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Abstract 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.

Based on the diversity and importance of phycospheric microorganisms as well as their complex interaction with algae, their ecological correlation with algal bloom events has become a research topic of great interest that remains unclear in the natural dynamic process of harmful macroalgal blooms (HMBs). The world’s largest green tides caused by macroalgae Ulva prolifera have occurred in the Yellow Sea for 16 consecutive years, and seriously affected the coastal ecosystem. Here, we monitored the spatiotemporal dynamics of the phycospheric bacterial community during the U. prolifera green tide bloom. The 73-day continuous field survey covered the whole process of initial invasion until the extinction of the green tide. The phycospheric bacterial community has a higher richness than the control seawater bacteria; in addition, it has more enriched taxa with organic nutrition preference and environmental pressure adaption, such as Bacteroidetes, Firmicutes, and Desulfobacterota. The same differences in populations and functional profiles were observed among the epiphytic bacterial and phycospheric seawater bacterial communities, which may be driven by the spatial heterogeneity of biogenic elements (e.g., organic nutrients and oxygen) among the habitats. Significant succession occurred in both the epiphytic bacterial and phycospheric seawater bacterial communities; their diversity and richness exhibited significant heterogeneity variation patterns, and their community structure became more similar in late phases. Our study revealed that macroalgal phycospheric microbial communities have a highly complex and dynamic composition and variable ecological functions, which indicate that phycospheric microorganisms are closely related to the fate of HMBs and may have the profound effect coastal biogeochemical cycles.

Marine macrophytes (seagrasses and macroalgae) and their epiphytic microorganisms play an important role in the ecological and biochemical processes of coastal oceans. However, simultaneous comparative studies on the biodiversity and functions of epiphytic bacteria and fungi associated with marine macrophytes have not been conducted. In this study, high-throughput sequencing technology was used to describe the epiphytic bacterial and fungal communities of 11 common macroalgae and 2 seagrasses from an intertidal zone of northern China and compare them with seawater communities. The results showed that Proteobacteria and Bacteroidota were the dominant bacterial phyla in marine macrophytes, whereas Ascomycota, Chytridiomycota, and Basidiomycota were the dominant fungal phyla. The alpha diversity of the bacterial and fungal communities in seagrasses was the highest of all macrophyte samples. This may have been related to their ability to recruit microorganisms from multiple sources. Host phylogeny may influence bacterial community structure, and geographical differences may influence fungal community structure. The FAPROTAX data indicated that C metabolic microbes were enriched in marine macrophytes, while the FUNGuild data indicated that undefined saprotroph, which participated in organic matter degradation, were also enriched in marine macrophytes. These findings provide a theoretical basis regarding the epiphytic microorganisms of macrophytes and may offer new insights to support the improved ecological restoration of seagrass and macroalgae beds.

Abstract 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.

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.

Large-scale green tides have occurred continuously in the Yellow Sea of China from 2007 to 2018, and the causative species of the Yellow Sea green tide (YSGT) is Ulva prolifera. The thalli form floated thallus mats, and the thalli from different layers of the thallus mat suffer significantly different environmental conditions. In the present study, the environmental conditions of the surface layer (SL), middle layer (ML), and lower layer (LL) of the thallus mat from mid-June (Stage I) to mid-July (Stage II) were simulated. Photosynthetic traits and antioxidant systems were measured. The results showed that (1) photoprotective [non-photochemical quenching (NPQ) and cyclic electron transport (CEF)] and antioxidant systems both play important roles in protecting against abiotic factors in U. prolifera. (2) Cooperation between NPQ and CEF was observed in the ML group; CEF and the antioxidant system in the SL group work synergistically to protect the thalli. Furthermore, an inferred spatiotemporal attribute regarding the YSGT is presented: the significant changes in abiotic factors on the sea surface can easily affect the thalli of SL and ML from mid-June to mid-July, and those of LL can be affected in mid-July. This cooperation combined with the spatiotemporal attributes offers an explanation for the annual occurrence of the YSGT.HIGHLIGHTS –Adaptive mechanisms of Ulva prolifera against abiotic factors. –Cooperation between photosynthetic and antioxidant systems. –Spatiotemporal attributes regarding the Yellow Sea green tide are presented.

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. [ABSTRACT FROM AUTHOR]

The aquatic toxicity and ecological risks of naphthenic acids (NAs) in marine environments have attracted an increasing amount of attention. However, there remains a lack of methodologies for the long-term risk assessment of NAs on marine ecosystems after high acid crude oil spill accidents. In this study, using the model microalgae Phaeodactylum tricornutum as the target object, the time-effect manner under NAs stress is investigated for a continuous 24–144 h. We found that: 1) NAs caused photosynthetic damage and persistent oxidative stress that slowed the growth rate and limited the maximum growth of P. tricornutum population within 24 h to 144 h of exposure, especially under the high concentration treatment; 2) Within 144 h, NAs can cause oxidative stress to P. tricornutum. The damage to cell membrane and radical oxidative species (ROS) accumulation of P. tricornutum were observed as obvious time-effect; 3) Under NAs stress, the two types of cell death (accidental cell death and regulated cell death) of P. tricornutum cell mediated by ROS played different roles in the population growth inhibition of P. tricornutum. Moreover, regulated cell death of the P. tricornutum cell was accompanied by PS externalization, DNA fragment and the G2/M phase stagnation acted as an adaptive regulatory mechanism under NAs stress. This explained the dose-time-effects of NAs on the population growth of P. tricornutum. Overall, the results suggested that NAs have a lasting effect on marine phytoplankton populations, and long-term risk assessments are required after high acid crude oil spill accidents. This is the first attempt to identify the different types of death at the cellular level to explain the time-effect toxicity at the population level of marine microalgae when exposed to NAs. This research will provide a new approach to facilitate further risk assessments for NAs and related contaminants in marine ecosystems. The toxic effects of naphthenic acids (NAs) on Phaeodactylum tricornutum were observed in consecutive 24–144 h. It was found that NAs has persistent photosynthetic damage and oxidative damage to P. tricornutum. Two different oxidative damage occurred in different time periods from 24 h to 144 h, causing accidental cell death and regulated cell death respectively. Two kinds of death together created the suppression of population density growth under the stress of NAs. [Display omitted] • Naphthenic acids caused photosynthetic damage and persistent oxidative stress. • Cell membrane damage and ROS accumulation showed time-effects. • ACD and RCD played different roles in population growth inhibition. • RCD acted as an adaptive mechanism that explained the dose-time effects of NAs. [ABSTRACT FROM AUTHOR]