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A molecular-based floristic survey of marine red algal biodiversity was conducted offshore Tanegashima Island, which is located at the northern end of mesophotic coral ecosystems (MCEs), in the Ryukyu Archipelago, Japan. This study provides the first comprehensive catalog of red algae comprising the sublittoral marine flora of offshore Tanegashima Island, Japan, and represents the first exhaustive molecular-assisted survey of red algal marine flora in Japan. Morphological and molecular analyses using plastid-encoded rbc L and mitochondrion-encoded cox 1 genes revealed a total of 129 species, which included nine newly recognized species in Japan. Morphologically, 82 species were assigned to known species. Among the 82 species, 17 included cryptic species, and 25 appeared to have misapplied names. The remaining 47 species could not be identified to the species level, which indicates the necessity of a detailed reference library containing validated DNA barcodes and further taxonomic studies based on morpho-molecular analyses.

This study investigated the effects of different light spectral qualities and temperature on the photosynthesis and pigment content of a subtidal edible red alga, Meristotheca papulosa. Photosynthesis–irradiance (P–E) experiments were carried out under red (660 nm), blue (450 nm), green (525 nm, light-emitting diodes), and white light (visible light, metal halide lamp), and at 12, 20, and 28 °C, respectively. Maximum net photosynthetic rates (NPmax) were highest under green light. Other P–E parameter estimates were similar among algae under red, blue, and green light, including their lower initial slope (α) and higher saturation irradiances (Ek) as compared to those under white light. Additionally, NPmax and Ek under white light were highest at 28 °C, and lowest at 12 °C with characteristic photoinhibition at irradiances greater than 150 μmol photons m−2 s−1. Photosynthesis–temperature (P–T) experiment revealed that the maximum gross photosynthetic rate (GPmax) occurred at 22.1 °C, which was within the optimal temperature range of Fv/Fm (21.5–23.6 °C). Exposures to the different light qualities at 100 μmol photons m−2 s−1 for 7 days showed increased phycoerythrin (PE) concentration of algae under blue and green light, while chlorophyll-a and phycocyanin (PC) showed little variation in all light qualities. Therefore, considering future management prospects for M. papulosa mariculture, we suggest that green light could be utilized to enhance photosynthesis. Furthermore, if the aim is to achieve high PE content for an improved reddish-color fresh product, exposure to blue or green light could be a good alternative.

This study examined the effects of temperature and irradiance on photosynthetic characteristics of the macroscopic sporophyte (SPO) and microscopic gametophyte (GAM) stages of a subarctic brown alga, Saccharina japonica var. japonica (Laminariales) from Hokkaido, Japan. In vitro measurements under short- and long-term exposures were carried out by using optical dissolved oxygen sensors and the pulse amplitude modulation (PAM)-chlorophyll fluorometer, respectively. The heteromorphic life history stages of S. japonica showed photosynthetic optima at 23–23.3 °C, derived from the gross photosynthesis–temperature model. Maximum quantum yields (Fv/Fm) of SPO and GAM after 72 h of temperature exposures in the dark were reduced to near zero above 24 °C, indicating PSII inactivation. Such similarity in their temperature characteristics suggests the co-occurrence of both generations in the habitat despite the seasonal growth and reproduction of the species. Net photosynthesis–irradiance experiments in the two life history stages at 8, 16, and 24 °C revealed similarity in their light-saturated photosynthetic rates (NPmax = 3.02–4.41 μg O2 gww−1 min−1, SPO; 2.87–3.73 μg O2 gww−1 min−1, GAM), but saturation irradiances of SPO (Ek = 81–102 μmol photons m−2 s−1) were higher than those of GAM (48–69 μmol photons m−2 s−1). A slight decrease in net photosynthetic rates of GAM above 500 μmol photons m−2 s−1 was likewise observed. This difference may be related to the light regime of their natural habitat, suggesting the low irradiance adaptation of the microscopic stage that settles on rock crevices beneath algal canopies.

We examined the effects of four stressors, irradiance, temperature, desiccation, and salinity on the photosynthesis of a red alga, Agarophyton vermiculophyllum (= Gracilaria vermiculophylla, Gracilariales, Rhodophyta) from its native distributional range in Hokkaido and Kagoshima, Japan. Photosynthesis–irradiance (P–E) curves at 8, 16, and 28°C showed that the maximum net-photosynthetic rates (NPmax) and saturation irradiance (Ek) were highest at 28°C for both strains. Gross-photosynthesis determined at 8–40°C at 200 μmol photons m-2 s-1 showed that the maximum gross-photosynthetic rate (GPmax) occurred at 25.3°C for Hokkaido and 28.0°C for Kagoshima (ToptGP), which is almost consistent with the summer-time seawater temperature at each habitat. The temperature responses (4–40°C) of effective quantum yields (ΔF/Fm') of photosystem II during 7-day exposures were similar to that of oxygenic photosynthesis and the optimum temperature (ToptΔF/Fm′) was 20.0°C for Hokkaido and 30.1°C for Kagoshima. In the desiccation experiment, the ΔF/Fm' decreased with decreasing absolute water content (AWC); nevertheless, for samples with an AWC above 20%, ΔF/Fm' retuned to initial levels after subsequent 1-day rehydration in seawater, suggesting relatively strong tolerance to desiccation. This alga also showed a broad range of tolerance to salinity ranging from 20 to 60 psu in 7-day exposures, and the ΔF/Fm' tolerated 0 psu at 3-day exposure. The adaptations of A. vermiculophyllum from its native range in Japan to relatively high irradiance, a broad range of temperature, and strong osmotic (desiccation and salinity) tolerance explain its potentially high invasive capacity.

The effects of temperature, irradiance, and desiccation on the photosynthesis of a cultivated Japanese green alga Caulerpa lentillifera (Caulerpaceae) were determined by a pulse amplitude modulation (PAM)-chlorophyll fluorometer and dissolved oxygen sensors. The photochemical efficiency in the photosystem II (Fv/Fm and ΔF/Fm') during the 72-h temperature exposures (8, 12, 16, 20, 24, 28, 32, 36, and 40°C) was generally stable at 16–32°C but quickly dropped at lower and higher temperatures. The photosynthesis–temperature curve at 200 μmol photons m−2 s−1 also revealed that the maximum gross photosynthesis (GPmax) occurred at 30.7°C (30.5–30.9, 95% highest density credible intervals). Photosynthesis–irradiance curves at 16, 24, and 32°C quickly saturated, then expressed photoinhibition, and revealed that the maximum net photosynthetic rates (NPmax) and saturation irradiance (Ek) were highest at 32°C and lowest at 16°C. Continuous 6-h exposure to irradiances of 200 (low) and 400 (high) μmol photons m−2 s−1 at 16, 24, and 32°C expressed greater declines in their ΔF/Fm' at 16°C, revealing chronic chilling-light stress. The response to continuous desiccation (~480 min) under 50% humidity at 24°C showed that ΔF/Fm' dropped to zero at 480-min aerial exposure, and the treatments of more than 60-min desiccation did not return to the initial level even after 24-h subsequent rehydration in seawater. Likewise, ΔF/Fm' fell when the absolute water content (AWC) of the frond dropped below AWC of 90% and mostly did not return to the initial level even after 24-h subsequent rehydration in seawater, signifying a low tolerance to desiccation.

We determined the effects of irradiance, temperature, and desiccation on the photosynthesis of a brown alga, Sargassum muticum (Fucales), from its native distributional range in Japan by using a pulse amplitude modulation (PAM)-chlorophyll fluorometer and optical dissolved oxygen sensors. Photosynthesis–irradiance curves at three temperatures (8, 20, and 28°C) showed that the maximum net photosynthetic rates (NPmax) and saturation irradiance were highest at 28°C. Gross photosynthesis determined at 8–36°C (every four increments) and 300 μmol photons m−2 s−1 showed that the maximum gross photosynthetic rate (GPmax) occurred at 19.5°C (ToptGP), which is consistent with the seawater temperature at its peaked abundance in Japan. The maximum quantum yields (Fv/Fm) of photosystem II (PSII) during the 72-h temperature exposures were above 0.60 at 8–28°C but dropped at higher temperatures. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m−2 s−1 at three temperatures showed remarkable decline in the effective quantum yields (ΔF/Fm') of PSII under high irradiance at 8°C only; the Fv/Fm measured after 12-h dark acclimation also did not recover to initial values, signifying its sensitivity to photoinhibition at 8°C. Furthermore, the alga exhibited tolerance to 2 h of desiccation with 80% of water loss from the thallus, and ΔF/Fm' recovered after 24 h of rehydration in seawater, suggesting potential of photosynthetic recovery of this alga at such low hydration threshold. In conclusion, the adaptation of S. muticum to relatively high irradiance, to broad range of temperature (8–28°C), and to desiccation explains its potentially high invasive capacity.

The effects of temperature on the photosynthesis and growth of a subtropical red alga, Phycocalidia tanegashimensis (= Pyropia tanegashimensis, Bangiaceae, Bangiales) from Tanegashima Island, Japan, were determined to better understand the ecology of the macroscopic gametophyte. Net photosynthesis–irradiance (P–E) curves, determined at 12, 20, and 28°C, revealed that the maximum net photosynthetic rates occurred at 28°C. A gross photosynthesis–temperature (P–T) curve, determined at temperatures between 8 and 36°C, supported this result and indicated that optimal temperature (ToptGP) for maximum gross photosynthetic rates was 27.3°C (25.9–28.8 95% highest density credible intervals, HDCI). However, after 4 days of incubation at constant temperature, the effective quantum yields (ΔF/Fm') of photosystem II, determined between 4 and 40°C under 50 μmol photons m−2 s−1 (12L:12D), were greatest at 22.1°C (20.9–23.4 HDCI) and declined quickly below 20°C and 32°C. The relative growth rate (RGR) was highest at 25.8°C (22.5–28.1 HDCI) at the conclusion of a 6-day culture (4 to 36°C) under 50 μmol photons m−2 s−1 (12L:12D). Compared to other temperate species of Bangiaceae, P. tanegashimensis appears to be well adapted to relatively high temperatures. The macroscopic gametophyte stage of Bangiaceae is regarded to be adapted to cold temperatures to overwinter; nevertheless, the significance of our study is that a year-round occurrence of P. tanegashimensis is enabled by its high optimal temperatures for photosynthesis and growth.

We determined the chronic effects of dehydration on the photochemical efficiency of a cultivated brown alga, Undaria pinnatifida (Alariaceae, Laminariales), in young sporophytes as cultivated sporelings. The effective quantum yields of photosystem II (ΔF/Fm′) at 50% humidity decreased markedly after 20 min of emersion and dropped almost zero after 60 min of emersion; the values did not restore even after subsequent 1-day immersion. The decreasing values coincided with a decrease in absolute water content of less than 40%. However, under 99% humidity up to 5-day emersion, the ΔF/Fm′ well remained and the last state exhibited a similar level to the initial value after a 48-h emersion at 20 °C and after 72 h of emersion at 10 °C, suggesting that the thalli were not truly dehydrated under saturated humidity and that photosynthetic activity was maintained for several days even without immersion in seawater. In addition, the subsequent growth of young sporophytes exposed to transportation storage stress featuring (1) immersed in seawater with aeration (ST1), (2) those without aeration (ST2), and (3) wrapped in paper towels moistened with seawater (ST3) revealed that the sporophytes exposed at lower temperature exhibited a higher subsequent growth than those at a higher temperature. The subsequent growth of ST3 was lower than ST1; however, the values of ST3 were higher than those of ST2 more than 12 °C, associating with poor seawater quality without aeration. These results indicated that the maintenance of moisture in the alga at low temperatures might be essential for the transportation of the sporelings.

The combined effects of temperature and irradiance and the influences of desiccation and salinity on the photochemical efficiency in a subtropical red alga, Phycocalidia tanegashimensis (= Pyropia tanegashimensis, Bangiaceae) from Tanegashima Island, Japan, were determined to reveal how this species has adapted to its habitat in the splash zone. Continuous 6-h exposure to irradiance of 200 (low) and 1000 (high) µmol photons m−2 s−1 at 12, 20, and 28 °C showed a decline in the effective quantum yields (ΔF/Fm′) of photosystem II (PSII) during the exposures; nevertheless, the maximum quantum yields (Fv/Fm) of PSII measured in subsequent 14-h dim-light acclimation almost returned to initial values at 20 and 28 °C, revealing its high capacity to recovery. In contrast, those under both low and high irradiances at 12 °C did not recover to initial values even after 14-h dim-light acclimation, signifying enhanced inhibition under irradiance at low temperature. The response to continuous desiccation (~ 480 min) under 50% humidity at 24 °C showed that the ΔF/Fm′ decreased with decreasing absolute water content (AWC). However, for the samples with an AWC above 10%, ΔF/Fm′ mostly recovered to initial levels after subsequent 1-day rehydration in seawater, suggesting relatively strong tolerance to desiccation. This alga also tolerated a broad range of salinity (i.e., 10–60 psu) under 3-day exposures. The adaptations of P. tanegashimensis to relatively high irradiance, warm temperature, and a strong osmotic (desiccation and salinity) tolerance may explain its high capacity to flourish in the splash zone in the subtropical environment of Japan.

We investigated the effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of juvenile (JS) and mature sporophytes (MS) of a temperate edible brown alga Undaria pinnatifida cultivated in northeastern Japan, using optical dissolved oxygen sensors and pulse amplitude modulation (PAM)–chlorophyll fluorometry. Photosynthesis–PAR experiments at 8, 15, and 22 °C revealed that the maximum net photosynthesis differed with temperature. Values for compensation and saturation PAR indicated a high affinity for a low-light environment. The photosynthetic response of U. pinnatifida to temperature revealed an optimum temperature for oxygenic photosynthesis of 18 °C, which was lower than other laminarian kelps and inconsistent with the wide distribution range of this U. pinnatifida. High temperature and irradiance tolerance differed between MS and JS. Notably, Fv/Fm of MS remained above 0.5 at 28 °C after 168 h of incubation under dark conditions, whereas Fv/Fm was reduced to 0 for the juveniles. Six hours of constant exposure to 200 (low) and 1000 μmol (high) photons m−2 s−1 at 8, 15, and 22 °C induced chronic photoinhibition in all both MS and JS. In contrast to MS, the JS did not recover from photoinhibition in either of the PAR treatments, regardless of a period of darkness. The physiological performance of U. pinnatifida sporophytes indicate an affinity to low light, especially for JS. We expect that these finding will lead to further improvements in commercial production methods to prevent the withering of juvenile sporophytes.