GROWTH PATTERN,REPRODUCTION, AND SELF-THINNING IN SEAWEEDS1

In unispecific plant stands, the logarithm of mean individual weight (w) depends on the logarithm of density (d) by the -3/4 power law (a slope of -1.5 and an intercept ranging from 2.3 to 5.0). The analysis of the w and d relationships in whole cohorts of two seaweed species from the Strait of Gibraltar shows deviations from the canonical equation. The kelp Phyllariopsis purpurascens (C. Agardh) Henry et South (Phaeophyta) growing at a 30-m depth has the lowest intercept value (0.6) recorded for any plant species and a slope not significantly different from -1.5. The slope value is in accordance with those found in species whose growth is not stopped by reproduction. Irradiance under a single layer of blades was lower than the photosynthetic light compensation point, and this could be due to overdispersion of the shoots and, in consequence, to the low intercept value of the self-thinning equation. The w to d relationships in Asparagopsis armata Harvey (Rhodophyta) show two different components: no dependence between these two variables (slope not significantly different from 0) at densities < 500 shoots·m^?2, and a slope more negative (-2.1) than proposed by the -3/2 power law at densities > 500 shoots·m^?2. The pattern at high densities could be due to intraspecific competition for light, whereas the slope ～0 at low densities could be related to inhibition of growth by reproduction (cystocarp and carpospore production). Therefore, rather than being considered exceptional, we suggest that a gradient of variability could be expected in the dependence of w on d when specific growth patterns and reproduction are considered.     

Is the tetrasporophyte of Asparagopsis armata (Bonnemaisoniales) limited by inorganic carbon in integrated aquaculture?1

Seaweeds cultivated in traditional land‐based tank systems usually grow under carbon‐limited conditions and consequently have low production rates, if no costly artificial source of inorganic carbon is supplied. In integrated aquaculture, the fish effluents provide an extra source of dissolved inorganic carbon (DIC) to seaweeds due to fish respiration. To evaluate if the tetrasporophyte of Asparagopsis armata (Harv.) F. Schmitz (the Falkenbergia stage) is carbon limited when cultivated with effluents of a fish (Sparus aurata) farm in southern Portugal, we characterized the DIC forms in the water, assessed the species photosynthetic response to the different DIC concentrations and pH values, and inferred for the presence of a carbonic anhydrase (CA)–mediated mechanism. Results showed that A. armata relies mainly on CO₂ to meet photosynthetic needs. Nevertheless, from pH 7.5 upward, the CO₂ supply to RUBISCO seems to derive also from the external dehydration of HCO₃^– mediated by CA. The contribution of this mechanism was essential for A. armata to attain fully saturated O₂‐evolution rates at the natural seawater DIC concentration (2–2.2 mM) and pH values (～8.0). We revealed in this study that seaweeds cultivated in fish‐farm effluents benefit not only from a rich source of ammonia but also from an important and free source of DIC for their photosynthesis. If supplied at relatively high turnover rates (～100 vol · d^?1), fish‐farm effluents provide enough carbon to maximize the photosynthesis and growth even for species with low affinity for HCO₃^–, avoiding the artificial and costly supply of inorganic carbon to seaweed cultures.     

Identification of reproductive sex-biased gene expression in Asparagopsis taxiformis (lineage 6) gametophytes

The sub-tropical red seaweed Asparagopsis taxiformis is of significant interest due to its ability to store halogenated compounds, including bromoform, which can mitigate methane production in ruminants. Significant scale-up of aquaculture production of this seaweed is required; however, relatively little is known about the molecular mechanisms that control fundamental physiological processes, including the regulatory factors that determine sexual dimorphism in gametophytes. In this study, we used comparative RNA-sequencing analysis between different morphological parts of mature male and female A. taxiformis (lineage 6) gametophytes that resulted in greater number of sex-biased gene expression in tips (containing the reproductive structures for both sexes), compared with the somatic main axis and rhizomes. Further comparative RNA-seq against immature tips was used to identify 62 reproductive sex-biased genes (59 male-biased, 3 female-biased). Of the reproductive male-biased genes, 46% had an unknown function, while others were predicted to be regulatory factors and enzymes involved in signaling. We found that bromoform content obtained from female samples (8.5 ± 1.0 mg·g⁻¹ dry weight) was ~10% higher on average than that of male samples (6.5 ± 1.0 mg·g⁻¹ dry weight), although no significant difference was observed (p > 0.05). There was also no significant difference in the marine bromoform biosynthesis locus gene expression. In summary, our comparative RNA-sequencing analysis provides a first insight into the potential molecular factors relevant to gametogenesis and sexual differentiation in A. taxiformis, with potential benefits for identification of sex-specific markers.     

THE INVASIVE GENUS ASPARAGOPSIS (BONNEMAISONIACEAE,RHODOPHYTA): MOLECULAR SYSTEMATICS,MORPHOLOGY, AND ECOPHYSIOLOGY OF FALKENBERGIA ISOLATES1

The genus Asparagopsis was studied using 25 Falkenbergia tetrasporophyte strains collected worldwide. Plastid (cp) DNA RFLP revealed three groups of isolates, which differed in their small subunit rRNA gene sequences, temperature responses, and tetrasporophytic morphology (cell sizes). Strains from Australia, Chile, San Diego, and Atlantic and Mediterranean Europe were identifiable as A. armata Harvey, the gametophyte of which has distinctive barbed spines. This species is believed to be endemic to cold‐temperate waters of Australia and New Zealand and was introduced into Europe in the 1920s. All isolates showed identical cpDNA RFLPs, consistent with a recent introduction from Australia. Asparagopsis taxiformis (Delile) Trevisan, the type and only other recognized species, which lacks spines, is cosmopolitan in warm‐temperate to tropical waters. Two clades differed morphologically and ecophysiologically and in the future could be recognized as sibling species or subspecies. A Pacific/Italian clade had 4–8° C lower survival minima and included a genetically distinct apomictic isolate from Western Australia that corresponded to the form of A. taxiformis originally described as A. sanfordiana Harvey. The second clade, from the Caribbean and the Canaries, is stenothermal (subtropical to tropical) with some ecotypic variation. The genus Asparagopsis consists of two or possibly three species, but a definitive taxonomic treatment of the two A. taxiformis clades requires study of field‐collected gametophytes.     

CARBON/NUTRIENT BALANCE IN RELATION TO BIOMASS PRODUCTION AND HALOGENATED COMPOUND CONTENT IN THE RED ALGA ASPARAGOPSIS TAXIFORMIS (BONNEMAISONIACEAE)1

We tested how the availability of carbon and nitrogen determines both the production of Asparagopsis taxiformis (Delile) V. Trevis. and content of the two major halocarbons, bromoform and dibromoacetic acid. The halogenated secondary metabolites of Asparagopsis species are particularly interesting from an applied perspective due to their remarkable antimicrobial activity. Terrestrial ecologists named the relationship between resources and secondary metabolites as the carbon (C)/nutrient balance (CNB) hypothesis. This relationship was tested both in the laboratory, with a factorial analysis using different concentrations of total ammonia (TAN) and dissolved inorganic carbon (DIC), and in an integrated aquaculture system where TAN and DIC fluxes of fish effluent were manipulated. The total C/N content of A. taxiformis biomass cultivated in laboratory was highly significantly linearly related to the content of both halocarbons, as predicted by the CNB hypothesis. A. taxiformis cultivated at low levels of carbon and high levels of nitrogen (N) (lowest C/N ratio) had the lowest content in both halogenated metabolites. Increased availability of CO₂ in the medium resulted in a general higher halocarbon content in the biomass, even though the effect was only statistically significant for bromoform at high levels of N. The farm experiments supported the results of the laboratory experiments. DIC fluxes had the highest effect on the production of both bromoform and biomass, as shown by multiple regression analysis. In A. taxiformis integrated aquaculture, C, rather than N, is the most important factor affecting the production of biomass and of valuable halocarbon secondary metabolites.     

Commercial farming of <Emphasis Type="BoldItalic">Asparagopsis armata</Emphasis> (Bonnemaisoniceae,Rhodophyta) in Ireland,maintenance of an introduced species?

The distribution of the gametophytes of Asparagopsis armata in Ireland currently encompasses a 75 km radius from a seaweed farm that commercially cultivates this species. This is a smaller range than the historic distribution shows since its arrival in Ireland in 1939. Fragmentation of this alga to seed ropes at a commercial seaweed farm seems to maintain the populations of this introduced species. Without this commercial seaweed farm A. armata would probably not flourish on the Irish west coast and this farm appears to act as a source pool for the populations. The sea surface temperatures (SSTs) of Irish waters are suitable for growth, survival and asexual reproduction of the tetrasporophyte and gametophyte; however, they may not be warm enough for sexual reproduction. The effect of temperature on the life cycle and distribution are discussed. Three species, i.e., Ulva lactuca, Plocamium cartilagineum and Cryptopleura ramosa, were hosts for epiphytic A. armata in summer in over 60% of the collected samples.     

Phylogeography of the invasive seaweed Asparagopsis (Bonnemaisoniales, Rhodophyta) reveals cryptic diversity

The rhodophyte seaweed Asparagopsis armata Harvey is distributed in the northern and southern temperate zones, and its congener Asparagopsis taxiformis (Delile) Trevisan abounds throughout the tropics and subtropics. Here, we determine intraspecific phylogeographic patterns to compare potential causes of the disjunctions in the distributions of both species. We obtained specimens throughout their ranges and inferred phylogenies from the hypervariable domains D1-D3 of the nuclear rDNA LSU, the plastid spacer between the large and small subunits of RuBisCo and the mitochondrial cox 2-3 intergenic spacer. The cox spacer acquired base changes the fastest and the RuBisCo spacer the slowest. Median-joining networks inferred from the sequences revealed the absence of phylogeographic structure in the introduced range of A. armata, corroborating the species' reported recent introduction. A. taxiformis consisted of three nuclear, three plastid and four mitochondrial genetically distinct, lineages (1-4). Mitochondrial lineage 3 is found in the western Atlantic, the Canary Islands and the eastern Mediterranean. Mitochondrial lineages 1, 2, and 4 occur in the Indo-Pacific, but one of them (lineage 2) is also found in the central Mediterranean and southern Portugal. Phylogeographic results suggest separation of Atlantic and Indo-Pacific lineages resulted from the emergence of the Isthmus of Panama, as well as from dispersal events postdating the closure event, such as the invasion of the Mediterranean Sea by mitochondrial lineages 2 and 3. Molecular clock estimates using the Panama closure event as a calibration for the split of lineages 3 and 4 suggest that A. taxiformis diverged into two main cryptic species (1 + 2 and 3 + 4) about 3.2-5.5 million years ago (Ma), and that the separation of the mitochondrial lineages 1 and 2 occurred 1-2.3 Ma.