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Ulva is the dominant genus in the green tide events and is considered to have efficient CO2 concentrating mechanisms (CCMs). However, little is understood regarding the impacts of ocean acidification on the CCMs of Ulva and the consequences of thalli's acclimation to ocean acidification in terms of responding to environmental factors. Here, we grew a cosmopolitan green alga, Ulva linza at ambient (LC) and elevated (HC) CO2 levels and investigated the alteration of CCMs in U. linza grown at HC and its responses to the changed seawater carbon chemistry and light intensity. The inhibitors experiment for photosynthetic inorganic carbon utilization demonstrated that acidic compartments, extracellular carbonic anhydrase (CA) and intracellular CA worked together in the thalli grown at LC and the acquisition of exogenous carbon source in the thalli could be attributed to the collaboration of acidic compartments and extracellular CA. Contrastingly, when U. linza was grown at HC, extracellular CA was completely inhibited, acidic compartments and intracellular CA were also down-regulated to different extents and thus the acquisition of exogenous carbon source solely relied on acidic compartments. The down-regulated CCMs in U. linza did not affect its responses to changes of seawater carbon chemistry but led to a decrease of net photosynthetic rate when thalli were exposed to increased light intensity. This decrease could be attributed to photodamage caused by the combination of the saved energy due to the down-regulated CCMs and high light intensity. Our findings suggest future ocean acidification might impose depressing effects on green tide events when combined with increased light exposure.

Copper (Cu) is an essential element for macroalgae and has been extensively studied, but the interactive effects of temperature and Cu on these organisms remain less understood. In this study, we measured the photosynthetic characteristics of Ulva lactuca exposed to varying Cu concentrations and different temperatures (10 °C, 15 °C, and 20 °C). The results indicated that at the same temperature, as the concentration of Cu increased, the relative growth rate of U. lactuca showed a decreasing trend. Under three different temperatures, the photosynthetic rate and chlorophyll content of the algae significantly decreased with the increase in Cu concentration. Under the same Cu concentration conditions, as the temperature rises, the RGR of the algae gradually increases. In the case of low Cu concentration (LCu), the net photosynthetic rate at 15 °C and 20 °C increased by 103.72% and 104.97%, respectively, compared to the rate at 10 °C. Under high Cu concentration (HCu), the net photosynthetic rate at 15 °C and 20 °C increased by 192.18% and 245.67%, respectively, compared to that at 10 °C. The pigment content showed a similar trend. These results indicated that under the same temperature conditions, high concentrations of Cu inhibited the growth of algae, while under the same Cu treatment conditions, a suitable increase in temperature could alleviate the toxic effects of Cu on the algae.

Species delineation in the genus Ulva is difficult due to a lack of distinguishing morphological characters and the high degree of phenotypic plasticity observed in these algae; thus, species descriptions are necessarily based on a limited set of characters. The present study uses molecular data to test species hypotheses for and explore species diversity in Ulva in the northeast Pacific. Samples of 21 taxa were collected from Valdez, Alaska to San Diego, California, additional Pacific locales outside this region, and Europe. Sequences from ITS nrDNA and the rbcL gene were analysed separately and simultaneously using maximum parsimony to reconstruct phylogeny. Molecular data resolve many of the species recognized by early and more recent treatments, reveal unanticipated potentially conspecific taxa and suggest that certain Ulva species are more widely distributed than may have been recognized previously. At least 12 species of Ulva were found in the northeast Pacific based on the present data: Ulva californica , U. intestinalis , U. lactuca , U. linza , U. lobata , U. pertusa , U. prolifera , U. pseudocurvata , U. rigida , U. stenophylla , U. taeniata and U. tanneri . Other Ulva species previously reported in this region were not encountered during the present study. In addition to providing insights into the systematics of Ulva , this paper indicates areas for additional research for this ubiquitous genus.

Green algae blooms of the genus Ulva are occurring globally and are primarily attributed to anthropogenic factors. At Los Tubos beach in Algarrobo Bay along the central Chilean coast, there have been blooms of these algae that persist almost year-round over the past 20 years, leading to environmental, economic, and social issues that affect the local government and communities. The objective of this study was to characterize the species that form these green tides based on a combination of ecological, morpho-anatomical, and molecular information. For this purpose, seasonal surveys of beached algal fronds were conducted between 2021 and 2022. Subsequently, the sampled algae were analyzed morphologically and phylogenetically using the molecular markers ITS1 and tufA, allowing for the identification of at least five taxa. Of these five taxa, three (U. stenophylloides, U. uncialis, U. australis) have laminar, foliose, and distromatic morphology, while the other two (U. compressa, U. aragoensis) have tubular, filamentous, and monostromatic fronds. Intertidal surveys showed that U. stenophylloides showed the highest relative coverage throughout the seasons and all intertidal levels, followed by U. uncialis. Therefore, we can establish that the green tides on the coast of Algarrobo in Chile are multispecific, with differences in relative abundance during different seasons and across the intertidal zone, opening opportunities for diverse future studies, ranging from ecology to algal biotechnology.

Macroalgal blooms occur worldwide and have the potential to cause severe ecological and economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have biphasic life cycles with separate haploid and diploid phases. In this study, we used flow cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom formation. Both haploid gametophytes and diploid sporophytes were present for both species. Sites ranged from a relative overabundance of gametophytes to a relative overabundance of sporophytes, compared to the null model prediction of √2 gametophytes: 1 sporophyte. We found significant differences in cell area between ploidy levels for each species, with sporophyte cells significantly larger than gametophyte cells in U. compressa and U. rigida. We found no differences in relative growth rate between ploidy levels for each species. Our results indicate the presence of both phases of each of the two dominant bloom forming species throughout the bloom season, and represent one of the first studies of in situ Ulva life cycle dynamics.

Axenic gametes of the marine green macroalga Ulva mutabilis Føyn (Ria Formosa, locus typicus) exhibit abnormal development into slow-growing callus-like colonies with aberrant cell walls. Under laboratory conditions, it was previously demonstrated that all defects in growth and thallus development can be completely abolished when axenic gametes are inoculated with a combination of two specific bacterial strains originally identified as Roseobacter sp. strain MS2 and Cytophaga sp. strain MS6. These bacteria release diffusible morphogenetic compounds (= morphogens), which act similar to cytokinin and auxin. To investigate the ecological relevance of the waterborne bacterial morphogens, seawater samples were collected in the Ria Formosa lagoon (Algarve, Southern Portugal) at 20 sampling sites and tidal pools to assess their morphogenetic effects on the axenic gametes of U. mutabilis. Specifically the survey revealed that sterile-filtered seawater samples can completely recover growth and morphogenesis of U. mutabilis under axenic conditions. Morphogenetic activities of free-living and epiphytic bacteria isolated from the locally very abundant Ulva species (i.e., U. rigida) were screened using a multiwell-based testing system. The most represented genera isolated from U. rigida were Alteromonas, Pseudoalteromonas and Sulfitobacter followed by Psychrobacter and Polaribacter. Several naturally occurring bacterial species could emulate MS2 activity (= induction of cell divisions) regardless of taxonomic affiliation, whereas the MS6 activity (= induction of cell differentiation and cell wall formation) was species-specific and is probably a feature of difficult-to-culture bacteria. Interestingly, isolated bacteroidetes such as Algoriphagus sp. and Polaribacter sp. could individually trigger complete Ulva morphogenesis and thus provide a novel mode of action for bacterial-induced algal development. This study also highlights that the accumulation of algal growth factors in a shallow water body separated from the open ocean by barrier islands might have strong implications to, for example, the wide usage of natural coastal seawater in algal (land based) aquacultures of Ulva.

The green-tide-forming macroalga Ulva linza was profiled by transcriptome sequencing to ascertain whether the alga carries both C sub(3) and C sub(4) photosynthesis genes. The key enzymes involved in C sub(4) metabolism including pyruvate orthophosphate dikinase (PPDK), phosphoenolpyruvate carboxylase (PEPC), and phosphoenolpyruvate carboxykinase (PCK) were found. When measured under normal and different stress conditions, expression of rbcL was higher under normal conditions and lower under the adverse conditions, whereas that of PPDK was higher under some adverse conditions, namely desiccation, high salinity, and low salinity. Both ribulose-1, 5-biphosphate carboxylase (RuBPCase) and PPDK were found to play a role in carbon fixation, with significantly higher PPDK activity across the stress conditions. These results suggest that elevated PPDK activity alters carbon metabolism in U. linza leading to partial operation of the C sub(4) carbon metabolism, a pathway that, under stress conditions, probably contributes to the hardy character of U. linza and thus to its wide distribution.

Light quality is a key factor affecting algal growth and biomass composition, particularly pigments such as carotenoids, known for their antioxidant properties. Light-emitting diodes (LEDs) are becoming a cost-effective solution for indoor seaweed production when compared to fluorescent bulbs, allowing full control of the light spectra. However, knowledge of its effects on Ulva biomass production is still scarce. In this study, we investigated the effects of LEDs on the phenotype of an Ulva lacinulata strain, collected on the Northern Portuguese coast. Effects of white (W), green (G), red (R), and blue (B) LEDs were evaluated for growth (fresh weight and area), photosynthetic activity, sporulation, and content of pigments and antioxidant compounds. The results showed that there were no significant differences in terms of fresh weight accumulation and reduced sporulation among the tested LEDs, while W light induced the highest expansion rate. Under G, U. lacinulata attained a quicker photoacclimation, and the highest content of pigments and total antioxidant activity; but with R and W, antioxidant compounds against the specific radicals O2•− and •NO were produced in a higher content when compared to other LEDs. Altogether, this study demonstrated that it is possible to modulate the bioactive properties of U. lacinulata by using W, R, and G light, opening the path to the production of biomass tailored for specific nutraceutical applications.

Microalgae with high growth rates have been considered as promising organisms to replace fossil resources with contemporary primary production as a renewable source. However, their microscopic size makes it hard to be harvested for industrial applications. In this regard, multicellular macroalgae are more suitable for harvesting. Here, we show that Ulva meridionalis has the highest growth rate ever reported for a multicellular autotrophic plant. Contrasted to the known bloom-forming species U. prolifera growing at an approximately two-fold growth rate per day in optimum conditions, U. meridionalis grows at a daily rate of over fourfold. The high growth ability of this multicellular alga would provide the most effective method for CO 2 fixation and biomass production.

Ulva spp. are among the most commonly encountered macroalgal species forming algal blooms and are responsible for many Green Tide events worldwide. Numerous adaptations have provided this genus with broad plasticity in carbon acquisition and metabolism, allowing rapid responses to nutrient inputs and sustained growth during the limiting conditions that arise during large bloom events. Because of this plasticity, the carbon isotopic composition of Ulva spp. may be highly variable. A compilation of 1,382 observations of Ulva δ¹³C signatures from literature sources and systematic surveys revealed an unusually wide range of values, much broader than what is typically reported for other marine macroalgae, with a bimodal distribution. This pattern suggests two distinct growth modes in Ulva spp.: one characteristic of non-limiting conditions, and another associated with bloom conditions where rapid growth and carbon limitation drive higher δ¹³C signatures. Analysis of environmental drivers in New England sites further showed that enriched Ulva δ¹³C values arise under the limiting conditions that develop within dense macroalgal canopies in eutrophic estuaries. Excess nutrient inputs stimulate large macroalgal accumulations, which then alter the surrounding physicochemical environment and shift isotopic signatures. These results confirm that Ulva δ¹³C is a valuable tool for assessing the trophic status of coastal environments.