Journal of Phycology最新文献

Development and growth of microalgae are mainly sustained by two essential nutrients: nitrogen (N) and phosphorus (P). Although single-nutrient limitation has been extensively studied, the balance between N and P availability remains less explored. Scrippsiella acuminata is one of the most abundant dinoflagellates in coastal ecosystems due to its physiological plasticity, making it a key species in the understanding of acclimation to unbalanced nutrient supply. To test the acclimation of S. acuminata, semicontinuous cultures were exposed to six N:P ratios (1.6, 8, 16, 32, 90, 180). Parameters such as photosynthetic response, biovolume, carbon excretion, lipids, reactive oxygen species production, cell cycle, and alkaline phosphatase activity were analyzed. Growth, regular cell cycle progression, balanced carbon allocation carbon resource, and high photosynthesis efficiency occurred at balanced N:P ratios (16, 32). At low ratios (1.6, 8), growth was reduced but cells maintained active photochemistry, whereas high ratios (90, 180) led to an extension of the G1 phase leading to biovolume increase and a limitation of the protective capacity of non-photochemical quenching leading to reactive oxygen species accumulation. Carbon allocation followed a stoichiometric gradient where more limiting N:P ratios favored soluble extracellular polymeric substances and a pool of cellular carbohydrates production as an overflow mechanism to protect cells, whereas moderate limitation led to lipid accumulation as a metabolic reserve. These results not only highlight the plasticity of S. acuminata to acclimate to nutrient stress but also suggest that this species may be more vulnerable in P-limited environments and has a competitive advantage where N is the primary limiting factor.

RNA interference (RNAi) is a powerful tool for protein knockdown and is widely used in model animals and plants. Here, we implemented RNAi in Bryopsis, a green feather alga that develops a coenocytic thallus >10 cm in length without cytokinesis. In vitro-transcribed double-stranded RNA (dsRNA) was either mixed with extruded cytoplasm in the presence of polyethylene glycol and then regenerated into thalli or directly introduced into the cytoplasm by perfusion. Within several days, target-gene transcript levels decreased, and the expected phenotypes emerged, indicating effective RNAi. We designed dsRNAs for all 34 kinesin superfamily genes of a model Bryopsis strain, delivered them by both methods, and monitored chloroplast distribution and motility by microscopy. Knockdown of KCBP-type kinesin-14VI (Kin14VIa and Kin14VIb), which drive retrograde chloroplast transport in the moss Physcomitrium patens, and of the apparently Bryopsis-specific kinesin-14II (Kin14IIb) suppressed retrograde motility, resulting in apical chloroplast accumulation. In addition, RNAi of the Bryopsidales-specific kinesin-14VI (Kin14VIc) reduced both retrograde and anterograde movements, implying that multiple Kin14 motors contribute to chloroplast transport in Bryopsis. In contrast, knockdown of Kin12c, a member of the kinesin-12 family that is known to be essential for cytokinesis in land plants, caused basal chloroplast enrichment, indicating divergent evolution of this motor family in the green lineage. Together, these results establish RNAi as a robust loss-of-function approach in a coenocytic alga and reveal lineage-specific kinesins as key drivers of chloroplast motility within its giant cytoplasm.

Ceratocorys armata, C. gourretii, and C. horrida have historically been treated as distinct species based on morphology. Both field samples and cultured strains, however, exhibit continuous morphological variation and intergradations between these taxa. Here, strains from Viet Nam, Japan (Pacific Ocean), and France (Mediterranean Sea, Atlantic Ocean) were analyzed using integrated morphomolecular approaches. Cultured strains revealed morphological intergradations from C. horrida to C. armata and from C. armata to C. gourretii. Phylogenetic analysis based on the ITS1-5.8S-ITS2 rDNA region sequences and the SSU (18S) and LSU (28S) nuclear rDNA gene sequences confirmed the genetic identity of these three taxa for these markers. The ITS2 secondary structure comparisons—including C. malayensis from Viet Nam and Malaysia—further supported their conspecificity, while validating C. malayensis as a distinct species. Based on these data, we have proposed to treat them as formae: C. horrida f. horrida, C. horrida f. armata, and C. horrida f. gourretii. Furthermore, our results indicated that C. malayensis strains from Viet Nam are non-toxic. We have also provided morphological descriptions and illustrations of C. bipes and C. magna, two species rarely reported in the Asia-Pacific region, enhancing the taxonomic clarity of the genus.

Exogenous application of methyl jasmonate (MeJA), a plant stress-responsive hormone, has been suggested to improve plant resistance, but its protective role in marine microalgae under pollutant exposure remains poorly understood. This study investigated the cytoprotective effects of MeJA on the marine green microalga Chlorella sp. exposed to 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), a highly biotoxic dibrominated diphenyl ether. Exposure to BDE-47 (0.075 mg · L⁻¹) significantly impaired cellular structure, inhibited growth, and induced oxidative stress, as evidenced by reactive oxygen species (ROS) overproduction, membrane lipid peroxidation, G2/M phase arrest, and DNA damage. Cotreatment with 0.1 μM MeJA was associated with a significant reduction in adverse effects (p < 0.05), with observed lower ROS levels and elevated activities of key antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). Additionally, MeJA-treated cells exhibited increased endogenous jasmonic acid (JA) content and upregulated lipoxygenase (LOX) activity and allene oxide synthase (AOS) content, both enzymes involved in JA biosynthesis. Partial least squares structural equation modeling suggested a potential link between MeJA-activated JA biosynthesis and antioxidant enzyme induction, indicating a self-amplifying ROS scavenging mechanism that may counteract BDE-47 toxicity. The findings have provided insights into the use of plant hormones to enhance microalgae tolerance to environmental pollutants, with potential applications in microalgae cultivation and biotechnology.

Sphaerochara canadensis, an aquatic macrophyte belonging to the Characeae, is described as a species with a circumpolar distribution and occurs in the polar (to boreal) zonobiomes, suggesting that it is cold-stenothermic. A recent report of an occurrence in Lake Wolfgangsee, Austria, contradicted this assumption, prompting this study to investigate the species' physiological adaptation capabilities and, consequently, ability to survive in non-polar environments. Field measurements at Lake Torneträsk, Sweden, indicated that S. canadensis is more adapted to low light compared to the co-occurring charophytes, with water temperatures in the lake ranging from 10 to 12°C during the experiment. Cultivation experiments also revealed clear temperature effects on growth, photosynthetic performance, and pigment composition at 5, 10, 15, and 20°C, with higher temperatures having a positive impact. Furthermore, it was shown that the species can adapt to different light intensities. A published occurrence of the species in Austria is probably erroneous. The photographic material of the original report was misidentified, and eDNA analyses of water samples taken from Lake Wolfgangsee and surrounding water bodies failed to confirm the presence of the species. Consequently, a factor other than temperature limits its distribution. Identifying the factor is crucial for conservation under climate change.

Raphidiopsis raciborskii is a diazotrophic cyanobacterium, globally distributed in aquatic environments and known for forming toxic blooms, thereby affecting ecosystem services. South American strains are producers of saxitoxins, potent neurotoxins harmful to humans and animals. This study examined the effect of nutrient availability on saxitoxin production in two toxic R. raciborskii strains. Reverse transcription–quantitative polymerase chain reaction (RT-qPCR) was used to investigate the transcriptional response of the saxitoxin sxtA4 gene under nitrogen and phosphorus gradients and the intracellular toxin concentration was measured by high performance liquid chromatography (HPLC). Results showed that the sxtA4 gene expression was generally upregulated at lower nutrient conditions. Positive correlations were observed among transcripts of sxtA4 and genes related to metabolic processes (ntcA, nifH, and pstS), an indication that nutrient stress may affect sxt gene regulation. Intracellular saxitoxin concentration increased slightly under moderate nitrogen reduction (10%), although not always significantly. Under phosphorus reduction, despite the observed upregulated transcription of sxtA4, total saxitoxin concentration significantly decreased, a possible consequence of hindered metabolic fitness. Interestingly, nutrient availability also affected the profiles of toxin analogs produced by R. raciborskii. Because different analogs exhibit variable toxicity, the presence of certain variants may enhance the toxic potential of an entire population under shifting environmental stressors. The responses observed in this study indicate the need for further investigations to identify the mechanisms controlling toxicity. This is particularly relevant as nutrient reduction may control cyanobacterial growth but not necessarily their toxin production.

Three new species of the freshwater diatom genus Rhopalodia from Yunnan province, China, have been described in this study based on a combination of morphological and molecular data. The diagnostic characteristics of Rhopalodia yunnanensis sp. nov. are the absence of secondary costae, and spines on its outer surface exhibit a flame-like morphology. Rhopalodia inflata sp. nov. is distinguished from other Rhopalodia species by the pronounced dorsal middle inflation, blunt-pointed spines, and areolae occluded by volae that consist of two or three c-shaped openings. The areolae of R. fuxianensis sp. nov. are covered by raised siliceous petal-like occlusions with one to two siliceous processes. Phylogenetic analysis based on the SSU rDNA and rbcL sequence data places the three new species within a distinct clade, clearly separated from other sequenced Rhopalodia species, including the generitype, R. gibba. In addition, the plastid, mitochondrial, and spheroid body genomes of the three novel species are presented and characterized in this paper.

It has been well established that warming beyond certain thresholds can negatively affect the growth of canopy-forming macroalgae. However, most studies evaluating these effects have been conducted under controlled laboratory conditions. Observational studies investigating the impact of extreme temperatures on growth rates have been rare and typically limited to events such as marine heatwaves or areas affected by thermal pollution. The decline in vertical growth could be detrimental to the development and recovery of macroalgal canopies, significantly impacting habitat complexity. This study examined spatiotemporal variations in thallus height and vertical growth rates of benthic Sargassum species near the Brazilian Nuclear Power Station (BNPS). Samples were collected from sites exposed and unexposed to the thermal effluent of the BNPS, across different periods. Throughout the sampling periods, sites exposed to the thermal effluent consistently exhibited lower vertical growth rates than unexposed sites. Reduced thallus height was also observed at impacted sites during the first sampling period, whereas in the second period, this stunted canopy was observed only at the site closest to the thermal effluent outfall. This decline in vertical growth could reduce habitat complexity, potentially altering the structure of shallow rocky macroalgal communities. Even acknowledging the limitations in establishing cause–effect relationships in in situ studies, these results can provide important insights into the potential effects of warming on shallow, rocky-bottom communities and may offer valuable guidance for managing and monitoring Sargassum populations in the face of thermal pollution and global climate change.

Prasinophytes are a globally distributed, diverse polyphyletic group of photosynthetic green algae, displaying a combination of presumably ancestral traits for the Chloroplastida. The Nephroselmidophyceae is a prasinophyte clade comprising small biflagellate algae that includes the marine mixoplankton Nephroselmis pyriformis, which was previously shown to feed on bacteria, particularly under low concentrations of dissolved inorganic nutrients. Given its mixotrophic proclivities and phylogenetic position, N. pyriformis has the potential to provide insights into early green algal evolution and ecophysiology. In this study, we have presented a highly contiguous nuclear genome assembly for N. pyriformis. We acquired both Illumina and PacBio long-read data to assemble a 70.8-Mbp nuclear genome annotated with a total of 19,330 protein-coding genes. The genome is inferred to be haploid based on the base frequency distribution at variable sites, together with prior cell biological information from a related species. When compared with four other green algal genomes, N. pyriformis displayed a relatively large proportion of ortholog genes shared with the Chlorophyta. The nuclear genomic data presented here will be valuable for a range of studies, including green algal phylogenomics, genome evolution, and phago-mixotrophy.