Acclimation of intertidal macroalgae Ulva prolifera to UVB radiation: the important role of alternative oxidase.

BACKGROUND: Solar radiation is primarily composed of ultraviolet radiation (UVR, 200 - 400 nm) and photosynthetically active radiation (PAR, 400 - 700 nm). Ultraviolet-B (UVB) radiation accounts for only a small proportion of sunlight, and it is the primary cause of plant photodamage. The use of chlorofluorocarbons (CFCs) as refrigerants caused serious ozone depletion in the 1980s, and this had led to an increase in UVB. Although CFC emissions have significantly decreased in recent years, UVB radiation still remains at a high intensity. UVB radiation increase is an important factor that influences plant physiological processes. Ulva prolifera, a type of macroalga found in the intertidal zone, is intermittently exposed to UVB. Alternative oxidase (AOX) plays an important role in plants under stresses. This research examines the changes in AOX activity and the relationships among AOX, photosynthesis, and reactive oxygen species (ROS) homeostasis in U. prolifera under changes in UVB and photosynthetically active radiation (PAR). RESULTS: UVB was the main component of solar radiation impacting the typical intertidal green macroalgae U. prolifera. AOX was found to be important during the process of photosynthesis optimization of U. prolifera due to a synergistic effect with non-photochemical quenching (NPQ) under UVB radiation. AOX and glycolate oxidase (GO) worked together to achieve NADPH homeostasis to achieve photosynthesis optimization under changes in PAR + UVB. The synergism of AOX with superoxide dismutase (SOD) and catalase (CAT) was important during the process of ROS homeostasis under PAR + UVB. CONCLUSIONS: AOX plays an important role in the process of photosynthesis optimization and ROS homeostasis in U. prolifera under UVB radiation. This study provides further insights into the response of intertidal macroalgae to solar light changes.

Structure-Function Covariation of Phycospheric Microorganisms Associated with the Typical Cross-Regional Harmful Macroalgal Bloom.

Unravelling the structure-function variation of phycospheric microorganisms and its ecological correlation with harmful macroalgal blooms (HMBs) is a challenging research topic that remains unclear in the natural dynamic process of HMBs. During the world's largest green tide bloom, causative macroalgae Ulva prolifera experienced dramatic changes in growth state and environmental conditions, providing ideal scenarios for this investment. Here, we assess the phycospheric physicochemical characteristics, the algal host's biology, the phycospheric bacterial constitutive patterns, and the functional potential during the U. prolifera green tide. Our results indicated that (i) variation in the phycosphere nutrient structure was closely related to the growth state of U. prolifera; (ii) stochastic processes govern phycospheric bacterial assembly, and the contribution of deterministic processes to assembly varied among phycospheric seawater bacteria and epiphytic bacteria; (iii) phycospheric seawater bacteria and epiphytic bacteria exhibited significant heterogeneity variation patterns in community composition, structure, and metabolic potential; and (iv) phycospheric bacteria with carbon or nitrogen metabolic functions potentially influenced the nutrient utilization of U. prolifera. Furthermore, the keystone genera play a decisive role in the structure-function covariation of phycospheric bacterial communities. Our study reveals complex interactions and linkages among environment-algae-bacterial communities which existed in the macroalgal phycosphere and highlights the fact that phycospheric microorganisms are closely related to the fate of the HMBs represented by the green tide. IMPORTANCE Harmful macroalgal blooms represented by green tides have become a worldwide marine ecological problem. Unraveling the structure-function variation of phycospheric microorganisms and their ecological correlation with HMBs is challenging. This issue is still unclear in the natural dynamics of HMBs. Here, we revealed the complex interactions and linkages among environment-algae-bacterial communities in the phycosphere of the green macroalgae Ulva prolifera, which causes the world's largest green tides. Our study provides new ideas to increase our understanding of the variation patterns of macroalgal phycospheric bacterial communities and the formation mechanisms and ecological effects of green tides and highlights the importance of phycospheric microorganisms as a robust tool to help understand the fate of HMBs.

Balancing Damage via Non-Photochemical Quenching, Phenolic Compounds and Photorespiration in Ulva prolifera Induced by Low-Dose and Short-Term UV-B Radiation.

The Yellow Sea green tide (YSGT) is the world's largest transregional macroalgal blooms, and the causative species Ulva prolifera (U. prolifera) suffers from ultraviolet-b radiation (UVBR) during the floating migration process. Previous study confirmed that U. prolifera displayed a wide variety of physiological responses characterized as acclimation to UVBR, while the response mechanisms against low-dose and short-term radiation (LDSTR) are not clear. A study with photosynthetically active radiation (PAR) and UVBR was designed: normal light (NL: 72 µmol photons m-2 s-1), NL+0.3 (UVBR: 0.3 W·m-2), and NL+1.6 (UVBR: 1.6 W·m-2). The results showed that high-dose UVBR inhibited photosynthesis in thalli, especially under long-term exposure, while a variety of physiological responses were observed under LDSTR. The inhibition of photosynthesis appeared to be ameliorated by the algae under LDSTR. Further analysis showed that U. prolifera achieved balancing damage by means of non-photochemical quenching (NPQ), accumulation of phenolic compounds coupled with the ASA-GSH cycle involved in the antioxidant process and enhanced photorespiratory metabolism under LDSTR. This study provides new insights into the balancing damage mechanisms of U. prolifera under LDSTR, enabling the thalli to adapt to the light conditions during the long duration and distance involved in floating migration.

Physiological functional traits explain morphological variation of Ulva prolifera during the drifting of green tides.

Ulva prolifera green tides, one of the greatest marine ecological disasters, originate in the southern Yellow Sea of China and obtain the highest biomass in Haizhou Bay (latitude around 35° N) during northward drift. U. prolifera shows different morphologies from southern Haizhou Bay (SH) to northern Haizhou Bay (NH). Owing to the distinct nutrient environments between SH and NH, we hypothesized that thalli in NH with poor nutrients increased the surface area to volume ratio (SA:VOL) to better absorb nutrients. Here, we tested this hypothesis by comparing the SA:VOL of thalli in SH and NH. The results showed that the thalli in NH had a lower SA:VOL than those in SH, and SA:VOL had positive relationships with temperature and nutrients, contrary to the general hypothesis. The novel results suggested that morphological differences of U. prolifera were the result of developmental state rather than environmental acclimation. Indicators of reproduction (reproductive allocation ratio) were negatively related to variation in tissue contents of C, N, P, and crude protein, whereas indicators of growth (tissue contents of C, N, P, and crude protein) showed significant positive influences on SA:VOL. The results indicated that a trade-off relationship between reproduction and growth existed in the northward drift. All the results suggested that physiological functional traits affected morphological variation of U. prolifera in different environmental conditions during the drifting of green tides. This study presents new insights into the opportunist species nature of U. prolifera through morphological variation and associated functional consequences.

Diel metabolism of Yellow Sea green tide algae alters bacterial community composition under in situ seawater acidification of coastal areas.

Ocean acidification in coastal seawaters is a complex process, with coastal pH being affected by numerous factors including watershed and biological processes that also support metabolically diverse bacterial communities. The world's largest macroalgal blooms have occurred consecutively in the Yellow Sea over the last 13 years. In particular, algal mats formed by Yellow Sea green tides (YSGT) significantly influence coastal environments. Herein, we hypothesized that 1) inorganic carbonate chemistry in coastal areas is altered by diel metabolism of these giant algal mats and that 2) bacterial community composition in diffusive boundary layers might be altered along diel cycles due to algal mat metabolism. In situ studies indicated that algal mat metabolism led to changes in diel pH and CO2 in affected seawaters. Such metabolic activities could intensify diel pH fluctuations in algal mat diffusive boundary layers, as noted by pH fluctuations of 0.22 ± 0.01 units, and pCO2 fluctuations of 214.62 ± 29.37 µatm per day. In contrast, pH fluctuations of 0.11 ± 0.02 units and pCO2 fluctuations of 79.02 ± 42.70 µatm were noted in unaffected areas. Furthermore, the bacterial community composition associated with diffusive algal boundary layers, including those of ambient bacteria and epiphytic bacteria, exhibited diel changes, while endophytic bacterial communities were relatively stable. Flavobacteriaceae were particularly highly abundant taxa in the ambient and epiphytic bacterial communities and exhibited increased abundances at night but sharp decreases in abundances during daytime. Flavobacteriaceae are heterotrophic taxa that could contribute to coastal area acidification at night due to the transformation of organic carbon to inorganic carbon. These results provide new insights to understand the variability in coastal ocean acidification via harmful algal blooms while providing a framework for evaluating the effects of YSGT on costal carbon cycling.

Bacteria associated with Ulva prolifera: a vital role in green tide formation and migration.

Ulva prolifera green tide in the Yellow Sea of China is a typical cross-regional marine ecological disaster. We hypothesized that the complex interactions between U. prolifera and its associated bacterial communities possibly impact the formation and outbreak of green tide. To test this hypothesis, the U. prolifera-associated bacterial community changes in the entire migration area were investigated through field sampling and high-throughput sequencing. The results showed that (1) with the green tide migration, the richness and diversity increased for U. prolifera epiphytic bacterial communities, while they decreased for seawater bacterial communities in the phycosphere. (2) The richness, diversity, and community composition of U. prolifera-associated bacteria changed more dramatically in the 35.00°N sea area. (3) Potential interactions between bacteria and U. prolifera existed during the entire long-distance migration of green tide, and six bacterial functional groups (BFGs) were defined. Growth-regulating BFG I and antibacterial and stress-resistance BFG II were the dominant communities in the early stage of the green tide migration, which have the role of regulating algal growth and synergistic protection. Heterotrophic BFG III and algicidal BFG IV were the dominant communities in the late stage of the green tide migration, and they were able to compete with algae for nutrients and inhibit algal growth. Nutritive BFG V and algae-derived nutritional type BFG VI symbiotically lived with algal host. Our study highlights the spatial and temporal complexity of U. prolifera-associated bacterial communities and provides valuable insights into the potential contribution of U. prolifera-associated bacterial communities to green tide outbreaks.

The complete mitochondrial genome of Mactra quadrangularis (Mactridae).

The whole mitochondrial genome sequence of Mactra quadrangularis (Reeve, 1854) was determined. It had a total length of 16,848 bp and it contained 12 protein coding genes, 2 ribosome RNA genes, and 22 transfer RNA genes. The base composition was 25.75% A, 20.82% G, 11.53% C, and 41.90% T, respectively. Furthermore, state codon of ND4 was ATT; ND1 and CYTB were ATA; COX1 was GTG; ND5, COX2, ND4L, ND6, ND2, COX3, ATP6, and ND3 were ATG. Phylogenetic analysis demonstrated that M. quadrangularis was most closely related to Mactra chinensis. The mitochondrial genome will provide reference for the further investigation and research of M. quadrangularis.

Competitive advantages of Ulva prolifera from Pyropia aquaculture rafts in Subei Shoal and its implication for the green tide in the Yellow Sea.

The physiological characteristics of Ulva prolifera and Blidingia sp. during two pre-bloom stages (March & May) were compared to evaluate the competitive advantage of U. prolifera on Pyropia aquaculture rafts in Subei Shoal. (1) Compared to Blidingia sp., U. prolifera had a lower growth rate, chlorophyll content, photosynthetic efficiency, and antioxidant capacity in March. (2) In May, various indicators of U. prolifera's physiological function improved significantly, while the antioxidant capacity of Blidingia sp. decreased significantly. Large lipidic globules in U. prolifera cells became scattered small lipidic globules in May, which indicated a decrease in lipid membrane peroxidation. (3) In U. prolifera, the ratio of buoyancy to gravity of per unit volume was 1.73, and the bubbles inside the thalli provided 60% of the total buoyancy. Buoyancy generated by the inflatable structure of U. prolifera allowed this species to float after being separated from the rafts.

Ecological effects of Ulva prolifera green tide on bacterial community structure in Qingdao offshore environment.

Ulva prolifera green tide is a serious marine ecological problem in China's coastal areas, with recurrent outbreaks occurring annually during late spring and summer since 2007. Marine bacteria communities are closely linked with important ecological functions in coastal environments. Hence, the succession of bacterial community structures in seawater and sediments during the outbreak and extinction stages of U. prolifera green tide were studied in this paper. The results revealed that: (1) the outbreak of a green tide led to changes in the bacterial community structure and a reduction in the abundance and diversity of the bacterial community in the Qingdao offshore region. This change was more significant in sediments than in seawater. (2) The bacterial assemblage appeared to be dominated by members of Deltaproteobacteria, Bacteroidia and Flavobacteria in sediment, and reductions in the relative abundance of Flavobacteria and Alphaproteobacteria were also observed in seawater after the green tide outbreak. There was also a high similarity in species composition of bacteria in sediment (extinction stage) and seawater (outbreak stage). (3) The outbreak of the U. prolifera affected the abundance of functional bacterial communities in the offshore environment such as SRB (sulfate-reducing bacteria), CFB (Cytophaga-Flavobacter-Bacteroides), heterotrophic bacteria and the potentially algicidal bacteria Alteromonadaceae. Overall, U. prolifera affects aquatic ecosystem secondary production, biogenic geochemical cycles and the growth of other seaweeds in Qingdao offshore areas by driving variations in functional bacterial communities.

Toxicological proteomic responses of halophyte Suaeda salsa to lead and zinc.

The long term (30 days) toxicological effects of environmentally relevant concentrations of Pb2+ (20µg/L) and Zn2+ (100µg/L) were characterized in Suaeda salsa using proteomics techniques. The responsive proteins were related to metabolism (Krebs cycle and Calvin cycle), protein biosynthesis, stress and defense, energy, signaling pathway and photosynthesis in Pb2+, Zn2+ and Pb2++ Zn2+ exposed groups in S. salsa after exposures for 30 days. The proteomic profiles also showed differential responses in S. salsa to metal exposures. In Pb2+-treated group, the proteins were categorized into cystein metabolism and pentose phosphate pathway. The responsive proteins were basically involved in glutathione metabolism, glycolysis, cystein and methane metabolism, and voltage-dependent anion channel in Zn2+-treated group. In Pb2++ Zn2+-treated group, the proecular mechanism at protein level remtein responses were devided into tyrosine metabolism and glycolysis. Our results showed that the two typical heavy metals, lead and zinc, could induce toxicological effects in halophyte S. salsa at protein level.