Transcriptomic Changes Induced by Low and High Concentrations of Heavy Metal Exposure in Ulva pertusa.

The impact of sewage and wastewater pollution on marine ecosystems is of increasing concern due to the rapid accumulation of heavy metals in seaweeds inhabiting near-shore environments. Seaweeds can be severely damaged by heavy metals throughout their life cycles. Although the physiological and ecological effects of heavy metal exposure have been studied, there is limited research on their molecular responses. Ulva pertusa is a prevalent seaweed species in South Korea and is ecologically significant in coastal ecosystems. We utilized high-throughput RNA sequencing to analyze changes in the transcriptome profiles of U. pertusa under low concentrations of heavy metals (MPS) and high concentrations of copper (MPS-Cu) and cadmium (MPS-Cd). Differential gene expression analysis revealed that 53 (control vs. MPS), 27 (MPS vs. MPS-Cd), and 725 (MPS vs. MPS-Cu) genes were expressed differentially. Differentially expressed genes identified in our study included those with protective roles against oxidative stress and those involved in metal transport to the vacuole. Furthermore, exposure to heavy metal stress had a negative impact on the photosynthetic apparatus structural proteins of U. pertusa, resulting in photosynthetic inhibition. Moreover, exposure to high concentrations of copper resulted in the activation of carbon-related metabolism. These findings contribute to our understanding of the molecular mechanisms underlying heavy metal toxicity in U. pertusa.

Temperature and high nutrients enhance hypo-salinity tolerance of the bloom forming green alga, Ulva prolifera.

The response of seaweeds to environmental stressors can be population-specific, and be related to the regime of their habitats. To explore the growth and physiological responses of Ulva prolifera, two strains of this alga (Korean and Chinese strains) were studied under an interaction of temperature (20 and 25 °C), nutrients (low nutrients: 50 µM of nitrate and 5 µM of phosphate; high nutrients: 500 µM of nitrate and 50 µM of phosphate) and salinity (20, 30 and 40 psu). The lowest growth rates of both strains were observed at 40 psu of salinity, independent of temperature and nutrient levels. At 20 °C and low nutrients condition, the carbon: nitrogen (C: N) ratio and growth rate in the Chinese strain were increased by 31.1% and 21.1% at a salinity of 20 psu in comparison to the salinity of 30 psu, respectively. High nutrients decreased the ratio of C:N in both strains with increasing tissue N content. At the same time, high nutrients also increased soluble protein and pigments contents, as well as photosynthetic and growth rates in both strains at the same salinity levels at 20 °C. Under 20 °C and high nutrients conditions, the growth rates and C:N ratio of both strains were significantly decreased with increasing salinity. The pigment, soluble protein and tissue N showed an inverse trend with the growth rate at all conditions. Moreover, the higher temperature of 25 °C inhibited the growth in both strains regardless of nutrients levels. The temperature of 25 °C enhanced the contents of tissue N and pigments in the Chinese strain only at the low nutrients level. The interaction of high nutrients and 25 °C led to the accumulation of tissue N and pigment contents in both strains under all salinity conditions compared to the 20 °C and high nutrients level. The temperature of 25 °C and high nutrients decreased the growth rate in the Chinese strain at both salinities of 30 and 40 psu more than the 20 °C, and low nutrients level at the same salinity. These results suggest that the Ulva blooms caused by the Chinese strain were more impacted at hypo-salinity levels compared to the Korean strain. Eutrophic or high nutrients level enhanced the salinity tolerance in both strains of U. prolifera. There will be a decline of U. prolifera blooms of the Chinese strain at hyper-salinity levels.

Effects of extraction methods for a new source of biostimulant from Sargassum horneri on the growth of economically important red algae, Neopyropia yezoensis.

Sargassum horneri is a major bloom forming species in Korea and China. It is important to find a way to utilize the huge biomass of Sargassum horneri in the region. Seaweed-derived biostimulants are primarily derived from brown algae and are known to improve terrestrial crop growth and tolerance to abiotic stresses. Neopyropia yezoensis is the most important seaweed cultured species in Korea, and research is required to increase heat resistance as a solution against climate change. In this study, various extraction methods were used to obtain Sargassum horneri extract, and it was applied to Neopyropia yezoensis to evaluate the effect on physiological activity. Metabolites of Sargassum horneri were extracted by using four different methods: boiling (SBE), soaking (SSE), autoclaving (SAE) and ethanol (SEE). The SBE, SSE and SAE derived extracts showed increased tolerance to high-temperature stress that had inhibited the growth of Neopyropia yezoensis, and show improved growth compared to the control group. The SBE and SSE extraction methods improved the content of phycobiliprotein, but also the SBE increased superoxide dismutase (SOD) activity. Based on the results of this study, the boiling extraction method appears to be the most suitable method for the extraction of plants stimulants from Sargassum horneri.

A comparison of physiological responses between attached and pelagic populations of Sargassum horneri under nutrient and light limitation.

Large-scale Sargassum blooms have been increasingly observed in coastal zones in recent years. Sargassum horneri (Turner) C. Agardh blooms (pelagic) have been observed in Jeju Island (Korea) and the southwest of the Korean Peninsula, causing serious problems for seaweed and abalone farms as well as for fisheries, tourism and recreational industries. The present study explored the physiological responses of attached and pelagic S. horneri populations cultivated under different nutrient concentrations (HN: 50 µM of nitrogen and 5 µM of phosphorus; LN: 5 µM of nitrogen and 0.5 µM of phosphorus) and photosynthetically active radiation (PAR) (H-PAR: 250; M-PAR: 150; L-PAR: 50 µmol photons m-2 s-1) for 25 days. Relative growth rates (RGR) were significantly lower in the pelagic population than that in the attached population. All thalli from the pelagic population died within 20 days. Chlorophyll a and c, and carotenoids were significantly higher at HN than at LN, and decreased as PAR increased for both populations. For the attached population, photosynthetic rate, tissue nitrogen, and carbon and nitrogen removal were also significantly higher at HN than at LN. These results suggest that high nutrient and lower PAR increased the biomass accumulation of attached populations in coastal areas. Nutrient limitation and high PAR may accelerate senescence of the pelagic populations while traveling on the sea surface from their point of origin.

Stable carbon and nitrogen isotopic characterization and tracing nutrient sources of Ulva blooms around Jeju coastal areas.

The present investigation was aimed to characterize the Ulva blooms and to identify the probable sources for Ulva blooms along the Jeju Island coast for pertinent control measures. Algal isotope signatures (δ13C, δ15N and δ18O) and tissue nitrogen and carbon were analyzed to map nutrient sources around the Jeju coastal areas. The algal δ13C values were ranged from -20.52 to -4.39‰, while δ15N and δ18O values ranged from 4.26 to 8.29‰ and 12.80-17.34‰, respectively. Moreover, site-specific significant differences were observed in algal stable isotope (δ13C, δ15N and δ18O) values. The bi-plot (δ15N vs δ18O) diagram indicated four dominant nitrogen sources along the Jeju coast, with 1) soil organic nitrogen mixed with livestock wastes (spring water samples and E), the 2) synthetic fertilizer input (A3 and B2), 3) sewage discharge (D1, D2 and I3) and 4) aquaculture waste (fish farm samples, A4, A5, B1, G and I2). Present findings revealed the different potential nitrogen sources for localized increase of algal growth along the Jeju coast. Finally, the present findings could be used as baseline data for efficient nutrient management to remediate Ulva blooms along Jeju coastal environment.

Reappraisal of the toxicity test method using the green alga Ulva pertusa Kjellman (Chlorophyta).

This study was aimed to develop an objective way of quantifying the reproductive status of the green macroalga, Ulva pertusa using a vital stain and programmed automated analysis (by Image J program). The EC50 values (with 95% CI), the concentrations of toxicants inducing a reduction of 50% in sporulation after 96 h exposure, from the newly developed method were similar to those obtained by the conventional method: 0.651 (0.598-0.705) mg l-1 for Cd, 0.144 (0.110-0.162) mg l-1 for Cu, 0.180 (0.165-0.195) mg l-1 for atrazine, 0.076 (0.049-0.094) mg l-1 for diuron and 30.6 (26.5-34.4) ml l-1 for DMSO, respectively. When the EC50 values from this study were compared to that those from literatures, the sensitivity for some toxicants was similar or higher than that of U. fasciata (1.930 mg l-1 for germination for Cd), U. armoricana (0.250 mg l-1 for Fv/Fm for Cu), U. reticulata (0.126-1.585 mg l-1 for growth for Cu), and U. intestinalis (0.650 mg l-1 for Fv/Fm for atrazine). The subjective views of the experimental performers can be eliminated using the newly developed method. The Ulva method gave consistent responses to Cu and Cd of internationally allowable ranges for effluents, implying that the method is a useful tool for monitoring industrial wastewaters containing these metals.

Comparing the acute sensitivity of growth and photosynthetic endpoints in three Lemna species exposed to four herbicides.

An ecological impact assessment of four herbicides (atrazine, diuron, paraquat and simazine) was assessed using the aquatic floating vascular plants, Lemna gibba, Lemna minor and Lemna paucicostata as test organisms. The sensitivity of several ecologically relevant parameters (increase in frond area, root length after regrowth, maximum and effective quantum yield of PSII and maximum electron transport rate (ETRmax), were compared after a 72 h exposure to herbicides. The present test methods require relatively small sample volume (3 mL), shorter exposure times (72 h), simple and quick analytical procedures as compared with standard Lemna assays. Sensitivity ranking of endpoints, based on EC50 values, differed depending on the herbicide. The most toxic herbicides were diuron and paraquat and the most sensitive endpoints were root length (6.0-12.3 µg L-1) and ETRmax (4.7-10.3 µg L-1) for paraquat and effective quantum yield (6.8-10.4 µg L-1) for diuron. Growth and chlorophyll a fluorescence parameters in all three Lemna species were sensitive enough to detect toxic levels of diuron and paraquat in water samples in excess of allowable concentrations set by international standards. CV values of all EC50s obtained from the Lemna tests were in the range of 2.8-24.33%, indicating a high level of repeatability comparable to the desirable level of <30% for adoption of toxicity test methods as international standards. Our new Lemna methods may provide useful information for the assessment of toxicity risk of residual herbicides in aquatic ecosystems.

Emersion induces nitrogen release and alteration of nitrogen metabolism in the intertidal genus Porphyra.

We investigated emersion-induced nitrogen (N) release from Porphyra umbilicalis Kütz. Thallus N concentration decreased during 4 h of emersion. Tissue N and soluble protein contents of P. umbilicalis were positively correlated and decreased during emersion. Growth of P. umbilicalis did not simply dilute the pre-emersion tissue N concentration. Rather, N was lost from tissues during emersion. We hypothesize that emersion-induced N release occurs when proteins are catabolized. While the δ(15)N value of tissues exposed to emersion was higher than that of continuously submerged tissues, further discrimination of stable N isotopes did not occur during the 4 h emersion. We conclude that N release from Porphyra during emersion did not result from bacterial denitrification, but possibly as a consequence of photorespiration. The release of N by P. umbilicalis into the environment during emersion suggests a novel role of intertidal seaweeds in the global N cycle. Emersion also altered the physiological function (nitrate uptake, nitrate reductase and glutamine synthetase activity, growth rate) of P. umbilicalis and the co-occurring upper intertidal species P. linearis Grev., though in a seasonally influenced manner. Individuals of the year round perennial species P. umbilicalis were more tolerant of emersion than ephemeral, cold temperate P. linearis in early winter. However, the mid-winter populations of both P. linearis and P. umbilicalis, had similar temporal physiological patterns during emersion.