Response mechanism of harmful algae Phaeocystis globosa to ocean warming and acidification

Nitrogen source type modulates heat stress response in coral symbiont (Cladocopium goreaui)

Ocean warming due to climate change endangers coral reefs, and regional nitrogen overloading exacerbates the vulnerability of reef-building corals as the dual stress disrupts coral-Symbiodiniaceae mutualism. Different forms of nitrogen may create different interactive effects with thermal stress, but the underlying mechanisms remain elusive. To address the gap, we measured and compared the physiological and transcriptional responses of the Symbiodiniaceae Cladocopium goreaui to heat stress (31°C) when supplied with different types of nitrogen (nitrate, ammonium, or urea). Under heat stress (HS), cell proliferation and photosynthesis of C. goreaui declined, while cell size, lipid storage, and total antioxidant capacity increased, both to varied extents depending on the nitrogen type. Nitrate-cultured cells exhibited the most robust acclimation to HS, as evidenced by the fewest differentially expressed genes (DEGs) and less ROS accumulation, possibly due to activated nitrate reduction and enhanced ascorbate biogenesis. Ammonium-grown cultures exhibited higher algal proliferation and ROS scavenging capacity due to enhanced carotenoid and ascorbate quenching, but potentially reduced host recognizability due to the downregulation of N-glycan biosynthesis genes. Urea utilization led to the greatest ROS accumulation as genes involved in photorespiration, plant respiratory burst oxidase (RBOH), and protein refolding were markedly upregulated, but the greatest cutdown in photosynthate potentially available to corals as evidenced by photoinhibition and selfish lipid storage, indicating detrimental effects of urea overloading. The differential warming nitrogen-type interactive effects documented here has significant implication in coral-Symbiodiniaceae mutualism, which requires further research.IMPORTANCERegional nitrogen pollution exacerbates coral vulnerability to globally rising sea-surface temperature, with different nitrogen types exerting different interactive effects. How this occurs is poorly understood and understudied. This study explored the underlying mechanism by comparing physiological and transcriptional responses of a coral symbiont to heat stress under different nitrogen supplies (nitrate, ammonium, and urea). The results showed some common, significant responses to heat stress as well as some unique, N-source dependent responses. These findings underscore that nitrogen eutrophication is not all the same, the form of nitrogen pollution should be considered in coral conservation, and special attention should be given to urea pollution.

The trade-offs associated with the adaptions of marine microalgae to high CO2 and warming

Trade-offs play vital roles in evolutionary theory, linking organism performance to changing environments in the context of global change. Marine microalgae, as one of the most important groups of primary producers in the biosphere, exhibit significant trade-offs across multiple traits in response to environmental changes, such as elevated CO2 (and consequent ocean acidification) and warming. In this review, we synthesize recent findings on the trade-offs associated with both short-term phenotypic acclimation and long-term genotypic adaptation of marine microalgae. Specifically, we discuss distinct classes of trade-offs (i.e., allocation trade-offs, acquisition trade-offs and specialist-generalist trade-offs) between multiple traits, such as growth rate, photosynthesis, nutrient acquisition, and stress tolerance. We also explored the underlying mechanisms driving these trade-offs. Finally, we discuss the broader ecological consequences of these trade-offs, such as potential shifts in species composition and ecosystem functions, and outline key research directions to better predict marine ecosystem responses to future global change scenarios.

A Phaeocystis bloom in the northern Beibu Gulf reflects nutrient-driven shifts in dominant phytoplankton taxa

Large-scale Phaeocystis globosa blooms in the Beibu Gulf have adversely affected marine ecosystems and the functioning of a nuclear power plant. To understand the formation mechanisms for such large-scale blooms, we examined the P. globosa bloom dynamics in the northern Beibu Gulf from September 2015 to March 2016 in association with changes in phytoplankton community and environmental factors. Statistical analysis methods, such as change-point method, principal component analysis (PCA), redundancy analysis (RDA) and an artificial neural network (ANN) model, were utilized to identify environmental drivers on the dynamics of phytoplankton community during the P. globosa bloom. The bloom evolution can be categorized into three stages, delineated by the fluctuation in colony abundance of P. globosa. Stage I, immediately before the Pheaocystis bloom, was characterized by the dominance of cyanobacterium Synechococcus (Syn) and the presence of the pigment 19'-hexanoyloxyfucoxanthin (Hex-fuco) in phytoplankton. Stage II, the development stage of the Phaeocystis bloom, was characterized by the appearance of P. globosa colonies and the pigment 19'-butanoyloxyfucoxanthin (But-fuco). Stage III, the decline stage of the bloom, was characterized by the increase of diatoms and pico-sized eukaryotes. Dynamics of the P. globosa bloom reflected the shift of dominant phytoplankton taxa from diatoms to haptophytes during winter, closely related to the change of turbidity, silicate limitation and rebalancing of nitrogen and phosphate. This study is the first to examine the relationship of a Phaeocystis bloom with phytoplankton structure and environmental factors in the Beibu Gulf, providing critical insights for the bloom forecasting and mitigation.

Tire microplastic particles and warming inhibit physiological functions of the toxic microalga Alexandrium pacificum

Previous studies have confirmed that the tire microplastic particles (TMPs) have a variety of toxic biological effects. However, the potential toxic mechanisms of TMPs remain to be elucidated, especially in the interaction between particle behavior and seawater warming. In this study, we investigated the effects of three different concentrations of TMPs suspensions (0 mg/L, 1 mg/L, and 500 mg/L) on Alexandrium pacificum in both the presence and absence of warming. Our results revealed significant differences in toxicity among different concentrations of TMPs towards A. pacificum, i.e., low concentrations promoting but high concentrations inhibiting, furthermore, warming exacerbated these toxicological responses. Specifically, under elevated temperature, high concentrations TMPs could inhibit photosynthetic pigment and chlorophyll fluorescence parameter, as well as the nutrient absorption, and induced oxidative stress. Furthermore, TMPs could adsorb onto microalgae surfaces and thus, forming heterogeneous aggregates through agglomeration with extracellular secretions. This is strongly correlated with biomarker response. Overall, these findings highlight the influence of warming on the toxicity of TMPs and provide valuable data for risk assessment.

Warming and polymetallic stress induce proteomic and physiological shifts in the neurotoxic Alexandrium pacificum as possible response to global changes

Harmful Algal Blooms involving the dinoflagellate Alexandrium pacificum continue to increase in ecosystems suffering the climate warming and anthropogenic pressure. Changes in the total proteome and physiological traits of the Mediterranean A. pacificum SG C10-3 strain were measured in response to increasing temperature (24 °C, 27 °C, 30 °C) and trace metal contamination (Cu2+, Pb2+, Zn2+, Cd2+). Warming reduced the cell densities and maximal growth rate (μmax), but the strain persisted at 30 °C with more large cells. The polymetallic stress increased cell sizes, reduced cell growth at 24 °C-27 °C and it increased this at 30 °C. Toxin profiles showed a predominance of GTX4 (32-38 %), then C2 (11-34 %) or GTX6 (18-24 %) among the total Paralytic Shellfish Toxins, however these were modified under warming, showing increased contents in GTX1 (among the most toxic), GTX5, C1 and NeoSTX, while dc-NeoSTX and STX (among the most toxic) only appeared at 30 °C. Under polymetallic contamination, warming also increased contents in GTX5 and NeoSTX. In contrast, polymetallic stress, or warming had harmful effects on C2 contents. Proteins were more quantitatively produced by A. pacificum SG C10-3 under warming in accordance with the high levels of up-regulated proteins found in the total proteome in this condition. Polymetallic stress, only or combined with warming, led to low proteomic modifications (1 % or 4 %), whereas warming induced strong 52 % modified proteomic response, mainly based on up-regulated proteins involved in photosynthesis (light harvesting complex protein), carbohydrate metabolism (arylsulfatase) and translation (ribosomal proteins), and with the lesser down-regulated proteins principally associated with the lipid metabolism (type I polyketide synthase). Our results show that warming triggers a strong up-regulated A. pacificum SG C10-3 proteomic response, which, coupled to modified cell sizes and toxin profiles, could help it to withstand stress conditions. This could presage the success of A. pacificum in anthropized ecosystems submitted to global warming in which this dinoflagellate also might be more toxic.

Predicting coastal harmful algal blooms using integrated data-driven analysis of environmental factors

Coastal harmful algal blooms (HABs) have become one of the challenging environmental problems in the world's thriving coastal cities due to the interference of multiple stressors from human activities and climate change. Past HAB predictions primarily relied on single-source data, overlooked upstream land use, and typically used a single prediction algorithm. To address these limitations, this study aims to develop predictive models to establish the relationship between the HAB indicator - chlorophyll-a (Chl-a) and various environmental stressors, under appropriate lagging predictive scenarios. To achieve this, we first applied the partial autocorrelation function (PACF) to Chl-a to precisely identify two prediction scenarios. We then combined multi-source data and several machine learning algorithms to predict harmful algae, using SHapley Additive exPlanations (SHAP) to extract key features influencing output from the prediction models. Our findings reveal an apparent 1-month autoregressive characteristic in Chl-a, leading us to create two scenarios: 1-month lead prediction and current-month prediction. The Extra Tree Regressor (ETR), with an R2 of 0.92, excelled in 1-month lead predictions, while the Random Forest Regressor (RFR) was most effective for current-month predictions with an R2 of 0.69. Additionally, we identified current month Chl-a, developed land use, total phosphorus, and nitrogen oxides (NOx) as critical features for accurate predictions. Our predictive framework, which can be applied to coastal regions worldwide, provides decision-makers with crucial tools for effectively predicting and mitigating HAB threats in major coastal cities.

Effect of acetochlor on the symbiotic relationship between microalgae and bacteria

In this study, two strains of symbiotic bacteria (SOB-1 and SOB-2) were isolated from Scenedesmus obliquus, and various algal-bacterial mutualistic systems were established under acetochlor (ACT) stress conditions. Following exposure to varying ACT concentrations from 2.0 to 25.0 μg/L, the capacity for co-cultured bacteria to degrade ACT was enhanced in 7 days by up to 226.9% (SOB-1) and 193.0% (SOB-2), compared with axenic algae, although bacteria exposed to higher ACT concentrations exacerbated algal metabolic stress, oxidative states, apoptosis and cellular lysis. ACT reduced carbohydrates in the phycosphere by up to 31.5%; compensatory nutrient plunder and structural damage by bacteria were the potential exploitation pathways determined based on the inhibition of bacterial infection using a glucanase inhibitor. The ACT-induced reduction in algal antimicrobial substances, including fatty acids and phenolics (by up to 58.1% and 56.6%, respectively), also facilitated bacterial exploitation of algae. ACT-dependent interspecific interaction coefficients between algae and bacteria generated from long-term symbiosis cultures implied that bacteria moved from mutualism (0 and 2.0 μg/L ACT) to exploitation (7.9 and 25.0 μg/L ACT). The population dynamic model under incremental ACT-concentration scenarios inferred that theoretical systematic extinction may occur in algal-bacterial systems earlier than in axenic algae. These outcomes provide interspecific insights into the distortion of algal-bacterial reciprocity due to the ecotoxicological effects of ACT.

Morphological, molecular, and life cycle characteristics of Phaeocystis globosa Scherffel (Prymnesiophyceae) in the Southeast China Sea

Phaeocystis globosa blooms frequently occur in the Southeast China Sea and cause significant negative impacts on coastal ecology and mariculture. The P. globosa blooms in southeastern China are very different compared to those of European strains, suggesting that differences may exist in their morphological, phylogenetic, and life history traits. In this study, seven strains of P. globosa isolated from Southeast China Sea that were typical strains of algal blooms in the region, in addition to one strain from the Gulf of Mexico (CCMP629), were comprehensively evaluated to better understand region-specific differences of the species. Significant differences were not observed in the internal cell structures and other characteristics compared to those of European strains, while differences in cell surface structures were apparent. For example, small and large flagellated Chinese P. globosa cells exhibited two flagella with slightly unequal lengths and a short haptonema, the surfaces of small flagellated cells were not covered by scales, and colony cell diameters were smaller. 18S rRNA sequence phylogenetic analysis also revealed that P. globosa comprised a species complex with two ecotypes (warm- and cold-water types), of which the strains from the southeastern coast of China and CCMP629 belonged to the warm-water type. In addition, the life cycles and variable modes of P. globosa colony formation were evaluated in detail. The algal bloom may be due to the rapid colonies formation by budding and colony fragments. These results provide new insights into the life cycle of P. globosa and highlight the differences in morphological and phylogenetic relationships between strains from the southeast coast of China and those from coastal European regions.

The cellular response to ocean warming in Emiliania huxleyi

Marine phytoplankton contribute substantially to the global flux of carbon from the atmosphere to the deep ocean. Sea surface temperatures will inevitably increase in line with global climate change, altering the performance of marine phytoplankton. Differing sensitivities of photosynthesis and respiration to temperature, will likely shift the strength of the future oceanic carbon sink. To further clarify the molecular mechanisms driving these alterations in phytoplankton function, shotgun proteomic analysis was carried out on the globally-occurring coccolithophore Emiliania huxleyi exposed to moderate- (23°C) and elevated- (28°C) warming. Compared to the control (17°C), growth of E. huxleyi increased under elevated temperatures, with higher rates recorded under moderate- relative to elevated- warming. Proteomic analysis revealed a significant modification of the E. huxleyi cellular proteome as temperatures increased: at lower temperature, ribosomal proteins and photosynthetic machinery appeared abundant, as rates of protein translation and photosynthetic performance are restricted by low temperatures. As temperatures increased, evidence of heat stress was observed in the photosystem, characterized by a relative down-regulation of the Photosystem II oxygen evolving complex and ATP synthase. Acclimation to elevated warming (28°C) revealed a substantial alteration to carbon metabolism. Here, E. huxleyi made use of the glyoxylate cycle and succinate metabolism to optimize carbon use, maintain growth and maximize ATP production in heat-damaged mitochondria, enabling cultures to maintain growth at levels significantly higher than those recorded in the control (17°C). Based on the metabolic changes observed, we can predict that warming may benefit photosynthetic carbon fixation by E. huxleyi in the sub-optimal to optimal thermal range. Past the thermal optima, increasing rates of respiration and costs of repair will likely constrain growth, causing a possible decline in the contribution of this species to the oceanic carbon sink depending on the evolvability of these temperature thresholds.