Acclimation of bloom-forming and perennial seaweeds to elevated pCO2 conserved across levels of environmental complexity

Loading of microplastics by two related macroalgae in a sea area where gold and green tides occur simultaneously

Macroalgae are important components of offshore ecosystems and can also cause algal blooms. Microplastics (MPs) have been found in macroalgae and exhibit interactions during algal blooms. Ulva prolifera and Sargassum horneri are common algae in the Yellow Sea in China and are also the major macroalgae that cause green and gold tides. However, the loading of MPs by S. horneri along the coast of China has not been investigated, and the loading characteristics of MPs by two related macroalgae in a sea area where gold and green tides occur simultaneously have not been reported. Because U. prolifera and S. horneri occur simultaneously in the Yellow Sea, we investigated the characteristics of MPs in U. prolifera and S. horneri at 9 stations in the Yellow Sea located along the migration paths of green and gold tides. It was found that U. prolifera exhibited a higher abundance of MPs (0.065 ± 0.047 items/g^-1 fresh weight) and ratio of the MP abundance in macroalgae to that in seawater (R_m/R_s = 97.33) than S. horneri (0.016 ± 0.016 items/g^-1 fresh weight, R_m/R_s = 24.56). Moreover, the MPs in U. prolifera showed increased diversity in shape, color and material than those in S. horneri, and the particle sizes in the former were also smaller. Fast-growing biomass, slender branches, hollow air sacs and soft epidermis may allow U. prolifera to load more MPs. However, these mechanisms have not been fully established and merit further study. The relationship between severe algal blooms and MP pollution should be considered.

Independent Evolution of the MYB Family in Brown Algae

Myeloblastosis (MYB) proteins represent one of the largest families of eukaryotic transcription factors and regulate important processes in growth and development. Studies on MYBs have mainly focused on animals and plants; however, comprehensive analysis across other supergroups such as SAR (stramenopiles, alveolates, and rhizarians) is lacking. This study characterized the structure, evolution, and expression of MYBs in four brown algae, which comprise the biggest multicellular lineage of SAR. Subfamily 1R-MYB comprised heterogeneous proteins, with fewer conserved motifs found outside the MYB domain. Unlike the SHAQKY subgroup of plant 1R-MYB, THAQKY comprised the largest subgroup of brown algal 1R-MYBs. Unlike the expansion of 2R-MYBs in plants, brown algae harbored more 3R-MYBs than 2R-MYBs. At least ten 2R-MYBs, fifteen 3R-MYBs, and one 6R-MYB orthologs existed in the common ancestor of brown algae. Phylogenetic analysis showed that brown algal MYBs had ancient origins and a diverged evolution. They showed strong affinity with stramenopile species, while not with red algae, green algae, or animals, suggesting that brown algal MYBs did not come from the secondary endosymbiosis of red and green plastids. Sequence comparison among all repeats of the three types of MYB subfamilies revealed that the repeat of 1R-MYBs showed higher sequence identity with the R3 of 2R-MYBs and 3R-MYBs, which supports the idea that 1R-MYB was derived from loss of the first and second repeats of the ancestor MYB. Compared with other species of SAR, brown algal MYB proteins exhibited a higher proportion of intrinsic disordered regions, which might contribute to multicellular evolution. Expression analysis showed that many MYB genes are responsive to different stress conditions and developmental stages. The evolution and expression analyses provided a comprehensive analysis of the phylogeny and functions of MYBs in brown algae.

Elevated CO2 affects kelp nutrient quality: A case study of Saccharina japonica from CO2 -enriched coastal mesocosm systems

Kelps provide critical services for coastal food chains and ecosystem, and they are important food source for some segments of human population. Despite their ecological importance, little is known about long-term impacts of elevated CO2 (eCO2 ) on nutrient metabolites in kelps and the underlying regulation mechanisms. In this study, the kelp Saccharina japonica was cultured in CO2 -enriched coastal mesocosm systems for up to 3 months. We found that, although eCO2 significantly increased the growth rate, carbon concentrations, and C/N ratio of S. japonica, and it had no effect on total nitrogen and protein contents at the end of cultivation period. Meanwhile, it decreased the lipid, magnesium, sodium, and calcium content and changed the amino acid and fatty acid composition. Combining the genome-wide transcriptomic and metabolic evidence, we obtained a system-level understanding of metabolic response of S. japonica to eCO2 . The unique ornithine-urea cycle (OUC) and aspartate-argininosuccinate shunt (AAS), coupled with TCA cycle, balanced the carbon and nitrogen metabolism under eCO2 by providing carbon skeleton for amino acid synthesis and reduced power for nitrogen assimilation. This research provides a major advance in the understanding of kelp nutrient metabolic mechanism in the context of global climate change, and such CO2 -induced shifts in nutritional value may induce changes in the structure and stability of marine trophic webs and affect the quality of human nutrition resources.

Effects of climate change factors on marine macroalgae: A review

Marine macroalgae, the main primary producers in coastal waters, play important roles in the fishery industry and global carbon cycles. With progressive ocean global changes, however, they are increasingly exposed to enhanced levels of multiple environmental drivers, such as ocean acidification, warming, heatwaves, UV radiation and deoxygenation. While most macroalgae have developed physiological strategies against variations of these drivers, their eco-physiological responses to each or combinations of the drivers differ spatiotemporally and species-specifically. Many freshwater macroalgae are tolerant of pH drop and its diel fluctuations and capable of acclimating to changes in carbonate chemistry. However, calcifying species, such as coralline algae, are very sensitive to acidification of seawater, which reduces their calcification, and additionally, temperature rise and UV further decrease their physiological performance. Except for these calcifying species, both economically important and harmful macroalgae can benefit from elevated CO₂ concentrations and moderate temperature rise, which might be responsible for increasing events of harmful macroalgal blooms including green macroalgal blooms caused by Ulva spp. and golden tides caused by Sargassum spp. Upper intertidal macroalgae, especially those tolerant of dehydration during low tide, increase their photosynthesis under elevated CO₂ concentrations during the initial dehydration period, however, these species might be endangered by heatwaves, which can expose them to high temperature levels above their thermal windows' upper limit. On the other hand, since macroalgae are distributed in shallow waters, they are inevitably exposed to solar UV radiation. The effects of UV radiation, depending on weather conditions and species, can be harmful as well as beneficial to many species. Moderate levels of UV-A (315-400nm) can enhance photosynthesis of green, brown and red algae, while UV-B (280-315nm) mainly show inhibitory impacts. Although little has been documented on the combined effects of elevated CO₂, temperature or heatwaves with UV radiation, exposures to heatwaves during midday under high levels of UV radiation can be detrimental to most species, especially to their microscopic stages which are less tolerant of climate change induced stress. In parallel, reduced availability of dissolved O₂ in coastal water along with eutrophication might favour the macroalgae's carboxylation process by suppressing their oxygenation or photorespiration. In this review, we analyse effects of climate change-relevant drivers individually and/or jointly on different macroalgal groups and different life cycle stages based on the literatures surveyed, and provide perspectives for future studies.

Microplastics in bloom-forming macroalgae: Distribution, characteristics and impacts

Macroalgal blooms and marine microplastics (MPs), as global challenges for oceans, are both showing a rising trend. However, none is known regarding the interaction of these two important issues. The Yellow Sea suffers the world's largest green tides and severe MPs pollution as well. Therefore, we tracked the trapping of MPs by drifting Ulva prolifera in the Yellow Sea during the green-tide period. The abundance of MPs in drifting U. prolifera was 595-3917 times higher than that in seawater and increased along the drifting path from south to north in the Yellow Sea. In addition, four mechanisms of trapping plastics (twining, attachment, embedment, and wrapping) on or in U. prolifera were unmasked, which explains why the plant has such strong capacity to trap MPs. Laboratory incubation experiments showed that MPs (0.025-25 mg L^-1) did not affect relative growth rate, effective photochemical efficiency of photosystem II (PSII), or saturating irradiance of U. prolifera until reaching an extremely high concentration (100 mg L^-1), indicating a high tolerance to MPs. Due to tremendous biomass and coverage of the green tide and increased frequency as well, the plastics trap in drifting macroalgae can alter the spatio-temporal distribution of MPs in the oceans.

Ulva prolifera green-tide outbreaks and their environmental impact in the Yellow Sea, China

The Ulva prolifera green tides in the Yellow Sea, China, which have been occurring since 2007, are a serious environmental problem attracting worldwide attention. Despite extensive research, the outbreak mechanisms have not been fully understood. Comprehensive analysis of anthropogenic and natural biotic and abiotic factors reveals that human activities, regional physicochemical conditions and algal physiological characteristics as well as ocean warming and biological interactions (with microorganism or other macroalgae) are closely related to the occurrence of green tides. Dynamics of these factors and their interactions could explain why green tides suddenly occurred in 2007 and decreased abruptly in 2017. Moreover, the consequence of green tides is serious. The decay of macroalgal biomass could result in hypoxia and acidification, possibly induce red tide and even have a long-lasting impact on coastal carbon cycles and the ecosystem. Accordingly, corresponding countermeasures have been proposed in our study for future reference in ecosystem management strategies and sustainable development policy.

Ocean acidification increases iodine accumulation in kelp-based coastal food webs

Kelp are main iodine accumulators in the ocean, and their growth and photosynthesis are likely to benefit from elevated seawater CO2 levels due to ocean acidification. However, there are currently no data on the effects of ocean acidification on iodine metabolism in kelp. As key primary producers in coastal ecosystems worldwide, any change in their iodine metabolism caused by climate change will potentially have important consequences for global geochemical cycles of iodine, including iodine levels of coastal food webs that underpin the nutrition of billions of humans around the world. Here, we found that elevated pCO2 enhanced growth and increased iodine accumulation not only in the model kelp Saccharina japonica using both short-term laboratory experiment and long-term in situ mesocosms, but also in several other edible and ecologically significant seaweeds using long-term in situ mesocosms. Transcriptomic and proteomic analysis of S. japonica revealed that most vanadium-dependent haloperoxidase genes involved in iodine efflux during oxidative stress are down-regulated under increasing pCO2 , suggesting that ocean acidification alleviates oxidative stress in kelp, which might contribute to their enhanced growth. When consumed by abalone (Haliotis discus), elevated iodine concentrations in S. japonica caused increased iodine accumulation in abalone, accompanied by reduced synthesis of thyroid hormones. Thus, our results suggest that kelp will benefit from ocean acidification by a reduction in environmental stress however; iodine levels, in kelp-based coastal food webs will increase, with potential impacts on biogeochemical cycles of iodine in coastal ecosystems.

Differential Photosynthetic Response of a Green Tide Alga Ulva linza to Ultraviolet Radiation, Under Short- and Long-term Ocean Acidification Regimes

Both ocean acidification (OA) and solar ultraviolet (UV) radiation can bring about changes in macroalgal physiological performance. However, macroalgal responses to UV radiation when acclimatized to OA under different time scales are rare. Here, we investigate the response of Ulva linza, a green tide alga, to UV radiation in the form of photosynthetically active radiation (PAR) or PAB (PAR+UVA+UVB) radiation. Radiation exposures were assessed following long-term (from spore to adult stage, 1 month) and short-term (adult stage, 1 week) OA treatments. Results showed that increased CO₂ decreased the damage rate (k) and repair rate (r) of thalli grown under short-term OA conditions with PAB treatment, the ratio of r:k was not altered. Following long-term OA conditions, r was not affected, although k was increased in thalli following PAB treatment, resulting in a reduced ratio of r:k. The relative level of UV inhibition increased and UV-absorbing compounds decreased when algae were cultured under long-term OA conditions. The recovery rate of thalli was enhanced when grown under long-term OA after UV radiation treatment. These results show that blooming algae may be more sensitive to UV radiation in marine environments, but it can develop effective mechanisms to offset the negative effects, reflecting acclimation to long-term OA conditions.

Photoperiod mediates the effects of elevated CO2 on the growth and physiological performance in the green tide alga Ulva prolifera

Ulva spp., an increasingly important food, are the dominant species of the large-scale green tides. In this study, both the growth and the physiological responses of the Ulva prolifera were studied after cultured in three different light and dark regimes (12:12, 14:10 and 16:8-h light/dark) in combination with current (420 μatm; LC) and increased (1000 μatm; HC) levels of atmospheric CO₂. Grown rate of U. prolifera was significantly enhanced by increased CO₂ under the three light:dark regimes, especially under 16:8 h-light:dark, indicating that growth was C-unsaturated at present CO₂ levels. U. prolifera showed a significantly higher growth rate and lower dark respiration rate (R_d) at 16:8 h-light:dark treatment than at 12:12 h-light/dark treatment, regardless of the CO₂ treatment. The photochemical performance was largely unaffected by elevated CO₂ and daylength. These results suggest that U. prolifera in a future CO₂ enriched coastal water, seems to be resilient to higher CO₂ concentrations, and this could be enhanced by longer daylength.

Evolution and Expansion of the Prokaryote-Like Lipoxygenase Family in the Brown Alga Saccharina japonica

Lipoxygenase (LOX) plays important roles in fatty acid oxidation and lipid mediator biosynthesis. In this study, we give first insights into brown algal LOX evolution. Whole genome searches revealed four, three, and eleven LOXs in Ectocarpus siliculosus, Cladosiphon okamuranus, and Saccharina japonica, respectively. In phylogenetic analyses, LOXs from brown algae form a robust clade with those from prokaryotes, suggesting an ancestral origin and slow evolution. Brown algal LOXs were divided into two clades, C1 and C2 in a phylogenetic tree. Compared to the two species of Ectocarpales, LOX gene expansion occurred in the kelp S. japonica through tandem duplication and segmental duplication. Selection pressure analysis showed that LOX genes in brown algae have undergone strong purifying selection, while the selective constraint in the C2 clade was more relaxed than that in the C1 clade. Furthermore, within each clade, LOXs of S. japonica evolved under more relaxed selection constraints than E. siliculosus and C. okamuranus. Structural modeling showed that unlike LOXs of plants and animals, which contain a β barrel in the N-terminal part of the protein, LOXs in brown algae fold into a single domain. Analysis of previously published transcriptomic data showed that LOXs in E. siliculosus are responsive to hyposaline, hypersaline, oxidative, and copper stresses. Moreover, clear divergence of expression patterns was observed among different life stages, as well as between duplicate gene pairs. In E. siliculosus, all four LOXs are male-biased in immature gametophytes, and mature gametophytes showed significantly higher LOX mRNA levels than immature gametophytes and sporophytes. In S. japonica, however, our RNA-Seq data showed that most LOXs are highly expressed in sporophytes. Even the most recently duplicated gene pairs showed divergent expression patterns, suggesting that functional divergence has likely occurred since LOX genes duplicated, which potentially contributes to the production of various oxylipins in brown algae.