Journal of Phycology最新文献

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.

The coccolithophore Helicosphaera carteri is an understudied, yet ecologically and biogeochemically important, marine calcifier. Hence, its response to ocean acidification has implications for ecosystem function and the marine carbon cycle. Here we employed dilute batch cultures featuring a coupled C-system manipulation (295, 444, and 600 μatm CO₂) to analyze the response of H. carteri in terms of growth rate and particulate carbon production, two key eco-physiological and biogeochemical parameters. We highlight that both growth rate and organic carbon production are CO₂ limited at 295 µatm but are not proton inhibited at 600 µatm of CO₂. This finding, combined with the maintenance of a stable inorganic production rate, places H. carteri among the coccolithophores less sensitive to seawater acidification. In addition, we tested a widely applied assumption underpinning the determination of carbon production, namely the constancy of particulate carbon quotas over the course of a dilute batch culture. We determined that the assumption holds true, an important validation of a method used in many publications.

Undaria pinnatifida is an economically important kelp that is extensively farmed in East Asia. Increasingly severe ocean warming makes it urgent to breed new cultivars that are resistant to higher temperatures. Breeding of triploid sporophytes is a potential way to achieve this purpose. Diploid gametophytes were previously produced via apospory from heterozygous sporophytes; however, their sex was not pure, making it difficult to conduct a precise cross. In the present study, we addressed this issue by means of obtaining aposporous diploid gametophytes from doubled haploid sporophytes that were produced by self-fertilization of a monoicous gametophyte. The sex of the aposporous diploid gametophytes was determined to be male. We crossed the male diploid gametophytes with three female haploid gametophyte clonal lines and obtained three hybrid sporophyte lines, which were identified to be triploid using flow cytometry analysis. We cultivated the three triploid lines in the open sea and evaluated their economic traits in April. Their total length and blade length were significantly higher than the control cultivar despite their cultivation period being ~2 months shorter than that of the control. Obvious decay was observed in the control, but not in the triploid sporophytes, suggesting that they sustained growth in higher temperatures. The triploid sporophytes formed sporophylls but were not able to release spores in the reproductive season, and thus, they were determined to be sterile. This study provides a novel and precise means for breeding triploid sporophytes in U. pinnatifida, which is expected to be useful for breeding triploid cultivars superior to the current diploid cultivars.

Asparagopsis taxiformis has high potential to mitigate enteric methane emissions from livestock due to its high bromoform content. However, the current supply of gametophytes is limited due to insufficient biomass. Year-round production of gametophyte can address insufficient biomass through a continuous supply of initial biomass from tetrasporophyte. In this study, we evaluated the effects of temperature (10, 20, and 30°C) and photoperiod (8:16, 12:12, and 16:8 h light:dark cycles), as well as irradiance (10, 20, 40, 80, 160 μmol photons · m⁻² · s⁻¹) and nutrient conditions (high nutrient: 500 μM nitrate and 30 μM phosphate; low nutrient: 50 μM nitrate and 3 μM phosphate) on the growth and reproduction of tetrasporophyte of A. taxiformis. Temperature was a key factor in both growth and reproduction, whereas photoperiod was a key factor in reproduction. Growth of tetrasporophyte was inhibited by 10°C regardless of photoperiod. The development of tetrasporangia was only observed at 20°C with an 8:16 h light:dark cycle. At 20°C with an 8:16 cycle, irradiance affected the development of tetrasporangia at high nutrient concentration. The development of tetrasporangia was observed at 20 and 40 μmol photons · m⁻² · s⁻¹, with the highest growth rate observed at 160 μmol photons · m⁻² · s⁻¹ without the development of tetrasporangia. These results suggest that controlling irradiance at 20°C on an 8:16 h light:dark cycle under high nutrient concentration can regulate the growth and reproduction of A. taxiformis tetrasporophyte during cultivation.

There has been an explosion of new Cyanobacterial taxa described within the last two decades. Cyanobacteria exhibit incredible ecological versatility and morphological variability, and thousands of species have already been described using “traditional” approaches (e.g., morphological features). However, DNA sequencing and other molecular tools have provided extensive evidence that the diversity of cyanobacteria is not necessarily congruent with morphology, as many morphological genera (e.g., Phormidium, Leptolyngbya, and Nostoc) are polyphyletic, and species within the genera are often morphologically indistinguishable, thus cryptic. Further confounding systematic assessments, newly erected taxa are often based on a single strain with one or two 16S rRNA gene sequences, may have incomplete formal descriptions, and lack indication of the employed species concepts. Here we have proposed a set of guidelines for cyanobacterial taxonomists. We have focused on the whole process of erecting new taxa: sampling, sequencing (including genomes), phylogenetic inference, phenotype characterization, species concepts, formal descriptions, and codes of nomenclature. Our hope is that these guidelines will help with the laborious but ever-rewarding task of identifying and describing the taxa within the world of cyanobacteria.

We sequenced the genome, transcriptome, and bacterial metagenome of Psammoneis japonica, a benthic, chain-forming, and araphid marine diatom. This combination of traits fills several gaps in genome sequencing coverage across diatoms. The nuclear genome (QPGO00000000) is an estimated 91.4 Mb in length, with 11,047 genes that comprise 18% of the total genome. Repetitive elements account for 33% of the genome, and other noncoding sequences comprise the remaining 49% of the genome. A global analysis of diatom genomes showed that repetitive elements are the principal driver of genome size variation in diatoms. Four complete genomes of Planctomycetota, ɑ-proteobacteria, and Bacteroidota were also recovered, and each had only moderate similarity to previously sequenced bacterial genomes. This finding supports the idea that bacterial species richness in the phycosphere is under-described and far exceeds the number of diatom host species, which themselves number in the tens to hundreds of thousands of species.

Dapis, along with other benthic cyanobacteria, forms large extensive proliferations (i.e., blooms, mats) that cover sand and seagrass beds across the coasts of Florida, United States. As these benthic cyanobacteria become more prevalent, especially in areas like the Indian River Lagoon (IRL), Sarasota Bay, Lemon Bay, Tampa Bay, and the Florida Keys, it is necessary to understand their diversity. To explore the diversity and identify the causative species of these nuisance marine benthic mats, growing and rafting mats were sampled. Three cyanobacterial isolates from marine mats across Florida were characterized and revealed to represent three novel species of Dapis, but with differing morphological features than those of previously described species. With the support of 16S rRNA gene sequence phylogeny, morphological evaluations, 16S–23S ITS rRNA region pairwise distances, and phylogenomic analyses, we have presented three novel species of Dapis. We have also provided taxonomically validated sets of Dapis genomes from several species.