Development of a new kappa-carrageenan hydrogel system to study benthic diatom vertical movements.

Benthic diatom vertical movement has been investigated mainly through indirect measurements based on chlorophyll a fluorescence and spectral reflectance signals. The presence of sediment hinders direct imaging and grazers activity renders the work under controlled conditions very difficult. This study provides a tool to study diatoms movement in a 3D hydrogel matrix. Synthetic and natural hydrogels were tested to find the best 3D transparent scaffold where diatoms could grow and freely move in all directions. Polyamidoamines (PAAm) hydrogels were no-cytocompatible and hyaluronic acid (HA) only allowed diatoms to survive for 2-days. Natural hydrogels made of gelatin/Na-alginate, Na-alginate and kappa-carrageenan (KC) were cytocompatible, with KC showing the best properties for diatom growth and movement on a long term (up to 2 months). Comparing Nitzschia spathulata, Gyrosigma limosum and Navicula phyllepta growth in liquid media vs in KC gels, we found that diatoms reached a significantly higher final biomass in the hydrogel condition. Hydrogels were also useful to isolate large size diatom species e.g., Nitzschia elongata, that did not survive in suspension. Finally, we showed three ways to study diatom species-specific movement in KC hydrogels: 1) controlled species mix; 2) natural diatom assemblages with grazers; and 3) natural diatom assemblages without grazers. With our system, single diatoms could be imaged, identified, and counted. In addition, different stimuli, e.g., light intensity and light composition can be applied and their effects on movement and physiology studied without being masked by sediment or impaired by meiofauna.

Temperature-dependent toxicity of fluoxetine alters the thermal plasticity of marine diatoms.

Anthropogenic activities have led to the emergence of pharmaceutical pollution in marine ecosystems, posing a significant threat to biodiversity in conjunction with global climate change. While the ecotoxicity of human drugs on aquatic organisms is increasingly recognized, their interactions with environmental factors, such as temperature, remain understudied. This research investigates the physiological effects of the selective serotonin reuptake inhibitor (SSRI), fluoxetine, on two diatom species, Phaeodactylum tricornutum and Thalassiosira weissflogii. Results demonstrate that fluoxetine significantly reduces growth rate and biomass production, concurrently affecting pigment contents and the thermal performance curve (TPC) of the diatoms. Fluoxetine reduces the synthesis of chlorophyll a (Chl a) and carotenoid (Car), indicating inhibition of photosynthesis and photoprotection. Furthermore, fluoxetine decreases the maximum growth rate (µmax) while increasing the optimum temperature (Topt) in both species, suggesting an altered thermal plasticity. This shift is attributed to the observed decrease in the inhibition rate of fluoxetine with rising temperatures. These findings emphasize the physiological impacts and ecological implications of fluoxetine on phytoplankton and underscore the significance of considering interactions between multiple environmental drivers when accessing the ecotoxicity of potential pollutants. The present study provides insights into crucial considerations for evaluating the impacts of pharmaceutical pollution on marine primary producers.

Multi-trait diversification in marine diatoms in constant and warmed environments.

Phytoplankton are photosynthetic marine microbes that affect food webs, nutrient cycles and climate regulation. Their roles are determined by correlated phytoplankton functional traits including cell size, chlorophyll content and cellular composition. Here, we explore patterns of evolution in interrelated trait values and correlations. Because both chance events and natural selection contribute to phytoplankton trait evolution, we used population bottlenecks to diversify six genotypes of Thalassiosirid diatoms. We then evolved them as large populations in two environments. Interspecific variation and within-species evolution were visualized for nine traits and their correlations using reduced axes (a trait-scape). Our main findings are that shifts in trait values resulted in movement of evolving populations within the trait-scape in both environments, but were more frequent when large populations evolved in a novel environment. Which trait relationships evolved was population-specific, but greater departures from ancestral trait correlations were associated with lower population growth rates. There was no single master trait that could be used to understand multi-trait evolution. Instead, repeatable multi-trait evolution occurred along a major axis of variation defined by several diatom traits and trait relationships. Because trait-scapes capture changes in trait relationships and values together, they offer an insightful way to study multi-trait variation.

Long-term response of interspecific competition among three typical bloom-forming species to changes in phosphorus and temperature.

Phosphorus and temperature play an important role in the succession of diatom-dinoflagellate blooms. However, there is little long-term research on interspecific competition based on phosphorus source and temperature. Here, interspecific competition among Skeletonema costatum, Prorocentrum donghaiense and Karenia mikimotoi was studied using trialgal laboratory co-cultures under different phosphorus and temperature conditions. These results suggest that S. costatum and P. donghaiense alternated as competing dominant species during the experimental period, which coincides with the different phosphorus conditions. However, K. mikimotoi growth was significantly inhibited throughout the experiment. We suggest that this may be due to different algal requirements for phosphorus, optimal growth temperatures, and possible allelopathic effects. This study provides a comprehensive mechanism of interspecific competition between diatom-dinoflagellate in response to phosphorus and temperature and elucidates the seasonal succession of diatom-dinoflagellate from late spring to early summer in the Changjiang River Estuary and the adjacent East China Sea.

The tolerance of two marine diatoms to diurnal pH fluctuation under dynamic light condition and ocean acidification scenario.

Coastal waters undergo dynamic changes in seawater carbonate chemistry due to natural and anthropogenic factors. Despite this, our current understanding of how coastal phytoplankton respond to fluctuating pH is limited. In the present study, we investigated the physiological responses of two coastal diatoms Thalassiosira pseudonana and Thalassiosira weissflogii to seawater acidification and diurnally fluctuating pH under natural solar irradiance. Seawater acidification did not significantly impact the growth, maximum and effective quantum yield of PSII, and photosynthetic rates of the two species. However, it did increase the maximum relative electron transport rate of T. weissflogii by 11%. Overall, fluctuating pH had neutral or positive effects on both species. It enhanced the light-saturated photosynthetic rate of T. weissflogii by 20% compared to cells grown under seawater acidification condition. Results from the short-term pH exposure experiment revealed that the photosynthetic rates of both species remained unaffected by acute pH changes, indicating their tolerance to varying pH. Nevertheless, it is crucial to consider dynamic pH when predicting changes in primary production in coastal waters, given the interplay of various environmental drivers.

Multiple-pathways light modulation in Pleurosigma strigosum bi-raphid diatom.

Ordered, quasi-ordered, and even disordered nanostructures can be identified as constituent components of several protists, plants and animals, making possible an efficient manipulation of light for intra- and inter- species communication, camouflage, or for the enhancement of primary production. Diatoms are ubiquitous unicellular microalgae inhabiting all the aquatic environments on Earth. They developed, through tens of millions of years of evolution, ultrastructured silica cell walls, the frustules, able to handle optical radiation through multiple diffractive, refractive, and wave-guiding processes, possibly at the basis of their high photosynthetic efficiency. In this study, we employed a range of imaging, spectroscopic and numerical techniques (including transmission imaging, digital holography, photoluminescence spectroscopy, and numerical simulations based on wide-angle beam propagation method) to identify and describe different mechanisms by which Pleurosigma strigosum frustules can modulate optical radiation of different spectral content. Finally, we correlated the optical response of the frustule to the interaction with light in living, individual cells within their aquatic environment following various irradiation treatments. The obtained results demonstrate the favorable transmission of photosynthetic active radiation inside the cell compared to potentially detrimental ultraviolet radiation.

Cu transport and complexation by the marine diatom Phaeodactylum tricornutum: Implications for trace metal complexation kinetics in the surface ocean.

Elucidating whether dissolved Cu uptake is kinetically or thermodynamically controlled, and the effects of speciation on Cu transport by phytoplankton will allow better modeling of the fate and impact of dissolved Cu in the ocean. To address these questions, we performed Cu physiological and physicochemical experiments using the model diatom, Phaeodactylum tricornutum, grown in natural North Atlantic seawater (0.44 nM Cu). Using competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV), we measured two organic ligand types released by P. tricornutum to bind Cu (L1 and L2) at concentrations of ~0.35 nM L1 and 1.3 nM L2. We also established the presence of two putative Cu-binding sites at the cell surface of P. tricornutum (S1 and S2) with log K differing by ~5 orders of magnitude (i.e., 12.9 vs. 8.1) and cell surface densities by 9-fold. Only the high-affinity binding sites, S1, exhibit reductase activity. Using voltammetric kinetic measurements and a theoretical kinetic model, we calculated the forward and dissociation rate constants of L1 and S1. Complementary 67Cu uptake experiments identified a high- and a low-affinity Cu uptake system in P. tricornutum, with half-saturation constant (Km) of 154 nM and 2.63 µM dissolved Cu, respectively. In the P. tricornutum genome, we identified a putative high-affinity Cu transporter (PtCTR49224) and a putative ZIP-like, low-affinity Cu transporter (PtZIP49400). PtCTR49224 has high homology to Homo sapiens hCTR1, which depending on the accessibility to extracellular reducing agents, the hCTR1 itself is involved in the reduction of Cu2+ to Cu+ before internalization. We combined these physiological and physicochemical data to calculate the rate constants for the internalization of Cu, and established that while the high-affinity Cu uptake system (S1) is borderline between a kinetically or thermodynamically controlled system, the low-affinity Cu transporters, S2, is thermodynamically-controlled. We revised the inverse relationship between the concentrations of inorganic complexes of essential metals (i.e., Ni, Fe, Co, Zn, Cd, Mn and Cu) in the mixed layer and the formation rate constant of metal transporters in phytoplankton, highlighting the link between the chemical properties of phytoplankton metal transporters and the availability and speciation of trace metals in the surface ocean.

Phytoplankton assemblage responses to nitrogen following COVID-19 stay-in-place orders in western Long Island Sound (New York/Connecticut).

This study evaluated water quality, nitrogen (N), and phytoplankton assemblage linkages along the western Long Island Sound (USA) shoreline (Nov. 2020-Dec. 2021) following COVID-19 stay-in-place (SIP) orders through monthly surveys and N-addition bioassays. Ammonia-N (AmN; NH3+NH4+) negatively correlated with total chlorophyll-a (chl-a) at all sites; this was significant at Alley Creek, adjacent to urban wastewater inputs, and at Calf Pasture, by the Norwalk River (Spearman rank correlation, p < 0.01 and 0.02). Diatoms were abundant throughout the study, though dinoflagellates (Heterocapsa, Prorocentrum), euglenoids/cryptophytes, and both nano- and picoplankton biomass increased during summer. In field and experimental assessments, high nitrite + nitrate (N + N) and low AmN increased diatom abundances while AmN was positively linked to cryptophyte concentrations. Likely N + N decreases with presumably minimal changes in AmN and organic N during COVID-19 SIP resulted in phytoplankton assemblage shifts (decreased diatoms, increased euglenoids/cryptophytes), highlighting the ecological impacts of N-form delivered by wastewater to urban estuaries.

Spring protistan communities in response to warming in the northeastern East China Sea.

The northeastern East China Sea is a highly dynamic marine ecosystem influenced by seasonally varying water mass properties. However, despite being among the world's fastest-warming ocean, there has been limited investigation into the impacts of warming on protistan communities. We collected seawater from two stations (E42 and E46) with different natural protist communities and environmental attributes to investigate the acclimation of the two communities to artificially elevated temperatures (ambient T, +2, and +4 °C). Nutrient and Chl-a conditions reflected oceanographic differences, providing insights into protistan community dynamics. Notably, small-sized autotrophic protists prevailed in the phosphate-deficient E42 community, with mid-incubation heterotrophic conversions. Higher temperatures exacerbated the effects of the P deficiency on the E42 community. While the proportions of Bacillariophyta increased only in the nutrient-balanced E46 communities, those of mixotrophic dinoflagellates increased with elevated temperature, regardless of P deficiency, suggesting that mixotrophy likely aids adaptation in changing marine environments. In summary, the findings of this microcosm study illuminate the potential modulation of spring protistan communities in the northeastern East China Sea under anticipated future warming.

Unveiling the evolution of phytoplankton communities: Decades-long insights into the southern Yellow Sea, China (1959-2023).

We obtained historical and observational data on phytoplankton communities from 1959 to 2023 to explore the responses of the phytoplankton community structure to long-term environmental changes in the southern Yellow Sea (SYS), China. The results revealed a decrease in the proportions of diatom cell abundance within the phytoplankton community by 8 %, accompanied by a corresponding increase in that of dinoflagellates. Dominant phytoplankton species were mainly chain-forming diatoms before 2000, and large dinoflagellate species from the genera Tripos and Noctiluca increased their dominance after 2000. Warm-water phytoplankton species have increased in dominance over the study period. Correlation analysis revealed that the ocean warming and alterations in nutrient structure (N/P and Si/N ratios) were mostly responsible for the long-term evolution trend, and these changes may result in an increase in dinoflagellate harmful algal blooms, reduced efficiency of the biological carbon pump, and heightened hypoxia in the future, which should draw our attention.

Short-term responses of temperate and subarctic marine diatoms to Irgarol 1051 and UV radiation: Insights into temperature interactions.

Phytoplankton face numerous pressures resulting from chemical and physical stressors, primarily induced by human activities. This study focuses on investigating the interactive effects of widely used antifouling agent Irgarol 1051 and UV radiation on the photo-physiology of marine diatoms from diverse latitudes, within the context of global warming. Our findings clearly shown that both Irgarol and UV radiation have a significant inhibitory impact on the photochemical performance of the three diatoms examined, with Irgarol treatment exhibiting more pronounced effects. In the case of the two temperate zone diatoms, we observed a decrease in the inhibition induced by Irgarol 1051 and UVR as the temperature increased up to 25°C. Similarly, for the subarctic species, an increase in temperature resulted in a reduction in the inhibition caused by Irgarol and UVR. These results suggest that elevated temperatures can mitigate the short-term inhibitory effects of both Irgarol and UVR on diatoms. Furthermore, our data indicate that increased temperature could significantly interact with UVR or Irgarol for temperate diatoms, while this was not the case for cold water diatoms, indicating temperate and subarctic diatoms may respond differentially under global warming.

The Photoprotective Behavior of a Motile Benthic Diatom as Elucidated from the Interplay Between Cell Motility and Physiological Responses to a Light Microgradient Using a Novel Experimental Setup.

It has long been hypothesized that benthic motile pennate diatoms use phototaxis to optimize photosynthesis and minimize photoinhibitory damage by adjusting their position within vertical light gradients in coastal benthic sediments. However, experimental evidence to test this hypothesis remains inconclusive, mainly due to methodological difficulties in studying cell behavior and photosynthesis over realistic spatial microscale gradients of irradiance and cell position. In this study, a novel experimental approach was developed and used to test the hypothesis of photosynthesis optimization through motility, based on the combination of single-cell in vivo chlorophyll fluorometry and microfluidic chips. The approach allows the concurrent study of behavior and photosynthetic activity of individual cells of the epipelic diatom species Craspedostauros britannicus exposed to a light microgradient of realistic dimensions, simulating the irradiance and distance scales of light microgradients in benthic sediments. Following exposure to light, (i) cells explored their light environment before initiating light-directed motility; (ii) cells used motility to lower their light dose, when exposed to the highest light intensities; and (iii) motility was combined with reversible non-photochemical quenching, to allow cells to avoid photoinhibition. The results of this proof-of-concept study not only strongly support the photoprotective nature of photobehavior in the studied species but also revealed considerable variability in how individual cells reacted to a light microgradient. The experimental setup can be readily applied to study motility and photosynthetic light responses of other diatom species or natural assemblages, as well as other photoautotrophic motile microorganisms, broadening the toolset for experimental microbial ecology research.

Effect of increased pCO2 and temperature on the phytoplankton community in the coastal of Yellow Sea.

In order to study the dynamics of marine phytoplankton communities in response to anticipated in temperature and CO2, a shipboard continuous culture experiment (Ecostat) was conducted. The experiment involved simulations under current atmospheric CO2 concentrations (400 ppm) and projected year-2100 CO2 levels (1000 ppm), as well as varying temperature under present (22 °C) versus increased temperature (26 °C) in the Yellow Sea during the summer of 2020. The results showed that both the increased pCO2 and temperature had significant effects on microphytoplankton and picophytoplankton, with the warming effect proving to be more significant. The different responses of various species to acidification and warming and their coupling effect led to the changes in microphytoplankton and picophytoplankton community structure. Elevated temperature and greenhouse treatments promoted the growth of dominant diatoms and Synechococcus, such as Guinardia flaccida and Pseudo-nitzschia delicatissima. This phenomenons widened the ecological niche, and the changes in the growth patterns of dominant species consequently influenced the content of cellular elements. Mantel's analysis further demonstrated that both warming and greenhouse promoted the growth of diatoms and Synechococcus. Projections of marine phytoplankton community trends by the end of the century based on Growth Rate Ratio (GRR), indicated that not only would species with GRR < 1 decrease, but also numerous species with growth rates >1 at elevated pCO2 levels would be ousted from competition. This experiment demonstrates the need to investigate whether extended exposure to increased pCO2 and temperature over more extended time scales would similarly induce shifts in the biological and biogeochemical dynamics of the Yellow Sea.

Phosphorus forms and zinc concentrations affect the physiological ecology and sinking rate of Thalassiosira weissflogii.

The combined effects of phosphorus (P) forms and zinc (Zn) concentrations on diatom silicification remain unclear. In this study, we investigate the effects of different Zn concentrations on the growth, cellular silicon content and sinking rate of Thalassiosira weissflogii under different P forms. The results showed that under the dissolved inorganic phosphorus (DIP) treatments, the specific growth rate of T. weissflogii in Zn limitation culture was significantly lower than that in Zn-replete culture. However, T. weissflogii cellular silicon content and sinking rate increased. Moreover, the reduced specific growth rate (7 %, p < 0.05), enhanced ALP activity (63 %, p < 0.05), and sinking rate (20 %, p < 0.05) for Zn-deplete T. weissflogii implied that the bioavailability of dissolved organic phosphorus (DOP) was depressed under Zn deplete medium. This study demonstrates that the physiological ecology and sinking rate of the diatom T. weissflogii were affected by both individual and combined changes in P forms and Zn concentrations.

Exposure to the mixotrophic dinoflagellate Lepidodinium sp. and its cues increase toxin production of Pseudo-nitzschia multiseries.

The present study examined the defense responses of toxigenic Pseudo-nitzschia species (P. multiseries) to a mixotrophic dinoflagellate, Lepidodinium sp., and its associated cues. We evaluated their responses to different predation risks, including direct physical contact and indirect interactions facilitated by cues from Lepidodinium sp. during active feeding on heterospecific prey (Rhodonomas salina), limited feeding on conspecific prey (P. multiseries) and non-feeding (autotrophic growth in f/2 medium) states. This study is the first investigation of these trophic interactions. Our results demonstrated a significant increase in cellular domoic acid (cDA) in P. multiseries when exposed to Lepidodinium sp. and its associated cues, which was 1.38 to 2.42 times higher than the non-induced group. Notably, this increase was observed regardless of Lepidodinium sp. feeding on this toxic diatom and nutritional modes. However, the most significant increase occurred when they directly interacted. These findings suggest that P. multiseries evaluates predation risk and increases cDA production as a defensive strategy against potential grazing threats. No morphological changes were observed in P. multiseries in response to Lepidodinium sp. or its cues. P. multiseries cultured in flasks of Group L+P-P showed a decrease in growth, but Group L-P and Group L+R-P did not exhibit any decrease. These results suggest a lack of consistent trade-offs between the defense response and growth, thus an increase in cDA production may be a sustainable and efficient defense strategy for P. multiseries. Furthermore, our findings indicate that P. multiseries had no significant impact on the fitness (cell size, growth and/or grazing) of Lepidodinium sp. and R. salina, which suggests no evident toxic or allelopathic impacts on these two phytoplankton species. This study enhances our understanding of the trophic interactions between toxic diatoms and mixotrophic dinoflagellates and helps elucidate the dynamics of Harmful Algal Blooms, toxin transmission, and their impact on ecosystem health.

Oxidative stress responses in two marine diatoms during sulfamethoxazole exposure and the toxicological evaluation using the IBRv2 index.

Sulfamethoxazole (SMX) is widely present in water systems, and its stable properties and poor biodegradability can result in high residues of SMX in the water environment. This, in turn, can have detrimental effects on the entire aquatic habitat and human life and health. This study aimed to investigate the toxic effects of SMX on the growth, photosynthetic pigment content, and oxidative stress of two marine microalgae species: Skeletonema costatum and Phaeodactylum tricornutum. SMX demonstrated a significant inhibitory effect on microalgae proliferation, with 96-h median effective concentration (EC50) values of 0.93 mg/L and 4.65 mg/L for S. costatum and P. tricornutum, respectively. At low concentrations, SMX significantly increased the production of Chl a in both microalgae species. However, in the higher concentration SMX treatment group, Chl a content in P. tricornutum experienced a significant decrease, whereas Chl c showed no sensitivity to SMX. The activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), along with the glutathione (GSH) content, exhibited a significant increasing trend in response to higher SMX concentrations. However, these changes effectively inhibited the accumulation of malondialdehyde (MDA) content. In the treatment group with the highest SMX concentration, MDA content in both microalgae species was significantly higher compared to the control group. The Integrated Biomarker Response Version 2 (IBRv2) index showed a significant positive correlation with SMX concentration, suggesting its potential for assessing the ecotoxicological effects of lower SMX concentrations on marine microalgae.

Meta-analysis reveals responses of coccolithophores and diatoms to warming.

A meta-analysis was conducted to explore the effects of warming on the physiological processes of coccolithophores and diatoms by synthesizing a large number of published literatures. A total of 154 studies consisting 301 experiments were synthesized in this study. Under a projected temperature increase of 3-5 °C by IPCC AR6 at the end of this century, our results suggest that the growth and photosynthetic rate of coccolithophores were significantly enhanced by the rising temperature, while the calcification of coccolithophores was only slightly promoted. Warming also had significantly positive effects on the growth but not photosynthesis of diatoms. In comparison, the effect size of warming on the growth rate of coccolithophores was larger than that of diatoms. However, there was no significant effect of warming on either the ratio of particulate inorganic carbon to particulate organic carbon (PIC:POC) of coccolithophores or the ratio of biogenic silica to carbon (BSi:C) of diatoms. Furthermore, the results reveal latitudinal and size-specific patterns of the effect sizes of warming. For diatoms, the effects of warming on growth were more prominent in high latitudes, specifically for the Southern Hemisphere species. In addition, the effect size of warming on the small-sized diatoms was larger than that of the large-sized diatoms. For coccolithophores, the growth of the Southern Hemisphere temperate strains was significantly promoted by warming. Overall, the results based on the meta-analysis indicate that the projected warming of the end of this century will be more favor to the growth of coccolithophores than that of diatoms, thus potentially affect the competitive advantages of coccolithophores over diatoms; while the mid-to high latitude species/strains of both coccolithophores and diatoms will benefit more than their counterparts in the lower latitudes. Therefore, this study offers novel insights into predicting both the inter- and intra-group competitive advantages of diatoms and coccolithophores under the future warming climate change scenario.

Disruption of diatom attachment on marine bioinspired antifouling materials based on Brill (Scophthalmus rhombus).

Bioinspired surfaces, due to their nano and micro topographical features, offer a promising approach for the development of novel antifouling solutions. The study of surface topography has gained popularity in recent years, demonstrating significant potential in mimicking natural structures that could be manufactured for application in the marine environment. This research focuses on investigating the antifouling (AF) performance of bio-inspired micro-textures inspired by Brill fish scales, Scophthalmus rhombus, under static laboratory conditions, using two common fouling diatom species, Amphora coffeaeformis and Nitzschia ovalis. In this study, we evaluate six engineered surfaces, inspired by Brill fish scales, fabricated through a 2-photon polymerization (2PP) process, for their potential as antifouling solutions. The investigation explores the settlement behaviour of microfouling organisms, comparing these mechanisms with theoretical models to guide the future design of antifouling materials. A key emphasis is placed on the impact of surface topography on the disruption of cellular response. Our results suggest that cells smaller than 10 µm, exceeding the peak-to-peak distances between surface features, comfortably position themselves between adjacent features. On the other hand, as peak-to-peak distances decrease, cells shift from settling within uniform gaps to resting on top of surface features. Surfaces with sharpened edges demonstrate a more substantial reduction in diatom attachments compared to those with rounded edges. Furthermore, all micro-textured surfaces exhibit a significant decrease in colony formation compared to control samples. In conclusion, this study shows the potential to manipulate cellular responses through topographical features, providing valuable insights for the design of effective antifouling materials. The results contribute to the growing body of knowledge in biomimetic antifouling strategies using a novel marine organism for inspiration to design practical structures that can be replicated.

Governance pathway for coastal eutrophication based on regime shifts in diatom-dinoflagellate composition of the Bohai and Baltic Seas.

Regime shifts in the diatom-dinoflagellate composition have occurred in the Baltic Sea (BS) and Bohai Sea (BHS) under eutrophication and have affected the entire coastal ecosystem, damaging the regulatory, provisioning, cultural, and supporting service functions of marine ecosystems. Therefore, finding a solution to restore the balance of phytoplankton community composition and mitigate eutrophication is of utmost importance. In this study, the Driver (per capita gross domestic product)-Pressure (terrestrial inputs)-State (seawater environmental parameters)-Impact (proportions of diatoms and dinoflagellates)-Response (eutrophication governance projects) framework served as a guide for our analysis of the causal relationship among various environmental components in the coastal system. The relevant data in BS and BHS spanning from the 1950s to the 2010s were collected and used to construct a diatom-dinoflagellate composition single index, which allowed us to identify the shifts in regimes (mutation points and phases) of the diatom-dinoflagellate composition and environmental factors using sequential t-test analysis. We also identified key environmental factors that moderated the diatom-dinoflagellate composition using redundancy analysis and analyzed the partial effects of the main environmental factors on the diatom-dinoflagellate composition using a generalized additive model. Finally, the regulation of the eutrophication governance investment on diatom-dinoflagellate composition was investigated. We found that (1) BS is a "time machine," with coastal eutrophication governance and regime shift of diatom-dinoflagellate composition and environmental factors two decades earlier than that in BHS; (2) in BS, the key moderation factor of diatom proportion is SiO3-Si and those of dinoflagellates are sea surface salinity and N:P ratio; in BHS, the key moderation factors of diatom proportion are PO4-P and Si:N ratio and those of dinoflagellate are dissolved inorganic nitrogen and N:P and Si:P ratios; (3) it is projected that BHS will enter its recovery phase from eutrophication after mid-2020s. In summary, the N/P/Si stoichiometric relationships should be given greater consideration, with the exception of the "dose-response" relationship in both sea areas. Our results indicate an urgent need for an improved mechanistic understanding of how phytoplankton biodiversity changes in response to changes in nutrient load and how we should ultimately deal with the challenges that arise.

Aeolian dust and hydro-biological characteristics: Decoding dust storm impacts on phytoplankton in the northern Arabian Gulf.

Aeolian dust is an essential source of growth-limiting nutrients for marine phytoplankton. Despite being at the core of the Global Dust Belt, the response of the Arabian Gulf ecosystem to such atmospheric forcing is rarely documented. Here, the hydro-biological effect of mineral dust was studied in the northern Arabian Gulf (NAG) off Kuwait through monthly water sampling (December 2020 to December 2021), dust-storm follow-up sampling, and mineral dust and nutrient addition in-situ experiments. The multivariate analysis of oceanographic data revealed pronounced hydro-biological seasonality. The mineral dust deposition during two severe dust storm events in March and June 2021 showed a spatially varying effect of dust on coastal waters. The dust storms elevated the surface dissolved iron levels by several magnitudes, increased the dissolved inorganic nitrogen and phosphorous levels, changed their stoichiometry, and offset the hydrobiological seasonality. In the microcosms, dust input temporarily reduced phytoplankton phosphorous limitation in a dose-dependent manner when mesozooplankton (copepods) grazing was minimal. The microphytoplankton response to mineral dust inputs was comparable to that with nitrogen and phosphorous treatment. While Both treatments increased diatom size structure and biomass, the abundance of single-celled diatoms was comparatively higher in dust treatment. Multivariate analysis indicated that dust deposition alters the hydrographical properties of the surface ocean during dust storm events. The effects, though transient, were traceable for 3-16 days post-storm in coastal waters. The response of the summer phytoplankton to these changes, if delayed or muted, should be interpreted with caution given the summer water column stratification, the high nitrogen: phosphorous ratio and the low phosphorous solubility of aerosol dust, and the complex pelagic microbial food web interactions in the NAG. This study thus underlines the importance of a multivariate approach in documenting the ecological implications of Aeolian dust storms on marine environments closer to the dust source regions.