Neusa Martins

Germplasm banks are important for preserving genetic diversity, yet the availability and long-term maintenance of macroalgal strains in germplasm banks is currently limited in European collections. To support future food security, restoration efforts, and biotechnological innovation, a strategy was recently developed for the long-term preservation of macroalgal genetic diversity. A foundational step in this effort is to assess the existing biodiversity, biogeographical distribution, and commercial relevance of seaweed cultures maintained in European collections. Using a universal data-collection template distributed widely across European seaweed networks, we compiled and analysed information on these seaweed cultures. As of November 2025, the resulting SeaStrains Database of European Seaweed Collections contains 2541 cultures representing 426 species, accounting for only 4% of global and 15% of European macroalgal diversity. The dataset reveals strong taxonomic bias: 74% are brown seaweeds (Ochrophyta), dominated by Laminaria , Saccharina , and Ectocarpus . Saccharina latissima is the most preserved species due to its scientific and commercial importance. Red and green algae are markedly underrepresented (11% and 14%, respectively), likely reflecting cultivation challenges and life-cycle knowledge gaps. Biogeographically, most cultures originate from cold- to warm-temperate zones, with limited representation of the South Atlantic, Indian Ocean, polar and remote regions. Over 60% of cultures are maintained at two institutions: the Culture Collection of Algae and Protozoa (Scotland) and the Alfred Wegener Institute (Germany). This work provides transparent, accessible information on Europe’s macroalgal germplasm resources and represents a crucial step toward a more robust and sustainable European seaweed sector aligned with UN SDG Target 2.5.

Anthropogenic climate change is causing the decline of seaweed forests in many parts of the world. Despite successful preservation efforts, their immense biodiversity is still severely underrepresented in germplasm biobanks throughout the world. These culture libraries can preserve genetic diversity and provide inoculum for marine forest restoration and mariculture ventures, and potentially accelerate the selection and breeding of climate-resilient and high-yielding strains. However, the complex life cycles and body plans of seaweeds pose a huge challenge for the development of standardized phenotyping and isolating protocols for microscopic stages, especially with the efficiency necessary to deal with the current pace of global climatic changes. Here, we present SAMMBA (Seaweed Automatable Microplate Microscopy for Breeding Approaches), an end-to-end pipeline for the high-throughput isolation, phenotyping and storage of macroalgal cells in 384-well plates (384WP). By optimizing fluorescence microscopy imaging and analysis, along with a novel fragmentation method and dilution-to-extinction isolation, different unialgal seaweed tissues could be regrown after thousand-fold dilutions. In a single plate, we successfully isolated 68 singlet gametophyte fragments of Laminaria ochroleuca (39 males, 29 females; 17.7% efficiency) and 60 spores of Phyllariopsis purpurascens (31.25% efficiency). Furthermore, the taxonomic versatility of SAMMBA was demonstrated through the successful isolation of 60 unialgal cultures of red algae ( Halymenia sp., Hydrolithon sp., Erythrotrichia sp. ) and 10 strains of the green alga Ulva sp, without cross-contamination. The viability and unialgal nature of the isolated strains were verified by distributing a single L. ochroleuca strain across an entire 384-well plate and imaging each well over 30 days. We found that the average specific daily growth rates (daily SGR) per well were 0.130 ± 0.006 and 0.117 ± 0.01 day ^-1 for males and females, respectively, showing a significant difference between sexes (n = 768; p = 1.27e ^-53 ), while edge effects significantly reduced daily SGR in males but not in females. This approach dramatically increases experimental reproducibility and statistical power compared to conventional methods. Due to its modular design and cost-effectiveness, SAMMBA is readily adaptable to macroalgal repositories globally. It supports high-throughput, selective recovery of unialgal strains without reliance on robotic platforms, while remaining fully compatible with automation. This system significantly expands the experimental and operational capacity in macroalgal hatcheries, providing a scalable foundation for phenomics, domestication programs, and standardized, verifiable biobanking of unialgal strains. Ultimately, SAMMBA could provide critical support for breeding strategies required to ensure the resilience of marine forests and aquaculture in a rapidly changing ocean.

Editorial: Opening the black box of kelps: Response of early life stages to anthropogenic stressors

Many organisms have endogenous clocks that synchronize biological processes with environmental changes, leading to optimized development and reproduction. However, certain environments, like the Arctic, pose a special challenge to circadian clocks, particularly due to extreme seasonal changes in daylength, ranging from permanent sunlight to complete darkness. Kelps seem to be well adapted to the variable environmental conditions characteristic of this region. However, daylength might affect kelp species that use circadian rhythms to control the timing of daily egg release from female gametophytes. We aimed to investigate how daylength and light intensity affect gametogenesis and reproductive success of summer-reproducing kelp species (using Alaria esculenta as a model). As daylength and temperature co-vary most of the year, we also investigated the thermal resilience of the sporophytes developed under different daylengths to understand if there is a cross-tolerance between light doses and temperature tolerance. Although continuous daylight, characteristic of Arctic summers, enhanced gametogenesis and increased gametophyte vegetative growth, and thereby the number of potential reproductive gametophyte cells, sporophyte production was higher under long (16 h light:8 h dark) and intermediate (12:12 h) days. Sporophyte growth was triggered by changing daylength from short to long days, suggesting a synchronization with annual daylength variation. High daily light doses during reproduction and early development improved subsequent sporophyte survival at high (sub)lethal temperatures, indicating cross-tolerance between light and temperature. Reproductive success in Arctic A. esculenta was hampered under continuous light, and we hypothesize that this might result from disturbance of synchronized egg release and subsequent fertilization.

Kelp forests are important habitats in the strongly environmentally and seasonally variable Arctic. There is a critical lack of knowledge about how seasonal conditions and climate change scenarios influence survival and reproduction of kelp early life stages. To better understand the regulation of kelp life cycle processes in this harsh environment we focused on the physiological performance and reproductive success of early life stages in Alaria esculenta and Laminaria digitata from Kongsfjorden, Spitsbergen. Gametophyte growth and survival during Arctic winter and subsequent sporophyte recruitment under spring conditions were investigated. Winter conditions (2°C, complete darkness) halted gametophyte growth and prevented the onset of gametogenesis in both species. The gametophytes of L. digitata but not A. esculenta became fertile after returning to spring conditions, suggesting that sporogenesis, sexual reproduction and recruitment in A. esculenta must occur successively during summer/autumn while in L. digitata a new generation of sporophytes could develop from over-wintering gametophytes. The effects of simulated canopy shading (offering protection against extreme irradiance stress, particularly as sea ice retreats), present-day and projected Arctic summer seawater temperatures, and nutrient levels on gametophyte survival, fertility and sporophyte recruitment success were also investigated in both species. A. esculenta gametophytes had greater survival and reproductive success than L. digitata, except under very low light (simulating dense canopy). In contrast, shading was required for reproductive success in L. digitata gametophytes. Predicted summer temperatures of 9°C reduced sexual reproduction in both species. Interactions observed between these environmental drivers probably reflect species-specific seasonal patterns of survival and reproduction. These differences between kelp species in response to abiotic factors and light levels (simulated canopy shading) suggest that climate change could alter community structure in the Arctic through effects on sexual reproduction and sporophyte recruitment success. HIGHLIGHTS• Gametophytes were able to endure long periods of darkness.• Parental kelp canopy is key for gametophyte survival and recruitment.• Climate change may alter kelp recruitment patterns.

The plasticity of different kelp populations to heat stress has seldom been investigated excluding environmental effects due to thermal histories, by raising a generation under common garden conditions. Comparisons of populations in the absence of environmental effects allow unbiased quantification of the meta-population adaptive potential and resolution of population-specific differentiation. Following this approach, we tested the hypothesis that genetically distinct arctic and temperate kelp exhibit different thermal phenotypes, by comparing the capacity of their microscopic life stages to recover from elevated temperatures. Gametophytes of Laminaria digitata (Arctic and North Sea) grown at 15°C for 3 years were subjected to common garden conditions with static or dynamic (i.e., gradual) thermal treatments ranging between 15 and 25°C and also to darkness. Gametophyte growth and survival during thermal stress conditions, and subsequent sporophyte recruitment at two recovery temperatures (5 and 15°C), were investigated. Population-specific responses were apparent; North Sea gametophytes exhibited higher growth rates and greater sporophyte recruitment than those from the Arctic when recovering from high temperatures, revealing differential thermal adaptation. All gametophytes performed poorly after recovery from a static 8-day exposure at 22.5°C compared to the response under a dynamic thermal treatment with a peak temperature of 25°C, demonstrating the importance of gradual warming and/or acclimation time in modifying thermal limits. Recovery temperature markedly affected the capacity of gametophytes to reproduce following high temperatures, regardless of the population. Recovery at 5°C resulted in higher sporophyte production following a 15°C and 20°C static exposure, whereas recovery at 15°C was better for gametophyte exposures to static 22.5°C or dynamic heat stress to 25°C. The subtle performance differences between populations originating from sites with contrasting local in situ temperatures support our hypothesis that their thermal plasticity has diverged over evolutionary time scales.

In haplodiplontic lineages, sexual reproduction occurs in haploid parents without meiosis. Although widespread in multicellular lineages such as brown algae (Phaeophyceae), haplodiplontic gametogenesis has been little studied at the molecular level. We addressed this by generating an annotated reference transcriptome for the gametophytic phase of the sugar kelp, Saccharina latissima. Transcriptional profiles of microscopic male and female gametophytes were analysed at four time points during the transition from vegetative growth to gametogenesis. Gametogenic signals resulting from a switch in culture irradiance from red to white light activated a core set of genes in a sex-independent manner, involving rapid activation of ribosome biogenesis, transcription and translation related pathways, with several acting at the post-transcriptional or post-translational level. Additional genes regulating nutrient acquisition and key carbohydrate-energy pathways were also identified. Candidate sex-biased genes under gametogenic conditions had potentially key roles in controlling female- and male-specific gametogenesis. Among these were several sex-biased or -specific E3 ubiquitin-protein ligases that may have important regulatory roles. Females specifically expressed several genes that coordinate gene expression and/or protein degradation, and the synthesis of inositol-containing compounds. Other female-biased genes supported parallels with oogenesis in divergent multicellular lineages, in particular reactive oxygen signalling via an NADPH-oxidase. Males specifically expressed the hypothesised brown algal sex-determining factor. Male-biased expression mainly involved upregulation of genes that control mitotic cell proliferation and spermatogenesis in other systems, as well as multiple flagella-related genes. Our data and results enhance genome-level understanding of gametogenesis in this ecologically and economically important multicellular lineage.