Combination of ocean acidification and warming enhances the competitive advantage of Skeletonema costatum over a green tide alga, Ulva linza

Marine heatwaves alter competition between the cultured macroalga Gracilariopsis lemaneiformis and the harmful bloom alga Skeletonema costatum

Seaweed cultivation can inhibit the occurrence of red tides. However, how seaweed aquaculture interactions with harmful algal blooms will be affected by the increasing occurrence and intensity of marine heatwaves (MHWs) is unknown. In this study, we run both monoculture and coculture systems to investigate the effects of a simulated heatwave on the competition of the economically important macroalga Gracilariopsis lemaneiformis against the harmful bloom diatom Skeletonema costatum. Coculture with G. lemaneiformis led to a growth decrease in S. costatum. Growth and photosynthetic activity (Fv/Fm) of G. lemaneiformis was greatly reduced by the heatwave treatment, and did not recover even after one week. Growth and photosynthetic activity of S. costatum was also reduced by the heatwave in coculture, but returned to normal during the recovery period. S. costatum also responded to the stressful environment by forming aggregates. Metabolomic analysis suggests that the negative effects on S. costatum were related to an allelochemical release from G. lemaneiformis. These findings show that MHWs may enhance the competitive advantages of S. costatum against G. lemaneiformis, leading to more severe harmful algal blooms in future extreme weather scenarios.

Environmentally degradable carbon dots for inhibiting P. globosa growth and reducing hemolytic toxin

Red tides not only destroy marine ecosystems but also pose a great threat to human health. The traditional anti-red tide materials are difficult to degrade effectively in the natural environment and there may be risks of environmental leakage and secondary pollution. Furthermore, they cannot reduce the toxicity of toxins released by algae. It is very important to prepare degradable materials that can effectively control red tide and reduce their toxins in the future. Herein, degradable CDs (De-CDs) with biocompatibility and non-toxicity is successfully prepared using the one-step electrolytic method. De-CDs can effectively inhibit P. globosa (algae associated with red tide) growth. More importantly, the De-CDs not only can attenuate the toxicity of toxins released by P. globosa, but also can be degraded under visible-light irradiation in the seawater and avoids environmental leakage. The successful preparation of De-CDs provides a new idea for degradable materials with anti-red tide algae in the future.

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.

Warming exacerbates the impacts of ultraviolet radiation in temperate diatoms but alleviates the effect on polar species

Under global change scenarios, the sea surface temperature is increasing steadily along with other changes to oceanic environments. Consequently, marine diatoms are influenced by multiple ocean global change drivers. We hypothesized that temperature rise mediates the responses of polar and temperate diatoms to UV radiation (UVR) to different extents, and exposed the temperate centric diatoms, Thalassiosira weissflogii and Skeletonema costatum, and a polar pennate diatom Entomoneis sp., to warming (+5°C) for 10 days, then performed short-term incubations under different radiation treatments with or without UVR. The effective quantum yields of the three diatoms were stable during exposure to PAR, but decreased when exposed to PAR + UVR, leading to significant UV-induced inhibition, which was 3% and 9%, respectively, for T. weissflogii and S. costatum under ambient temperature but increased to 12% and 17%, respectively, in the cells acclimated to the warming treatment. In contrast, UVR induced much higher inhibition, by about 45%, in the polar diatom Entomoneis sp. at ambient temperature, and the warming treatment alleviated the UV-induced inhibition, which dropped to 36%. The growth rates were significantly inhibited by UVR in S. costatum under the warming treatment and in Entomoneis sp. under ambient temperature, while there was no significant effect for T. weissflogii. Our results indicate that the polar diatom was more sensitive to UVR though warming could alleviate its impact, whereas the temperate diatoms were less sensitive to UVR but warming exacerbated its impacts, implying that diatoms living in different regions may exhibit differential responses to global changes.

Shift in algal blooms from micro- to macroalgae around China with increasing eutrophication and climate change

Blooms of microalgal red tides and macroalgae (e.g., green and golden tides caused by Ulva and Sargassum) have caused widespread problems around China in recent years, but there is uncertainty around what triggers these blooms and how they interact. Here, we use 30 years of monitoring data to help answer these questions, focusing on the four main species of microalgae Prorocentrum donghaiense, Karenia mikimotoi, Noctiluca scintillans, and Skeletonema costatum) associated with red tides in the region. The frequency of red tides increased from 1991 to 2003 and then decreased until 2020, with S. costatum red tides exhibiting the highest rate of decrease. Green tides started to occur around China in 1999 and the frequency of green tides has since been on the increase. Golden tides were first reported to occur around China in 2012. The frequency of macroalgal blooms has a negative linear relationship with the frequency and coverage of red tides around China, and a positive correlation with total nitrogen and phosphorus loads as well as with atmospheric CO2 and sea surface temperature (SST). Increased outbreaks of macroalgal blooms are very likely due to worsening levels of eutrophication, combined with rising CO2 and SST, which contribute to the reduced frequency of red tides. The increasing grazing rate of microzooplankton also results in the decline in areas affected by red tides. This study shows a clear shift of algal blooms from microalgae to macroalgae around China over the past 30 years driven by the combination of eutrophication, climate change, and grazing stress, indicating a fundamental change in coastal systems in the region.

Attached Ulva meridionalis on nearshore dikes may pose a new ecological risk in the Yellow Sea

Green tides have been reported to occur in many sea areas worldwide. In China, most of them are caused by Ulva spp., such as Ulva prolifera and Ulva meridionalis. Green tide algae shed are frequently the initial biomass for the formation of green tide. Human activities and seawater eutrophication are the fundamental causes of the formation of the green tides in the Bohai Sea, Yellow Sea, and South China Sea, but other environmental factors may also have an impact on the shedding of green tide algae, such as typhoons and currents. Algae shedding is divided into artificial shedding and natural shedding. However, few studies have explored the relationship between algal natural shedding and environmental factors. pH, sea surface temperature, and salinity are critical environmental factors affecting the physiological state of algae. Therefore, based on field observations of the shedding of attached green macroalgae in Binhai Harbor, this study assessed the correlation between the shedding rate and environmental factors (pH, sea surface temperature, and salinity). The green algae that shed from Binhai Harbor in August 2022 were all identified as U. meridionalis. The shedding rate range was 0.88% ± 0.11% d^-1 to 4.78% ± 1.76% d^-1, and was not correlated with pH, sea surface temperature, or salinity; however, the environmental conditions were very suitable for the proliferation of U. meridionalis. This study provided a reference for the shedding mechanism of green tide algae and revealed that with the frequent human activities along the coast, U. meridionalis may pose a new ecological risk in the Yellow Sea.

Metabolites and metabolic pathways associated with allelochemical effects of linoleic acid on Karenia mikimotoi

Linoleic acid (LA) shows great potential in inhibiting the growth of multiple red tide microalgae by disturbing algal physio-biochemical processes. However, our knowledge on the mechanisms of algal mortality at metabolic level remains limited. Herein, the response of K. mikimotoi to LA was evaluated using metabolomics, stable isotope techniques (SIT), and physiological indicators. Results showed that 100 μg/L LA promoted the growth of K. mikimotoi, which was significantly inhibited by 500 μg/L LA, along with a significant reduction of photosynthetic pigments and a significant increase of reactive oxygen species (ROS). SIT showed that LA entered algal cells, and 56 isotopologues involved in ferroptosis, carotenoid biosynthesis, and porphyrin metabolism were identified. Non-targeted metabolomics identified 90 and 111 differential metabolites (DEMs) belonging to 11 metabolic pathways under the 500 μg/L and 100 μg/L LA exposure, respectively. Among them, 34 DEMs were detected by SIT. Metabolic pathway analysis showed that 500 μg/L LA significantly promoted ferroptosis, and significantly inhibited carotenoid biosynthesis, porphyrin metabolism, sphingolipid metabolism, and lipopolysaccharide biosynthesis, presenting changes opposite to those observed in 100 μg/L LA-treated K. mikimotoi. Overall, this study revealed the metabolic response of K. mikimotoi to LA, enriching our understanding on the allelochemical mechanism of LA on K. mikimotoi.

Light history modulates growth and photosynthetic responses of a diatom to ocean acidification and UV radiation

To examine the synergetic effects of ocean acidification (OA) and light intensity on the photosynthetic performance of marine diatoms, the marine centric diatom Thalassiosira weissflogii was cultured under ambient low CO₂ (LC, 390 μatm) and elevated high CO₂ (HC, 1000 μatm) levels under low-light (LL, 60 μmol m^-2 s^-1) or high-light (HL, 220 μmol m^-2 s^-1) conditions for over 20 generations. HL stimulated the growth rate by 128 and 99% but decreased cell size by 9 and 7% under LC and HC conditions, respectively. However, HC did not change the growth rate under LL but decreased it by 9% under HL. LL combined with HC decreased both maximum quantum yield (F _V/F _M) and effective quantum yield (Φ _PSII), measured under either low or high actinic light. When exposed to UV radiation (UVR), LL-grown cells were more prone to UVA exposure, with higher UVA and UVR inducing inhibition of Φ _PSII compared with HL-grown cells. Light use efficiency (α) and maximum relative electron transport rate (rETR_max) were inhibited more in the HC-grown cells when UVR (UVA and UVB) was present, particularly under LL. Our results indicate that the growth light history influences the cell growth and photosynthetic responses to OA and UVR.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-022-00138-x.

The need for unrealistic experiments in global change biology

Climate change is an existential threat, and our ability to conduct experiments on how organisms will respond to it is limited by logistics and resources, making it vital that experiments be maximally useful. The majority of experiments on phytoplankton responses to warming and CO₂ use only two levels of each driver. However, to project the characters of future populations, we need a mechanistic and generalisable explanation for how phytoplankton respond to concurrent changes in temperature and CO₂. This requires experiments with more driver levels, to produce response surfaces that can aid in the development of predictive models. We recommend prioritising experiments or programmes that produce such response surfaces on multiple scales for phytoplankton.

Monitoring, modeling and projection of harmful algal blooms in China

This review assesses monitoring, modeling and projection of harmful algal blooms (HABs) in China, with a focus on near-term warning and long-term trend projection. We reviewed phytoplankton monitoring as early warning, remote sensing for offshore environments, building models to describe ecological processes, modeling and forecasting near-term or seasonal HAB events, and projection of long-term HAB trend in China. Over the past 40 years, great progresses were made in traditional observation capability of HABs, and some reliable remote sensing algorithms were developed for HABs in optically complex coastal waters in Chinese seas. Numerical models have been applied to simulating real-world algal bloom events successfully; and these models, to some degree, are capable of predicting the time and place of HAB occurrence. In terms of long-term trend, HABs appeared to have shown diversified forms, being more miniaturized and more harmful. The development of an integrated monitoring and early-warning system of algal blooms and HABs should be a necessary first step to provide an effective management tool for mitigating damages associated with the occurrence of HABs in China.

Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming

The occurrence of green-tides, whose bloom potential may be increased by various human activities and biogeochemical process, results in enormous economic losses and ecosystem collapse. In this study, we investigated the ecophysiology of the subtropical green-tide forming alga, Ulva ohnoi complex (hereafter: U. ohnoi), under simulated future ocean conditions in order to predict its bloom potential using photosynthesis and growth measurements, and stable isotope analyses. Our mesocosm system included four experimental conditions that simulated the individual and combined effects of elevated CO₂ and temperature, namely control (450 μatm CO₂ & 20 °C), acidification (900 μatm CO₂ & 20 °C), warming (450 μatm CO₂ & 25 °C), and greenhouse (900 μatm CO₂ & 25 °C). Photosynthetic electron transport rates (rETR) increased significantly under acidification conditions, but net photosynthesis and growth were not affected. In contrast, rETR, net photosynthesis, and growth all decreased significantly under elevated temperature conditions (i.e. both warming and greenhouse). These results represent the imbalance of energy metabolism between electron transport and O₂ production that may be expected under ocean acidification conditions. This imbalance appears to be related to carbon and nitrogen assimilation by U. ohnoi. In particular, ¹³C and ¹⁵N discrimination data suggest U. ohnoi prefers CO₂ and NH₄⁺ over HCO₃^- and NO₃^- as sources of carbon and nitrogen, respectively, and this results in increased N content in the thallus under ocean acidification conditions. Together, our results suggest a trade-off in which the bloom potential of U. ohnoi could increase under ocean acidification due to greater N accumulation and through the saving of energy during carbon and nitrogen metabolism, but that elevated temperatures could decrease U. ohnoi's bloom potential through a decrease in photosynthesis and growth.

Ultraviolet Radiation Stimulates Activity of CO2 Concentrating Mechanisms in a Bloom-Forming Diatom Under Reduced CO2 Availability

The diatom Skeletonema costatum is cosmopolitan and forms algal blooms in coastal waters, being exposed to varying levels of solar UV radiation (UVR) and reduced levels of carbon dioxide (CO₂). While reduced CO₂ availability is known to enhance CO₂ concentrating mechanisms (CCMs) in this diatom and others, little is known on the effects of UV on microalgal CCMs, especially when CO₂ levels fluctuate in coastal waters. Here, we show that S. costatum upregulated its CCMs in response to UVR (295–395 nm), especially to UVA (320–395 nm) in the presence and absence of photosynthetically active radiation (PAR). The intensity rise of UVA and/or UVR alone resulted in an increase of the activity of extracellular carbonic anhydrase (CAe); and the addition of UVA enhanced the activity of CCMs-related CAe by 23–27% when PAR levels were low. Such UV-stimulated CCMs activity was only significant at the reduced CO₂ level (3.4 μmol L⁻¹). In addition, UVA alone drove active HCO₃⁻ uptake although it was not as obvious as CAe activity, another evidence for its role in enhancing CCMs activity. In parallel, the addition of UVA enhanced photosynthetic carbon fixation only at the lower CO₂ level compared to PAR alone. In the absence of PAR, carbon fixation increased linearly with increased intensities of UVA or UVR regardless of the CO₂ levels. These findings imply that during S. costatum blooming period when CO₂ and PAR availability becomes lower, solar UVR (mainly UVA) helps to upregulate its CCMs and thus carbon fixation, enabling its success of frequent algal blooms.

Disentangling effects of river inflow and marine diffusion in shaping the planktonic communities in a heavily polluted estuary

Estuarine ecosystems are important in terms of biodiversity processes because there are intense interactions between the river and sea environments. Phytoplankton and zooplankton have been shown to be ecological indicators of the water quality status in estuary ecosystems. Therefore, a comprehensive evaluation of the effects that multiple pressures have on the phytoplankton and zooplankton communities in estuarine ecosystems is essential. In this study, water samples from 29 stations were collected from the Liaohe Estuary over three different seasons, and biotic factors (i.e., phytoplankton and zooplankton) were obtained and compared. The results showed that there were significant temporal and spatial variations in the phytoplankton and zooplankton communities from the Liaohe Estuary. The correlation analyses showed that water temperature was the most important factor regulating the variation in phytoplankton communities, whereas the main driving force for the zooplankton was nutrient concentrations. Large amounts of nutrients entered the estuary in spring and summer due to intensive human activities in the Liaohe River basin. The inflows by the Liaohe River introduced some phytoplankton and zooplankton into the estuary, such as Coscinodicus asteromphalus, Chaetoceros decipiens, and Schmacheria poplesia. The impacts of Liaohe inflows on the estuary region gradually decreased as the distance from the inlet increased and this change was mediated by marine diffusion. The results from this study will improve knowledge about planktonic communities in estuarine ecosystems and provide a theoretical foundation for estuary environmental management.