Volume 68, Issue 1 - Special Issue: Phylogeny and Ecology of Ulva. Part II; Issue Editors: Sophie Steinhagen, Thomas Wichard and Jonas Blomme

Ulva green tides have adversely affected coastal ecosystems. In June 2023 in Changhua County, Taiwan, an unprecedented floating tubular Ulva bloom spanning about 30 km of coastline caused about 30 % reduction in oyster yield as reported by local farmers. Understanding its taxonomic and ecological basis is crucial for preemptive and remedial measures. Based on molecular ( rbc L and tuf A) and morphological evidence, U lva meridionalis was the cause of this green tide. Haplotype network analysis (based on ITS) suggests that this green tide originated from northern China via the China Coastal Current. Historical survey data indicate that U . meridionalis arrived in Taoyuan Algal Reef (about 150 km north of the bloom area) as far back as 2018. Our ecophysiological experiments revealed that U . meridionalis exhibited a rapid daily growth rate with biomass increment up to 13–21 % when subjected to local nutrient-rich waters under lower salinity and spring-like conditions. Although historical ecological and poultry/livestock data analyses showed no noticeable change in sea surface temperature and rainfall over the past decade, a gradual rise in agricultural nitrogen and phosphorus output was observed. This taxonomic and ecological background lays the groundwork for long-term ecological monitoring. Moreover, this study exemplifies the detrimental impact of an unforeseen Ulva bloom on oyster farming.

Fouling seaweeds on nori cultivation rafts in Subei radial sand ridges (Jiangsu, China) are believed to be the source of the Yellow Sea green tide (YSGT). Although major species are consistent across years, their biomass and proportions undergo significant annual changes, which was speculated to be related to the size of YSGT. Therefore, early detection and biomass assessment of the bloom-forming algae are crucial, increasing the need to develop a morphological identification method suitable for large sample sizes in the field. In this study, fouling seaweeds were collected from this area in spring 2017. Five major species, Ulva aragoënsis , Ulva compressa , Ulva linza , Ulva prolifera , and Blidingia sp. were molecularly identified, and confirmed as the predominant fouling seaweeds on nori rafts over the past decade. In this unique ecological niche, the morphological features of species changed significantly yet stably. Based on morphological observations, an identification key for these five species was established. Furthermore, a simplified version containing only two indices was developed for practical field use, with an average accuracy of 81 % ( δ 2  = 0.003) in empirical accuracy assessments. This method can be used to monitor the community structure and succession of fouling seaweeds at the geographical source of YSGT.

The genus Ulva Linnaeus 1753 is well known for its global distribution and containing many taxonomically debated species. Their morphological plasticity and cryptic nature overrepresent and underrepresent, respectively, the biodiversity of these species. The difficulty in morphologically identifying Ulva species has led to the accumulation of many species names that are currently considered synonyms. The correct identification of these species is crucial due to their significant role in marine ecosystems and mariculture. In the last 30 years, several checklists and taxonomic contributions have been made from the Italian coast on Ulva species, almost entirely based on morphological studies and only a few papers with molecular data have been published. In this study, samples deposited at the Institute of Marine Sciences (ISMAR) and Phycological Lab (PHL) herbaria have been reviewed by sequencing the chloroplast-encoded elongation factor Tu ( tuf A) barcode marker. The results obtained provide an update of the DNA barcode inventory of Ulva species for the Lagoon of Venice and the Strait of Messina, two ecosystems characterized by a high algal biogeographical diversity and continuous introduction due to anthropogenic activities. Here, ten Ulva species were identified based on molecular data. Furthermore, we record the presence of Ulva chaugulei on the coast of Tunisia.

The genus Ulva , described as a good source of antioxidants known for its antibacterial properties and associated with the capacity to adapt to different environments and high growth rates, has justified the increasing interest in its large-scale production. While extensive research has been done on optimizing the extraction of Ulv a’s bioactive compounds, few studies were conducted on increasing or optimizing antioxidant activity (AA) of Ulva spp. during cultivation. Our study aimed to investigate an optimization method of Ulva lacinulata by testing the impact of light dose and irradiance on its AA. Two geographically different strains (NE-Atlantic and Mediterranean) were observed for 5 days under two irradiances (70 or 185 µmol photons m −2  s −1 ) with the same light dose (4 mol photons m −2  d −1 ). Samples were collected at different times (0, 3, 24, 48 and 120 h) to evaluate their antioxidant activity (with 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) radical decolorization assay) and photosynthetic performance (with Pulse Amplitude Modulated fluorometer). A strain-dependent response was observed in the NE-Atlantic strain which had significantly higher AA after 5 days (89 %) under the photosynthetic saturating irradiance, while the Mediterranean strain was not impacted, suggesting that light dose may significantly affect AA in certain Ulva spp.

The rising global significance of sea lettuce ( Ulva spp.) in aquaculture stems from its versatility, rapid growth, and nutritional benefits. Cultivation expansion into lower salinity areas, like the Baltic Sea, is crucial for advancing aquaculture beyond traditional environments. This study investigated the impact of long-term (8 weeks) low salinity treatments on the biochemical content of eight Ulva strains – encompassing some of the most common Ulva crop species ( Ulva lacinulata , Ulva linza , Ulva intestinalis , Ulva fenestrata ) of the wider Baltic Sea area – from varying source salinities (30, 14, 9, 7). Most strains exhibited significantly higher growth rates and contents of crude protein under low salinity treatments, irrespective of where they came from (i.e. euhaline or mesohaline environments). However, effects on pigments and phenolic contents were strain-specific. Ulva lancinulata showed high resilience to salinity changes. Cultivating Ulva under low salinity conditions enhances its nutritional attributes and identifies the broader Baltic Sea as a viable cultivation environment. Nevertheless, careful selection of strains is crucial due to significant inter- and intraspecific differences. This research underscores the importance of tailored cultivation strategies for optimizing Ulva biomass production, particularly in the context of the expanding Blue Economy industry.