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Harmful algal blooms (HABs) represent a significant global marine ecological disaster. In the Yellow Sea, green and golden tides often occur simultaneously or sequentially, suggesting that interspecific competition involves not only spatial and resource competition but also allelopathy. This study investigated the allelopathic interactions between Ulva prolifera and Sargassum horneri using physiological and biochemical parameters, including relative growth rate (RGR), cell ultrastructure, chlorophyll fluorescence, enzyme activity, and metabolomics analysis. The results showed that S. horneri filtrate significantly inhibited U. prolifera growth, while U. prolifera filtrate had no significant effect on S. horneri. Both algal filtrates caused cellular damage and affected photosynthesis, enzyme activities, and metabolism. However, their allelopathic responses differed: U. prolifera may rely on internal compensatory mechanisms, while S. horneri may depend on defense strategies. These findings provide insights into the dynamics of green and golden tides and support the scientific control of HABs through allelopathy.

IntroductionHeat stress is one of the main threats for the seaweed aquaculture, leading to the decay of the blades and causing serious economic losses. A newly developed biostimulant, Sargassum horneri extract (SHE), can enhance the heat-tolerance in Neopyropia yezoensis, which can be applied in the seaweed aquaculture. However, the molecular mechanism underlying this acquired heat-tolerance is still unclear.MethodIn this study, we examined the transcriptional regulation of N. yezoensis under heat stress after the SHE pre-treatment by performing the RNA-Seq on illumina Novaseq platform and differentially expressed gene analysis.Result and discussionOur results show that, under heat stress, the SHE pre-treatment can alleviate the suppression of genes involved in normal cell activities, such as ribosome biogenesis, photosynthesis, respiration and cell cycle. On the other hand, SHE pre-treatment can also induce some key genes involved in the heat-tolerance. We also found that the combination of SHE pre-treatment and heat stress can induce some genes related to epigenetic modification, suggesting that SHE may facilitate the formation of heat stress memory in N. yezoensis. Our study provides information for understanding the molecular mechanism of heat-tolerance in N. yezoensis and for promoting the application of seaweed-derived biostimulants in the seaweed aquaculture.

Climate change directly influences species composition and distribution of macroalgae on a global scale. Meanwhile, eutrophication and heavy metal pollution continue to be major concerns in coastal areas. The objective of this study was to explore interactive effects of these complex environmental conditions (high temperature, eutrophication and heavy metal pollution) in the bloom forming green alga, Ulva prolifera. This study evaluated the physiological characteristics of two strains of U. prolifera (Korean strain and Chinese strain) at two temperature levels (20 and 25°C), two nutrient concentrations (low nutrient: 50 μM of N and 5 μM of P; high nutrient: 250 μM of N and 25 μM of P) and three copper concentrations [Control (0.03 μM); 0.1 μM; 1 μM]. Under the 20°C and low nutrient condition, the Chinese strain grew significantly slower at 1μM copper group compared to copper control group. Whereas there was no significant change in the growth rate of the Korean strain. High nutrient significantly increased the growth rate, photosynthetic and respiration rates, soluble protein and tissue N in both strains at all copper groups under 20°C compared to copper groups under 20°C and low nutrient condition. Both strains grew significantly slower at 1 μM copper group compare to copper control group under high nutrient and 20°C condition. 25°C significantly reduced the growth rate of both strains at each nutrient condition. Under 25°C and low nutrient condition, 1 μM copper had no effects on growth rate and all photosynthetic parameters of both strains. Meanwhile, under 25°C and high nutrient conditions, 1 μM copper significantly inhibited growth and photosynthetic rates and increased soluble protein content in the Chinese strain compared to copper control treatment. These results suggest that the Korean strain had higher tolerance to the copper stress than the Chinese strain. Higher nutrient levels enhanced the temperature tolerance and the copper stress in U. prolifera. Increased temperature reduced the negative effects caused by copper stress in U. prolifera.

Acadian marine plant extract powder (AMPEP) and Kelpak are commercial biostimulants derived from brown algae Ascophyllum nodosum. This study was to determine if AMPEP and Kelpak can induce thermal resistance in Pyropia yezoensis. P. yezoensis blades were exposed to different concentrations (control: 0, low: 0.001, high: 1 ppm) of AMPEP and Kelpak at 10°C for 6 and 7 days, respectively. Those blades were then cultivated in von Stosch enriched seawater medium at different temperatures (10, 15, 20, and 25°C) with 12: 12 L: D photoperiod and 100 µmol m-2 s-1 of photosynthetically active radiation for additional 15 days. Results showed that P. yezoensis reproduced archeospores at 20 and 25°C at all biostimulant conditions within 15 days. At lower temperatures (10 and 15°C), only AMPEP-treated P. yezoensis reproduced archeospores. P. yezoensis exposed to 1 ppm Kelpak exhibited higher phycoerythrin and phycocyanin contents than control and 0.001 ppm conditions at 15°C. AMPEP-treated conditions showed higher phycoerythrin and phycocyanin contents than control at 10°C. These results suggest that AMPEP and Kelpak may not enhance the thermal resistance of P. yezoensis. However, AMPEP stimulated archeospores release at lower temperatures. The treatment of AMPEP and Kelpak also increased the pigment contents in P. yezoensis. These results suggest that the use of seaweed-derived biostimulants can provide some economic benefits in P. yezoensis aquaculture. The enhancement of archeospores formation by AMPEP at lower temperature may also increase the productivity since Pyropia farming relies on the accumulation of secondary seedings via asexual reproduction. [ABSTRACT FROM AUTHOR]

Grateloupia turuturu is a red alga with a flat but firm slippery thallus. Throughout its lifetime, this alga experiences a wide range of environmental stresses in the intertidal rocky shores. The aim of this study is to investigate the effect of salinity and irradiance on the early developmental stages of G. turuturu tetrasporophytes. The released carpospores were cultivated at different salinities (S = 15, 25, and 35) and irradiances (50, 100, and 200 µmol photons m-2 s-1). Germination of carpospores and development of juvenile tetrasporophytes were observed every 5 days and recorded by a digital camera. Discoid crusts were formed at all conditions within 5 days. The discoid crusts at 200 µmol photons m-2 s-1 died within 20 days regardless the salinity. The discoid crusts at S = 35 also died at all irradiance conditions within 25 days. Except for those at S = 35 and 200 µmol photons m-2 s-1, the discoid crusts reached about 8,000-9,000 µm² by day 20. Regardless of irradiance, the upright thalli formation rate from discoid crusts was 85 and 10% at S = 15 and S = 25, respectively. These results suggest that salinity and irradiance are important factors influencing early developmental stages of G. turuturu. [ABSTRACT FROM AUTHOR]