Abstract Projections for climate change potentially mean that tidally exposed seaweeds will experience greater air temperature extremes than those that remain immersed. This study investigated the combined effects of temperature and humidity on the relative growth rate of Fucus vesiculosus through two controlled experiments: (1) evaluating responses to various temperature and humidity regimes, and (2) assessing the impact of three consecutive days of heatwave conditions (25 °C). Samples from the upper and lower shore were subjected to 24, 30 and 40 °C at relative humidities of 30–90 % for 4 or 8 h, using an incubator oven to generate six vapour pressure deficit (VPD) levels (0.3–5.3 kPa). A VPD of 5.3 kPa proved detrimental to the seaweeds regardless of duration. In contrast, F. vesiculosus tolerated a VPD of 2.4 kPa for up to 8 h. Little difference was detected between upper and lower shore residing specimens. Heatwave conditions did not negatively affect F. vesiculosus growth irrespective of shore location. Blades could recover from proportional water biomass loss of up to 0.6 but not from a loss of 0.7 regardless of VPD. Nonetheless, greater VPD values tended to coincide with increased water biomass loss and decreased post-exposure growth. These findings suggest that increased humidity may mitigate thermal stress, enhancing resilience under projected climate scenarios.

Abstract Species of Dictyota with dentate margins exhibit notable diversity and a broad distribution across the Greater Caribbean. Oceanic islands such as Saint Kitts (Lesser Antilles) and the Archipelago of San Andrés (Colombia) play a key role in expanding our understanding of these taxa. In this study, we report Dictyota canariensis as a new record for both countries and confirm the presence of Dictyota ciliolata and Dictyota jamaicensis using molecular and morphological evidence. Phylogenetic analyses based on psb A and cox 1 sequences support the identity of these species, and the cox 1 haplotype network reveals some geographic structure. Overall, our findings underscore the value of molecular data in addressing taxonomic complexity and highlight the importance of oceanic islands in shaping the regional diversity of dentate Dictyota species in the Caribbean basin.

Abstract Species in the green algal order Cladophorales inhabit a wide range of aquatic ecosystems and display significant morphological variation. Identifying these species can be challenging due to their simple morphologies, which offer few diagnostic features. Furthermore, several characters exhibit extensive intraspecific variation and overlap between species. As a result, DNA-based approaches have become essential for accurate species identification. Cladophora , the most diverse genus in the order, is widely distributed and functions as an ecological engineer by shaping aquatic habitats. Accurate species identification is therefore important for sustainable management and effective conservation strategies. This study combines morphological analysis with DNA-based methods, using partial nuclear large subunit ribosomal DNA (LSU rDNA) sequences, to identify Cladophora species collected from various regions of the Aegean Sea (Turkey), and some locations in Greece. Our findings confirm the presence of six Cladophora species ( C. albida , C. hutchinsiae , C. laetevirens , C. prolifera , C. ruchingeri , and C. vagabunda ), along with Cladophoropsis membranacea . While these species have been previously reported from Turkey, our study highlights the continued importance of DNA-based methods for accurate identification within the genus Cladophora .

Abstract Prorocentrum includes ∼80 species inhabiting benthic and planktonic marine environments. Some species, such as Prorocentrum cassubicum and P. norrisianum, have similar morphology, which complicates their identification based solely on morphological characters. This study describes the morphometrics of two Prorocentrum strains (PNCETMAR-1 and PNCETMAR-2) isolated from the Ensenada de La Paz, Gulf of California. Sequences of the 28S and ITS regions of rDNA of both strains and the growth curve of strain PNCETMAR-1 were obtained. Strain PNCETMAR-1 was grown in modified GSe medium and f/2 medium at a salinity of 34, at 24 °C and 120–150 μmol m−2 s−1 in a 12 h light: 12 h dark cycle. There was no significant difference in growth between the culture media. In f/2 medium the maximum abundance was 74,312 ± 2,008 cells ml−1, while in GSe medium it reached 69,766 ± 2,154 cells ml−1. Morphometric and molecular analyses indicate that both strains belong to the same species. However, because the morphological and molecular information on P. cassubicum and P. norrisianum is contradictory, in this study they are reported as Prorocentrum cf. cassubicum, contributing to the first report of the occurrence and the growth of this species in the Gulf of California.

Abstract The Arabian Sea hosts one of the major perennial, oceanic oxygen-depleted environments (ODE) in the water column, influencing the seabed beneath it. These regions harbor a vast diversity of microbial communities, which play a significant role in the global budgets of key biogenic elements like carbon, nitrogen and oxygen. This study assessed the relative importance of bacterial and fungal communities in the Arabian Sea ODE using eDNA metabarcoding tools. Sediment samples from four stations at ∼200–1,200 m depths were analyzed across a hypoxic gradient, with 5–35 μM dissolved oxygen levels. Species diversity and richness indices revealed that bacterial and fungal communities were the highest at an oxygen threshold of 10–20 μM, with variations in their community composition along the hypoxic gradient. The bacterial phylum Proteobacteria was dominant in the sediment of all stations. However, the relative abundances of phyla such as Firmicutes, Chloroflexi, Bacteroidetes, Planctomycetes and Actinobacteria varied across the stations. The phyla Ascomycota and Basidiomycota dominated the fungal communities at all the stations. However, Mucoromycota was present only at stations with 20–35 μM oxygen levels. These results enhance our understanding of microbial community distribution in the Arabian Sea ODE.

Abstract We measured baseline growth rates of perennial understory kelps, Agarum clathratum and Hedophyllum nigripes, and canopy-forming Eualaria fistulosa from March 1977 through October 1978 in Jakolof Bay, Kachemak Bay, Alaska. We measured density and biomass of several kelp species in the entrance to Jakolof Bay, at Seldovia Point, and at Latouche Point, Latouche Is., in an ocean entrance into Prince William Sound. We used Kachemak Bay data to estimate primary productivity in these kelp beds. In view of current records demonstrating increasing sea-surface temperatures in the Gulf of Alaska, these baseline data provide a useful basis for evaluating effects of climate change or other environmental effects. Growth rates, highest from late winter/early spring and lowest from late summer until mid-winter, changed seasonally. Rates were fastest in Eualaria, the largest of these kelps, and slowest in Agarum, the smallest. Rates and seasonality of stipe growth varied among species. Stipe growth, observed seasonally in Agarum and Hedophyllum, occurred from spring through mid-summer; rates were slowest in Agarum. Stipe growth in the annual Eualaria occurred throughout its life span. Tissue growth and primary production varied among species; all produced appreciable quantities of fixed carbon annually. Estimated production for understory kelps off Seldovia Point varied from 73 g C m−2 yr−1 for Agarum to 87 g C m−2 yr−1 for Hedophyllum. Estimated production for canopy-forming kelps was an order of magnitude higher. Value of these decades-old baseline data cannot be over-emphasized in view of growing awareness of effects of climate change on kelps.

Abstract In this study, we introduce a novel saprobic taxon Paraloratospora edkuensis, from a decaying stem of Phragmites australis collected at Lake Edku, El-Behera Governorate, Egypt. Multi-locus analyses of the combined ITS (internal transcribed spacers of rDNA), LSU (nuclear large subunit rDNA) and tef1-α (translation elongation factor) sequence data placed the new taxon within Paraloratospora as a phylogenetically distinct species in a clade containing two aquatic species P. fructigena and P. marina. The new species is characterized by having globose to subglobose, dark brown to black ascomata with a short neck, 4–8-spored, cylindrical to obclavate asci, and 3-septate, hyaline and fusiform ascospores. Paraloratospora edkuensis differs from P. fructigena and P. marina by having smaller ascomata with a single neck, asci with four or eight ascospores, different mycelial colony morphology, and nucleotide differences of 2.1–3 %, 0.6–1.4 %, and 1.3–6.9 % in the ITS, LSU and tef1-α genes, respectively.

Abstract 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.

Abstract The Subantarctic brown alga Cladochroa chnoosporiformis had been collected only on one occasion by Carl Skottsberg in 1907 from Port Philomel, West Falkland, Falkland Islands, resulting in its formal taxonomic description. Due to the lack of reports since then, doubts remained about its existence and identity. Within the framework of this study, Cladochroa was rediscovered 106 years after its original description at its type locality, confirming its existence and the morphological features described by Skottsberg. The re-collection enabled molecular studies on field material and isolated cultures. Sequences of nrDNA, rbcL, and 5′-COI suggest that C. chnoosporiformis is conspecific with Utriculidium durvillei, which has nomenclatural priority. However, the morphologies of C. chnoosporiformis and U. durvillei are significantly different. Utriculidium may be the gametophyte generation of Cladochroa, or hitherto unknown factors cause highly different morphologies without alternating generations.