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The important role of vegetated ecosystems in the sequestration of carbon has gained strong interest across a wide variety of disciplines. With evidence growing of the potential for macroalgae ecosystems to capture carbon, there is burgeoning interest in applying newfound knowledge of carbon capture rates to better understand the potential for carbon sequestration. Seaweed farms are expected to play a significant role in carbon capture; advocates for the expansion of seaweed farms are increasing in many countries. In general, seaweed farms are expected to be highly productive, although whether they are autotrophic or heterotrophic ecosystems and hence potential exporters of carbon, is under debate. Therefore, we present our investigation of three seaweed farms, two in northern Japan and one in southern Japan. We examine the frequency of autotrophic days and compare potential rates of carbon capture of the seaweed farms with two natural macroalgae ecosystems and one degraded site. We estimated potential carbon capture rates by calculating the net ecosystem productivity from continuous recordings of dissolved oxygen concentrations under natural environmental conditions. The net ecosystem production rates for the natural ecosystems in Arikawa Bay and Omura Bay were equivalent to 0.043 and 0.054 [g C m-2 d-1] m-1, respectively. Whereas, for the degraded ecosystem in Tainoura Bay, it was -0.01 [g C m-2 d-1] m-1. We reveal that the Undaria pinnatifida farm in Matsushima Bay experience autotrophy more often than natural ecosystems, although for seaweed farms producing U. pinnatifida in Hirota Bay and Cladospihon okamuranus at Bise Point, autotrophy was less frequently observed. Nevertheless, up to 14.1 g C m-2 (0.110 g C m-2 d-1) was captured by the production of U. pinnatifida and 3.6 g C m-2 (0.034 g C m-2 d-1) was captured by C. okamuranus, and the total yield of carbon captured during 2021 production season for these farms was 43,385 kg C.

Domoic acid (DA), the causative agent of amnesic shellfish poisoning, is produced by select organisms within two distantly related algal clades: planktonic diatoms and red macroalgae. The biosynthetic pathway to isodomoic acid A was recently solved in the harmful algal bloom-forming diatom Pseudonitzschia multiseries, establishing the genetic basis for the global production of this potent neurotoxin. Herein, we sequenced the 507-Mb genome of Chondria armata, the red macroalgal seaweed from which DA was first isolated in the 1950s, identifying several copies of the red algal DA (rad) biosynthetic gene cluster. The rad genes are organized similarly to the diatom DA biosynthesis cluster in terms of gene synteny, including a cytochrome P450 (CYP450) enzyme critical to DA production that is notably absent in red algae that produce the simpler kainoid neurochemical, kainic acid. The biochemical characterization of the N-prenyltransferase (RadA) and kainoid synthase (RadC) enzymes support a slightly altered DA biosynthetic model in C. armata via the congener isodomoic acid B, with RadC behaving more like the homologous diatom enzyme despite higher amino acid similarity to red algal kainic acid synthesis enzymes. A phylogenetic analysis of the rad genes suggests unique origins for the red macroalgal and diatom genes in their respective hosts, with native eukaryotic CYP450 neofunctionalization combining with the horizontal gene transfer of N-prenyl-transferases and kainoid synthases to establish DA production within the algal lineages. [ABSTRACT FROM AUTHOR]

Photosynthetic responses to photosynthetically active radiation (PAR) and temperature were investigated in the macroscopic sporophyte (SPO) and microscopic gametophyte (GAM) stages of the brown alga Costaria costata from Hokkaido, Japan. Measurements of net photosynthesis, respiration rates and photochemical efficiency were carried out using dissolved oxygen sensors and a pulse amplitude-modulated fluorometer. Net photosynthesis-irradiance curves of the two life history stages revealed a similarity in their light-saturated photosynthesis rates but a difference in initial slopes, with lower efficiency in oxygenic evolution for SPO than for GAM. Consequently, compensation PAR (Ec = 9 μmol photons m-2 s-1) and saturation PAR (Ek = 243 μmol photons m-2 s-1) of the sporophyte were higher than for the gametophyte (Ec = 1 μmol photons m-2 s-1; Ek = 44 μmol photons m-2 s-1). This reflects the low-PAR adaptation of the microscope stage that is commonly found on the underside of rocks or in crevices where light exposure is limited. Continuous exposures to low (6 h, 100 μmol photons m-2 s-1) and high PAR (1000 μmol photons m-2 s-1) induced photoinhibition of photosynthesis, with larger declines in gametophyte effective quantum yields (ΦPSII). Gametophyte maximum quantum yields (Fv/Fm) also did not fully recover even after 12 h of dark acclimation, indicating photodamage. As for the temperature response in C. costata, gross photosynthesis and Fv/Fm characteristics were similar for both developmental stages, with temperature optima ranging from 14°C to 22°C. Their Fv/Fm and oxygenic net photosynthesis were relatively insensitive to low temperatures but declined above 24°C. This may explain the disappearance of C. costata sporophytes from their habitat during summer as well as their southern distributional limit in the western Pacific. [ABSTRACT FROM AUTHOR]

The effect of photosynthetically active radiation (PAR) and temperature on the photosynthesis of a Japanese brown alga, Ecklonia radicosa, was determined by using dissolved oxygen sensors and pulse-amplitude modulated (PAM) fluorometry. The effective quantum yield (ΦPSII) of the sporophytes, in a natural population in shallow water, showed a characteristic noontime decline that indicated the influence of high PAR. However, ΦPSII of sporophytes in deeper water (-10 to -20 m) did not appear to be affected and maintained relatively high values of ΦPSII. In laboratory measurements of ΦPSII, after 12 h of continuous low and high PAR (100 and 1000 µmol photon m-2 s-1), a relatively greater decrease in ΦPSII in the high PAR treatment was observed. Furthermore, despite an overnight dark acclimation, the maximum quantum yield (Fv/Fm) under the high PAR treatment did not fully recover. A net photosynthesis-irradiance (P-E) curve was generated at 20°C, which revealed low compensation and saturation irradiances (Ec, and Ek) of 14 and 45 µmol photon m-2 s-1, suggesting adaptation to a low PAR environment. Results from experiments to determine the net/gross photosynthesis and dark respiration rates revealed that the maximal gross photosynthetic rates occurred at 27.5°C and were 5.18 µg O2 gww -1min-1; whereas, the dark respiration rate increased exponentially with a mean value of 1.35 µg O2 gww-1 min-1 at 22°C. Fv/Fm was relatively stable at low temperature, and the highest value (0.73) occurred at 16.5°C. The field survey and laboratory experiments revealed that E. radicosa is well adapted to relatively low PAR and water temperatures typical of warm temperate coastal regions in Japan. The poor response to high PAR and the inability of sporophytes to fully recover overnight from high PAR exposure indicates that the vertical distribution of this species is driven by the PAR environment. [ABSTRACT FROM AUTHOR]

The effect of irradiance and temperature on the photosynthesis of a Japanese native brown alga, Sargassum fusiforme, was determined by using dissolved oxygen sensors and pulse amplitude-modulated fluorometry. In situ measurements of the effective quantum yield (ΦPSII) decreased as irradiance increased at 12:00 PM and recovered in the late afternoon, indicating that a proportionately greater number of reaction centres were closed. Likewise, laboratory measurement after 6 hours of exposure to low and high (300 and 1000 μmol photons m-2 s-1, respectively) irradiances showed a relatively greater decrease in ΦPSII at high irradiance, and both treatments recovered overnight. A model of the net photosynthesis-irradiance relationship determined at 20°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 391 μmol photons m-2 s-1 [95% Bayesian credible interval (BCI) of 299-515 μmol photons m-2 s-1]. Gross photosynthesis and dark respiration rates determined over a range of temperatures (8-36°C) revealed that the maximal gross photosynthetic rate was 24.1 (BCI: 21.5-26.8) μg O2 gww-1 min-1 at 22.9°C (BCI: 19.8-25.8), and the dark respiration rate increased nonlinearly from a low of 0.5 μg O2 gww-1 min-1 at 8°C to a high of 8.9 μg O2 gww-1 min-1 at 36°C. The highest value of the quantum yield (Fv/Fm = ΦPSII at 0 μmol photons m-2 s-1) was estimated to occur at 22.9°C (BCI: 21.8-23.9) and was 0.72 (BCI: 0.70-0.73). The specimens examined appeared to be well adapted to the natural temperature range of the study site. Although the irradiances on the fronds were strong enough to induce a drop in quantum yield, our evidence clearly suggests that full recovery occurs overnight. [ABSTRACT FROM AUTHOR]

The incidence of Gracilaria bursa-pastoris, which is found mainly in Europe, in the northwest region of the Pacific Ocean was recently brought into question following the descriptions of G. parvispora and G. chouae from Hawaii and China, respectively. We examined the morphology ofthalli previously identified as G. bursa- pastoris in Korea and Japan, and analyzed the plastid rbcL gene sequences of Korean and Japanese samples, and a G. parvispora sample from Hawaii. The morphological and sequence data both indicate identity between the taxon from Korea/Japan and G. parvispora from Hawaii, but significant differences exist between G. parvispora and G. bursa-pasloris from Europe. We propose that the thalli from Korea and Japan should be renamed G. parvispora. The rbcL tree grouped G. parvispora and G. damaecornis together, as a sister to the G. tikvahiae/G. isabellanalG. cune~fo1ia dade. This is the first recorded occurrence of G. parvispora outside Hawaii. [ABSTRACT FROM AUTHOR]

The effects of temperature (20, 24 and 28 °C) and irradiance (15 and 40 μmol photon m−2 s−1) on the nitrate and ammonium uptake rates of the subtropical red alga, Laurencia brongniartii, were investigated to prepare for tank cultivation. Nitrate uptake followed saturation kinetics and was faster at higher irradiances and temperatures. In contrast, ammonium uptake was linear over the experimental range and was not affected by an increase in temperature. A parameter, β, was calculated to compare substrate uptake rates of nitrate along the linear portion of the uptake curve with that of ammonium. For nitrate, β was lower at low irradiance and higher at high irradiance (β = 0.007 ± 0.003 and 0.030 ± 0.002 [μmol N L−1 (μmol N gww−1 d−)−1], respectively). However, β was 0.023 ± 0.002 and 0.034 ± 0.002 [μmol N L−1 (μmol N gww−1 d−1)−1] for ammonium, suggesting a preference for ammonium over nitrate. [ABSTRACT FROM AUTHOR]

Abstract Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (Pmax) was 4.41 µmol photon m-2 s-1 at 26 °C and 4.07 µmol photon m-2 s-1 at 28 °C. Saturating irradiance occurred at 95 µmol photon m-2 s-1 at 26 °C and 65 µmol photon m-2 s-1 at 28 °C. In contrast, growth experiments at 41.7 µmol photon m-2 s-1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day-1 at 28 °C and 25 µmol photon m-2 s-1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation. [ABSTRACT FROM AUTHOR]