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Harmful microalgal blooms such as red-tide or brown-tide events lead to abrupt light reductions and consequently cause immediate damage to seagrass beds. Because red tide algal blooms usually occur unexpectedly, seagrass responses to the microalgal blooms have rarely been documented. A red tide caused by a dense bloom of Heterosigma akashiwo, a noxious red-tide-causing alga of temperate and subtropical waters, occurred at a study site on the south coast of Korea in late May 2002. Because the red-tide event occurred on an eelgrass bed where seagrass monitoring was being conducted, pre-event conditions were well documented. Nearly all eelgrass shoots disappeared rapidly because of the reduction in light caused by the algal bloom. Additionally, a thick layer of mucilaginous material secreted from algal cysts suffocated eelgrass plants for weeks, directly causing eelgrass death. Eelgrass seedlings were found in the die-off area from December 2002; <1 yr after its destruction, the site was completely re-established by seedling recruitment via germination from the seed bank. Seedling mortality was very low. Seedlings grew exponentially during the spring, and their fast growth also contributed to rapid eelgrass recolonization. During the second year of recolonization, asexual reproduction through lateral shoot production by rhizome elongation and branching played the main role in the persistence and growth of the eelgrass bed. Seed density in the seed bank varied seasonally, increasing to a maximum after seed release and decreasing to nearly zero after seed germination. Many more seedlings were found and nearly all seedlings established successfully in the first year after the bloom, when no adult eelgrass shoots were observed, suggesting significant effects of shoot density on rates of seed germination and seedling establishment. This was a unique opportunity to examine eelgrass responses to dense microalgal blooms, which provided valuable information on the die-off process caused by red tide and the natural recolonization of seagrass after its destruction.KEY WORDS: Recolonization · Red tide algal bloom · Eelgrass · Zostera marina · Die-off · Seed bank · Seedling Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 342: [105][106][107][108][109][110][111][112][113][114][115] 2007 been reported (Onuf 1996). Red tide algal blooms usually occur unexpectedly; accordingly, seagrass conditions both before and after the occurrence of blooms have seldom been documented. On the coasts of Korea, red tide algal blooms occur frequently during the summer period; the frequency of bloom events has been increasing from 5 to 40 times yr -1 during the 1980s to 30 to 120 times yr -1 during the 1990s (Kim et al. 2000). At a study site on the south coast of Korea, where monitoring of seagrass beds was in progress, a red-tide event caused by a very dense bloom of Heterosigma akashiwo occurred in late May 2002 and lasted for about 2 wk. The seagrass bed and abiotic factors, such as un...

Resale or republication not permitted without written consent of the publisherAn eelgrass population near its maximum depth limit was reestablished by seedling recruitment (left), and nearly all the surviving seedlings flowered (right).

In recent years, drifting and inundating brown seaweed ( Sargassum horneri ) biomass, called ‘golden tides’, has frequently drifted and accumulated along the southern coastlines of Korea, causing devastating impacts on the local economy and coastal ecosystems. In this study, based on combined analyses of mitochondrial DNA cox3 gene and seven microsatellites, we investigated the genetic makeup of the floating S . horneri populations ( N = 14) in comparison to Korean benthic populations ( N = 5), and tracked their genetic sources. Given a shared mtDNA haplotype and oceanic circulation systems, the floating populations may have been originated from the southeastern coast of China (e.g. Zhoushan, Zhejiang province). Population structure analyses with microsatellites revealed two distinct genetic clusters, each comprising floating and benthic populations. High levels of inter-population differentiation were detected within Korean benthic samples. The floating populations from the same periods during a 2015–2018 year were genetically more different from one another than those from different periods. These results suggest that the floating populations might be of multiple genetic sources within geographic origin(s). This study will inform management efforts including the development of “ S . horneri blooming forecasting system”, which will assist in mitigating ecological and economic damages on the Korean coastal ecosystems in the future.

Ultraviolet B (UVB) radiation-induced oxidative skin cell damage is a major cause of photoaging. In the present study, a low molecular weight fucoidan fraction (SHC4) was obtained from Sargassum horneri by Celluclast-assisted extraction, followed by step gradient ethanol precipitation. The protective effect of SHC4 was investigated in human keratinocytes against UVB-induced oxidative stress. The purified fucoidan was characterized by Fourier-transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), agarose gel-based molecular weight analysis and monosaccharide composition analysis. SHC4 had a mean molecular weight of 60 kDa, with 37.43% fucose and 28.01 ± 0.50% sulfate content. The structure was mainly composed of α-L-Fucp-(1→4) linked fucose units. SHC4 treatment dose-dependently reduced intracellular reactive oxygen species (ROS) levels and increased the cell viability of UVB exposed HaCaT keratinocytes. Moreover, SHC4 dose-dependently inhibited UVB-induced apoptotic body formation, sub-G1 accumulation of cells and DNA damage. Inhibition of apoptosis was mediated via the mitochondria-mediated pathway, re-establishing the loss of mitochondrial membrane potential. The UVB protective effect of SHC4 was facilitated by enhancing intracellular antioxidant defense via nuclear factor erythroid 2–related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) signaling. Further studies may promote the use of SHC4 as an active ingredient in cosmetics and nutricosmetics.

Photoacclimatory responses of the seagrass Zostera marina in the intertidal and subtidal zones were investigated by measuring chlorophyll a fluorescence parameters, photosynthetic pigments, leaf δ13C values, and shoot morphology in two bay systems. Intertidal plants had higher carotenoid concentrations than subtidal plants to avoid photodamage under excess light conditions during the day. The maximum relative electron transport rate (rETRmax) and minimum saturation irradiance (Ek) of the intertidal plants were higher than those of the subtidal plants, whereas photosynthetic efficiency (α) and maximum quantum yield (Fv/Fm) were higher in subtidal plants. The intertidal plants also had significantly greater Stern–Volmer non-photochemical quenching (NPQ) than that of the subtidal plants. These results suggest that the subtidal plants photoacclimated to use limited light more efficiently, and the intertidal plants exhibited photosynthetic responses to minimize photodamage at excess irradiance. The δ13C values of leaf tissues were more negative in the intertidal plants than those in the subtidal plants, suggesting that the intertidal plants used atmospheric or dissolved CO2 for photosynthesis during emersion. Effective quantum yield (ΔF/Fm´) in the intertidal plants decreased more slowly after emersion than that in the subtidal plants, indicating higher desiccation tolerance of the intertidal plants. The intertidal plants also recovered more rapidly from desiccation damage than the subtidal plants, suggesting photosynthetic adaptation to desiccation stress. The photosynthetic plasticity of Z. marina in response to variable environmental conditions most likely allows this species to occur in the intertidal and subtidal zones.

The coastal ecosystems are considered as one of the most dynamic and vulnerable environments under various anthropogenic developments and the effects of climate change. Variations in the composition and diversity of microbial communities may be a good indicator for determining whether the marine ecosystems are affected by complex forcing stressors. DNA sequence-based metagenomics has recently emerged as a promising tool for analyzing the structure and diversity of microbial communities based on environmental DNA (eDNA). However, few studies have so far been performed using this approach to assess the impacts of human activities on the microbial communities in marine systems. In this study, using metagenomic DNA sequencing (16S ribosomal RNA gene), we analyzed and compared seawater and sediment communities between sand mining and control (natural) sites in southern coastal waters of Korea to assess whether anthropogenic activities have significantly affected the microbial communities. The sand mining sites harbored considerably lower levels of microbial diversities in the surface seawater community during spring compared with control sites. Moreover, the sand mining areas had distinct microbial taxonomic group compositions, particularly during spring season. The microbial groups detected solely in the sediment load/dredging areas (e.g., Marinobacter, Alcanivorax, Novosphingobium) are known to be involved in degradation of toxic chemicals such as hydrocarbon, oil, and aromatic compounds, and they also contain potential pathogens. This study highlights the versatility of metagenomics in monitoring and diagnosing the impacts of human disturbance on the environmental health of marine ecosystems from eDNA.