortality event in Danish and Swedish marine mammals?

An unusual mass mortality event (MME) of harbour seals (Phoca vitulina) and harbour porpoises (Phocoena phocoena) occurred in Denmark and Sweden in June 2007. Prior to this incident, the region had experienced two MMEs in harbour seals caused by Phocine Distemper Virus (PDV) in 1988 and 2002. Although epidemiology and symptoms of the 2007 MME resembled PDV, none of the animals examined for PDV tested positive. Thus, it has been speculated that another - yet unknown - pathogen caused the June 2007 MME. To shed new light on the likely cause of death, we combine previously unpublished veterinary examinations of harbour seals with novel analyses of algal toxins and algal monitoring data. All harbour seals subject to pathological examination showed pneumonia, but were negative for PDV, influenza and coronavirus. Histological analyses revealed septicaemia in multiple animals, and six animals tested positive for Klebsiella pneumonia. Furthermore, we detected the algal Dinophysis toxin DTX-1b (1-115 ng g-1) in five seals subject to toxicology, representing the first time DTX-1b has been detected in marine vertebrates. However, no animals tested positive for both Klebsiella and toxins. Thus, while our relatively small sample size prevent firm conclusions on causative agents, we speculate that the unexplained MME may have been caused by a chance incidence of multiple pathogens acting in parallel in June 2007, including Dinophysis toxin and Klebsiella. Our study illustrates the complexity of wildlife MMEs and highlights the need for thorough sampling during and after MMEs, as well as additional research on and monitoring of DTX-1b and other algal toxins in the region.

Chattonella marina blooms in a trophic gradient system: Interaction with environmental drivers.

For coastal eutrophication, lots of studies focused on the influence from environmental factors, especially nitrogen and phosphorus, on algae blooms. The interaction between algae and environmental factors has been often ignored. Using Chattonella marina, a dominant species in marine algal blooms, we established a trophic gradient system that simulated C. marina blooms at three trophic levels: eutrophic, mesotrophic, and oligotrophic, and examined the life history patterns of C. marina and the interactions with environmental factors. Increased trophic levels influenced the growth potential of C. marina, while its unique cyst reproduction allowed it to thrive in nutrient-limited environments. Adequate nutrients caused changes in dissolved oxygen (DO) and pH led by C. marina, with the carbonate system playing a crucial role in regulating pH under nutrient-limited conditions. Limiting the growth of C. marina in areas with low nutrient by manipulating reactive silicate (SiO32-) availability may prove effective. Nitrate (NO3-) was the preferred nutrient for C. marina when its concentration exceeded that of ammonium (NH4+). Phosphorus played a crucial role in the growth and proliferation of C. marina, especially when other nutrients were scarce. The findings of this study may provide valuable insights into the effective management and prevention of algae blooms.

Pilot-Scale Fermentation of Pseudoalteromonas sp. Strain FDHY-MZ2: An Effective Strategy for Increasing Algicidal Activity.

The role of microorganisms in effectively terminating harmful algal blooms (HABs) is crucial for maintaining environmental stability. Recent studies have placed increased emphasis on bio-agents capable of inhibiting HABs. The bacterium Pseudoalteromonas sp. strain FDHY-MZ2 has exhibited impressive algicidal abilities against Karenia mikimotoi, a notorious global HAB-forming species. To augment this capability, cultures were progressively scaled from shake flask conditions to small-scale (5 L) and pilot-scale (50 L) fermentation. By employing a specifically tailored culture medium (2216E basal medium with 1.5% soluble starch and 0.5% peptone), under precise conditions (66 h, 20 °C, 450 rpm, 30 L/min ventilation, 3% seeding, and constant starch flow), a notable increase in algicidal bacterial biomass was observed; the bacterial dosage required to entirely wipe out K. mikimotoi within a day decreased from 1% to 0.025%. Compared to an unoptimized shake flask group, the optimized fermentation culture caused significant reductions in algal chlorophyll and protein levels (21.85% and 78.3%, respectively). Co-culturing induced increases in algal malondialdehyde and H2O2 by 5.98 and 5.38 times, respectively, leading to further disruption of algal photosynthesis. This study underscores the unexplored potential of systematically utilized microbial agents in mitigating HABs, providing a pathway for their wider application.

Nitrogen liberated via allelopathy can promote harmful algal blooms.

Allelopathy is a biological mechanism that can promote harmful algal blooms (HAB) via the inhibition of sympatric phytoplankton. While nutrient loading can also promote HABs, the ability of allelopathy to stimulate HABs via the regeneration of nutrients has yet to be explored. To examine the impacts of allelopathically liberated N on HAB species, a series of experiments were performed using multiple allelopathic HAB species including the dinoflagellates Alexandrium catenella and Margalefidinium polykrikoides, and the pelagophyte, Aureoumbra lagunensis. These HAB species were paired with the cosmopolitan dinoflagellate, Akashiwo sanguinea, that was labeled with 15NO3- or 15NH4+, allowing the release and transfer of N to be traced as a time course during allelopathic interactions. During all experiments, the allelopathic inhibition of Akashiwo was accompanied by increases in cell densities, growth rates, and the δ15N content of the HAB species, evidencing the transfer of N liberated from Akashiwo. The cellular transfer of 15N and release of dissolved N was higher when Akashiwo was grown with 15NO3- compared to 15NH4+ suggesting a differential subcellular-compartmentalization of N sources. Regardless of the type of N, HAB species obtained 60 - 100% of their cellular N from lysed Akashiwo cells and there was an enrichment of the δ15N content of the dissolved NH4+ pool post-lysis of Akashiwo. Collectively, the results demonstrate that beyond facilitating species succession, allelopathy can supply HABs with N and, therefore, is likely important for promoting and sustaining HABs. Given that allelopathy is known to be a dose-dependent process, allelopathy may induce a positive feedback loop, whereby competitors are lysed, N is liberated, HABs are intensified and, in turn, become more strongly allelopathic.

The relationship between salmon (Salmo salar) farming and cell abundance of harmful algal taxa.

The effects of nutrient effluents from fish cage aquaculture are an important eutrophication concern. It has been proposed that marine fish farm derived nutrients have the potential to increase phytoplankton abundance and lead to intensification of Harmful Algal Blooms (HABs), and that these blooms may negatively impact both the finfish and the shellfish industry. This study addressed this hypothesis using farmed salmon biomass in Scottish marine waters (as a proxy for nutrient load added to the water column as a consequence of fish farming) cell abundance of HAB taxa that most frequently impact shellfish farms and human health in the region (Dinophysis spp., Alexandrium spp. and Pseudo-nitzschia spp.), and cell abundance of one phytoplankton species of particular concern to the salmon farming industry (Karenia mikimotoi). Data from a 15-year weekly HAB monitoring programme and parallel national monitoring data relating to salmon farm stocking biomass were summarised in 5 km per 5 km aggregation boxes. Linear regression models were used to assess (i) inter-annual variation in cell abundance and total annual farmed salmon biomass; (ii) intra-annual (monthly) variation in harmful phytoplankton cell abundance and salmon biomass; (iii) a further analysis included seasonal effects within the intra-annual analysis. Farmed salmon biomass alone had a non-significant effect on cell abundance of any of the studied phytoplankton taxa. In contrast, a significant effect on cell abundance was found when using location, month or season as the predictive variable. Despite the non-significant impact of fish biomass on phytoplankton counts, the relationship varied seasonally, with a different response of Dinophysis spp. indicating a taxa specific interaction. A possible explanation for the lack of a significant relationship between farmed salmon and harmful phytoplankton cell abundance is that aquaculture farms are generally located in hydrodynamically energetic locations where recurrent flushing likely allows efficient dilution of nutrients. Overall, the analyses suggest that current levels of salmon farming activities do not markedly impact the abundance of routinely monitored biotoxin producing or fish killing phytoplankton taxa in Scottish waters.

Operational modelling to assess advective harmful algal bloom development and its potential to impact aquaculture.

A particle tracking model is described and used to explore the role of advection as the source of harmful algal blooms that impact the Shetland Islands, where much of Scotland's aquaculture is located. The movement of particles, representing algal cells, was modelled using surface velocities obtained from the 1.5 km resolution Atlantic Margin Model AMM15. Following validation of model performance against drifter tracks, the model results recreate previously hypothesised onshore advection of harmful algal cells from west of the archipelago during 2006 and 2013, when exceptional Dinophysis spp. abundances were measured at Shetland aquaculture sites. Higher eastward advection of Dinophysis spp. cells was also suggested during 2018. Wind roses explain this higher eastward advection during 2006, 2013 and 2018. The study suggests that the European Slope Current is important for the transport of harmful algal blooms, particularly those composed of dinoflagellates.