A Rise in ROS and EPS Production: New Insights into the Trichodesmium erythraeum Response to Ocean Acidification

The Effecting Mechanisms of 100 nm Sized Polystyrene Nanoplastics on the Typical Coastal Alexandrium tamarense

Due to the increase in nanoplastics (NPs) abundance in aquatic environments, their effects on phytoplankton have aroused large research attention. In this study, 100 nm sized polystyrene NPs were chosen to investigate their effecting performance and mechanisms on a typical dinoflagellates Alexandrium tamarense. The results indicated the population growth and photosynthetic efficiencies of A. tamarense were significantly inhibited by NPs exposure, as well as the increase in cellular total carotenoids and paralytic shellfish toxins (PSTs). Meanwhile, the cellar ROS levels increased, corresponding to the increased activities or contents of multiple antioxidant components, including SOD, CAT, GPX, GR, GSH and GSSG. The transcriptional results support the physiological-biochemical results and further revealed the down-regulation of genes encoding the light reaction centers (PSI and PSII) and up-regulation of genes encoding the antioxidant components. Up-regulation of genes encoding key enzymes of the Calvin cycle and glycolytic pathway together with the TCA cycle could accelerate organic carbon and ATP production for A. tamarense cells resistant to NPs stress. Finally, more Glu and acetyl-CoA produced by the enhanced GSH cycle and the glycolytic pathway, respectively, accompanied by the up-regulation of Glu and Arg biosynthesis genes supported the increase in the PST contents under NPs exposure. This study established a data set involving physiological-biochemical changes and gene information about marine dinoflagellates responding to NPs, providing a data basis for further evaluating the ecological risk of NPs in marine environments.

Rapid mode switching facilitates the growth of Trichodesmium: A model analysis

Trichodesmium is one of the dominant dinitrogen (N2) fixers in the ocean, influencing global carbon and nitrogen cycles through biochemical reactions. Although its photosynthetic activity fluctuates rapidly, the physiological or ecological advantage of this fluctuation is unclear. We develop a metabolic model of Trichodesmium that can perform daytime N2 fixation. We examined (1) the effect of the duration of switches between photosynthetic and non-photosynthetic cellular states and (2) the effect of the presence and absence of N2 fixation in photosynthetic states. Results show that a rapid switch between photosynthetic and non-photosynthetic states increases Trichodesmium growth rates by improving metabolic efficiencies due to an improved balance of C and N metabolism. This provides a strategy for previous paradoxical observations that all Trichodesmium cells can contain nitrogenase. This study reveals the importance of fluctuating photosynthetic activity and provides a mechanism for daytime N2 fixation that allows Trichodesmium to fix N2 aerobically in the global ocean.

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.

A laboratory study of the increasing competitiveness of Karenia mikimotoi under rising CO2 scenario

Ocean acidification (OA) driven by elevated carbon dioxide (CO2) levels is expected to disturb marine ecological processes, including the formation and control of harmful algal blooms (HABs). In this study, the effects of rising CO2 on the allelopathic effects of macroalgae Ulva pertusa to a toxic dinoflagellate Karenia mikimotoi were investigated. It was found that high level of CO2 (1000 ppmv) promoted the competitive growth of K. mikimotoi compared to the group of present ambient CO2 level (420ppmv), with the number of algal cell increased from 32.2 × 104 cells/mL to 36.75 × 104 cells/mL after 96 h mono-culture. Additionally, rising CO2 level weakened allelopathic effects of U. pertusa on K. mikimotoi, as demonstrated by the decreased inhibition rate (50.6 % under the original condition VS 34.3 % under the acidified condition after 96 h co-culture) and the decreased reactive oxygen species (ROS) level, malondialdehyde (MDA) content, antioxidant enzymes activity (superoxide dismutase (SOD), peroxidase (POD), glutathione peroxidase (GPX), glutathione reductase (GR) and catalase (CAT) and non-enzymatic antioxidants (glutathione (GSH) and ascorbic acid (ascorbate, vitamin C). Indicators for cell apoptosis of K. mikimotoi including decreased caspase-3 and -9 protease activity were observed when the co-cultured systems were under rising CO2 exposure. Furthermore, high CO2 level disturbed fatty acid synthesis in U. pertusa and significantly decreased the contents of fatty acids with allelopathy, resulting in the allelopathy weakening of U. pertusa. Collectively, rising CO2 level promoted the growth of K. mikimotoi and weakened allelopathic effects of U. pertusa on K. mikimotoi, indicating the increased difficulties in controlling K. mikimotoi using macroalgae in the future.

Reactive oxygen species mediated extracellular polymeric substances production assisting the recovery of Thalassiosira pseudonana from polystyrene micro and nanoplastics exposure

As emerging pollutants in the aquatic environments, micro- and nano-plastics (MNPs) aroused widespread environmental concerns for their potential threats to the ecological health. Previous research has proved that microalgae growth could recover from the MNPs toxicities, in which the extracellular polymeric substances (EPS) might play the key role. In order to comprehensively investigate the recovery process of microalgae from MNPs stress and the effecting mechanisms of EPS therein, this study conducted a series of experiments by employing two sizes (0.1 and 1 μm) of polystyrene (PS) MNPs and the marine model diatom Thalassiosira pseudonana during 14 days. The results indicated: the pigments accumulations and photosynthetic recovery of T. pseudonana under MPs exposure showed in the early stage (4-5 days), while the elevation of reactive oxygen species (ROS) and EPS contents lasted longer time period (7-8 days). EPS was aggregated with MNPs particles and microalgal cells, corresponding to the increased settlement rates. More increase of soluble (SL)-EPS contents was found than bound (B)-EPS under MNPs exposure, in which the increase of the protein proportion and humic acid-like substances in SL-EPS was found, thus facilitating aggregates formation. ROS was the signaling molecule mediating the overproduction of EPS. The transcriptional results further proved the enhanced EPS biosynthesis on the molecular level. Therefore, this study elucidated the recovery pattern of microalgae from MNPs stress and linked "ROS-EPS production changes-aggregation formation" together during the growth recovery process, with important scientific and environmental significance.

Aerobic methane production by phytoplankton as an important methane source of aquatic ecosystems: Reconsidering the global methane budget

Biological methane, a major source of global methane budget, is traditionally thought to be produced in anaerobic environments. However, the recent reports about methane supersaturation occurring in oxygenated water layer, termed as "methane paradox", have challenged this prevailing paradigm. Significantly, growing evidence has indicated that phytoplankton including prokaryotic cyanobacteria and eukaryotic algae are capable of generating methane under aerobic conditions. In this regard, a systematic review of aerobic methane production by phytoplankton is expected to arouse the public attention, contributing to the understanding of methane paradox. Here, we comprehensively summarize the widespread phenomena of methane supersaturation in oxic layers. The remarkable correlation relationships between methane concentration and several key indicators (depth, chlorophyll a level and organic sulfide concentration) indicate the significance of phytoplankton in in-situ methane accumulation. Subsequently, four mechanisms of aerobic methane production by phytoplankton are illustrated in detail, including photosynthesis-driven metabolism, reactive oxygen species (ROS)-driven demethylation of methyl donors, methanogenesis catalyzed by nitrogenase and demethylation of phosphonates catalyzed by CP lyase. The first two pathways occur in various phytoplankton, while the latter two have been specially discovered in cyanobacteria. Additionally, the effects of four crucial factors on aerobic methane production by phytoplankton are also discussed, including phytoplankton species, light, temperature and crucial nutrients. Finally, the measures to control global methane emissions from phytoplankton, the precise intracellular mechanisms of methane production and a more complete global methane budget model are definitely required in the future research on methane production by phytoplankton. This review would provide guidance for future studies of aerobic methane production by phytoplankton and emphasize the potential contribution of aquatic ecosystems to global methane budget.

Better together? Lessons on sociality from Trichodesmium

The N2-fixing cyanobacterium Trichodesmium is an important player in the oceanic nitrogen and carbon cycles. Trichodesmium occurs both as single trichomes and as colonies containing hundreds of trichomes. In this review, we explore the benefits and disadvantages of colony formation, considering physical, chemical, and biological effects from nanometer to kilometer scale. Showing that all major life challenges are affected by colony formation, we claim that Trichodesmium's ecological success is tightly linked to its colonial lifestyle. Microbial interactions in the microbiome, chemical gradients within the colony, interactions with particles, and elevated mobility in the water column shape a highly dynamic microenvironment. We postulate that these dynamics are key to the resilience of Trichodesmium and other colony formers in our changing environment.

Hormetic Effect of Pyroligneous Acids on Conjugative Transfer of Plasmid-mediated Multi-antibiotic Resistance Genes within Bacterial Genus

Spread of antibiotic resistance genes (ARGs) by conjugation poses great challenges to public health. Application of pyroligneous acids (PA) as soil amendments has been evidenced as a practical strategy to remediate pollution of ARGs in soils. However, little is known about PA effects on horizontal gene transfer (HGT) of ARGs by conjugation. This study investigated the effects of a woody waste-derived PA prepared at 450°C and its three distillation components (F1, F2, and F3) at different temperatures (98, 130, and 220°C) on conjugative transfer of plasmid RP4 within Escherichia coli. PA at relatively high amount (40-100 μL) in a 30-mL mating system inhibited conjugation by 74-85%, following an order of PA > F3 ≈ F2 ≈ F1, proving the hypothesis that PA amendments may mitigate soil ARG pollution by inhibiting HGT. The bacteriostasis caused by antibacterial components of PA, including acids, phenols, and alcohols, as well as its acidity (pH 2.81) contributed to the inhibited conjugation. However, a relatively low amount (10-20 μL) of PA in the same mating system enhanced ARG transfer by 26-47%, following an order of PA > F3 ≈ F2 > F1. The opposite effect at low amount is mainly attributed to the increased intracellular reactive oxygen species production, enhanced cell membrane permeability, increased extracellular polymeric substance contents, and reduced cell surface charge. Our findings highlight the hormesis (low-amount promotion and high-amount inhibition) of PA amendments on ARG conjugation and provide evidence for selecting an appropriate amount of PA amendment to control the dissemination of soil ARGs. Moreover, the promoted conjugation also triggers questions regarding the potential risks of soil amendments (e.g., PA) in the spread of ARGs via HGT.

Transcriptional responses of Trichodesmium to natural inverse gradients of Fe and P availability

The filamentous diazotrophic cyanobacterium Trichodesmium is responsible for a significant fraction of marine di-nitrogen (N₂) fixation. Growth and distribution of Trichodesmium and other diazotrophs in the vast oligotrophic subtropical gyres is influenced by iron (Fe) and phosphorus (P) availability, while reciprocally influencing the biogeochemistry of these nutrients. Here we use observations across natural inverse gradients in Fe and P in the North Atlantic subtropical gyre (NASG) to demonstrate how Trichodesmium acclimates in situ to resource availability. Transcriptomic analysis identified progressive upregulation of known iron-stress biomarker genes with decreasing Fe availability, and progressive upregulation of genes involved in the acquisition of diverse P sources with decreasing P availability, while genes involved in N₂ fixation were upregulated at the intersection under moderate Fe and P availability. Enhanced N₂ fixation within the Fe and P co-stressed transition region was also associated with a distinct, consistent metabolic profile, including the expression of alternative photosynthetic pathways that potentially facilitate ATP generation required for N₂ fixation with reduced net oxygen production. The observed response of Trichodesmium to availability of both Fe and P supports suggestions that these biogeochemically significant organisms employ unique molecular, and thus physiological responses as adaptations to specifically exploit the Fe and P co-limited niche they construct.

In contrast to diatoms, cryptophytes are susceptible to iron limitation, but not to ocean acidification

Previous field studies in the Southern Ocean (SO) indicated an increased occurrence and dominance of cryptophytes over diatoms due to climate change. To gain a better mechanistic understanding of how the two ecologically important SO phytoplankton groups cope with ocean acidification (OA) and iron (Fe) availability, we chose two common representatives of Antarctic waters, the cryptophyte Geminigera cryophila and the diatom Pseudo-nitzschia subcurvata. Both species were grown at 2°C under different pCO₂ (400 vs. 900 μatm) and Fe (0.6 vs. 1.2 nM) conditions. For P. subcurvata, an additional high pCO₂ level was applied (1400 μatm). At ambient pCO₂ under low Fe supply, growth of G. cryophila almost stopped while it remained unaffected in P. subcurvata. Under high Fe conditions, OA was not beneficial for P. subcurvata, but stimulated growth and carbon production of G. cryophila. Under low Fe supply, P. subcurvata coped much better with OA than the cryptophyte, but invested more energy into photoacclimation. Our study reveals that Fe limitation was detrimental for the growth of G. cryophila and suppressed the positive OA effect. The diatom was efficient in coping with low Fe, but was stressed by OA while both factors together strongly impacted its growth. The distinct physiological response of both species to OA and Fe limitation explains their occurrence in the field. Based on our results, Fe availability is an important modulator of OA effects on SO phytoplankton, with different implications on the occurrence of cryptophytes and diatoms in the future.

The responses of harmful dinoflagellate Karenia mikimotoi to simulated ocean acidification at the transcriptional level

The HAB-forming, toxic dinoflagellate Karenia mikimotoi, previously found to benefit from ocean acidification (OA), was cultivated to investigate its transcriptional response to simulated OA for 30 generations. Batch cultures were grown under two CO₂ concentrations, 450 (control) and 1100 (simulated OA) μatm, and physiological parameters [growth, pigments, catalase (CAT), glutathione reductase (GR), and superoxide dismutase (SOD) activity], as well as transcriptomes (obtained via RNA-seq), were compared. Chlorophyll a (Chl a) and carotenoid (Caro) contents, as well as CAT and GR activities, were significantly increased under OA conditions. Transcriptomic analysis revealed 2,490 differentially expressed unigenes in response to OA, which comprised 1.54% of all unigenes. A total of 1,121 unigenes were upregulated, and 1,369 unigenes were downregulated in OA compared to control conditions. The downregulated expression of bicarbonate transporter and carbonic anhydrase genes was a landmark of OA acclimation. Key genes involved in energy metabolism, e.g., photosynthesis, tricarboxylic acid cycle, oxidative phosphorylation, and nitrogen metabolism, were highly upregulated under OA, contributing to increases in the Chl a (55.05%) and Caro (28.37%). The enhanced antioxidant enzyme activities (i.e. CAT, GR) and upregulated genes (i.e. glutathione peroxidase, ascorbate peroxidase, heat shock protein, 20S proteasome, aldehyde dehydrogenase, and apolipoprotein) benefit cells against the potential lower pH stress condition under OA. In addition, the downregulation of four genes associated with motility suggested that the preserved energy could further boost growth. In conclusion, the present study suggests that K. mikimotoi exhibits efficient gene expression regulation for the utilization of energy and resistance to OA-induced stress. Taken together, K. mikimotoi appeared as a tolerant species in response to OA. Thus, more extensive algal blooms that threaten marine organisms are likely in the future. These findings expand current knowledge on the gene expression of HAB-forming species in response to future OA.

The genetic and ecophysiological diversity of Microcystis

Microcystis is a cyanobacterium that forms toxic blooms in freshwater ecosystems around the world. Biological variation among taxa within the genus is apparent through genetic and phenotypic differences between strains and via the spatial and temporal distribution of strains in the environment, and this fine-scale diversity exerts strong influence over bloom toxicity. Yet we do not know how varying traits of Microcystis strains govern their environmental distribution, the tradeoffs and links between these traits, or how they are encoded at the genomic level. Here we synthesize current knowledge on the importance of diversity within Microcystis and on the genes and traits that likely underpin ecological differentiation of taxa. We briefly review spatial and environmental patterns of Microcystis diversity in the field and genetic evidence for cohesive groups within Microcystis. We then compile data on strain-level diversity regarding growth responses to environmental conditions and explore evidence for variation of community interactions across Microcystis strains. Potential links and tradeoffs between traits are identified and discussed. The resulting picture, while incomplete, highlights key knowledge gaps that need to be filled to enable new models for predicting strain-level dynamics, which influence the development, toxicity and cosmopolitan nature of Microcystis blooms.