Two harmful red tide causative raphidophytes Chattonella antiqua (Hada) Ono and C. marina (Subrahmanyan) Hara & Chihara occurred in Isahaya Bay, Japan in 2007. A mesocosm was installed in Isahaya Bay where outbreaks of Chattonella red tides (10³–10⁴ cells mL^-1) have frequently been observed. In order to examine the controlling factor of the diurnal vertical migration of Chattonella spp., vertical profiles of water temperature, salinity, chlorophyll fluorescence, concentration of DIN, PO₄-P and cell density of Chattonella spp. were continuously measured in the mesocosm during two periods (8–9 and 23–24 August in 2007). These data indicated two vertical profile types of Chattonella spp. For one type, cells of Chattonella spp. vertically migrated to the lower layer in shallower waters, and its cell density reached more than 10³ cells mL^-1. For another type, cells of Chattonella spp. continuously aggregated more than 10⁴ cells mL^-1 in the upper layer (0.5–2.0 m depth) until a night without the diurnal vertical migtation (moving to deeper waters during the day) in the condition of a sufficient nutirient in the upper layer (DIN: >1 µM, PO₄-P: >0.1 µM).

The expansion of blooming in the toxic dinoflagellate Alexandrium tamarense in Osaka Bay, eastern Seto Inland Sea, Japan, is examined from investigations at 13 or 20 sampling stations from 2002 to 2016, as well as long-term environmental monitoring from 1975 to 2016. Alexandrium tamarense blooms were mainly observed from March to May in Osaka Bay, at approximately 15˚C and 30 PSU. High cell densities were observed at low nutrient conditions, especially low phosphate, and low chlorophyll a. Temperatures in April and May gradually rose over the study period, but salinity from February to May showed no trend. Dissolved inorganic nitrogen (DIN) declined from the 1980s onward, and the downward trend became conspicuous after the 2000s. Phosphate drastically declined until the 1990s and then remained at a low level. Alexandrium tamarense blooms in Osaka Bay increased in scale approximately from the 2000s onward. Long-term trends in the dynamics of A. tamarense populations were considered to relate to the concentration of DIN and the cell density of the dominant diatom in Osaka Bay, Skeletonema spp. Short-term blooms of A. tamarense in Osaka Bay may have been induced by declines in phosphate concentration. However, the long-term expansion of A. tamarense appears to be related to a decline in DIN concentrations and subsequent decreased spring blooms, especially of Skeletonema spp.

We investigated the summer–autumn distributions of the subarctic copepod Neocalanus plumchrus (Marukawa) as food for young chum salmon, Oncorhynchus keta (Walbaum) in the surface waters of southwestern Okhotsk Sea, using zooplankton samples collected by vertical hauls (0–50 m depth) with Norpac net (mouth diameter: 45 cm, mesh opening: 0.315 mm) in July 2007, August 2012, September 2007, October 2004 and 2005. From investigation of the zooplankton samples, almost all 5th stage copepodids (CV) N. plumchrus were not observed during late September to early October due to the seasonal descent into deep layers. This disappearance from surface layer was 1 to 4 (mean: 2.5) months later than those in the adjacent waters (Oyashio region and Japan Sea). They developed lipid accumulation from L (low lipid store) or M (medium lipid store) type in summer into H (high lipid store) type in early autumn, maintaining high abundance as observed in late July. The development of lipid accumulation was considered to be closely related with the food availability for CV, sustained by the stably high primary production at the subsurface chlorophyll a maximum from summer to autumn, under the lower temperature due to the presence of the Intermediate Cold Water. Thus, the lipid-rich CV may serve as a large-sized and high-caloric food source for seasonal migratory fishes (e.g. young chum salmon) in the summer–autumn pelagic ecosystem in the Okhotsk Sea.

Phytoplankton is a potentially important component for attenuation of light in seawater, together with seawater itself, colored dissolved organic matter, and non-algal particles. In coastal areas, relative contribution of each optically active component, including phytoplankton, to total light attenuation differs largely among the areas due to wide variety of the water quality. In this study, we examined attenuation coefficient of downwelling photosynthetically active radiation (K_d) during a three-year period at the shallow cove, Shido Bay, eastern Seto Inland Sea in Japan. In parallel, we also measured proxies for abundance of the optically active components in the water column. The objective of this study was to reveal how phytoplankton contributes to light attenuation in this shallow cove, where light reaches the sea bottom. K_d through the water column varied seasonally between 0.235 and 0.838 m^-1 (mean±S.D.＝0.419±0.134 m^-1) with 0.7–24％ of the surface irradiance reaching on the seafloor (6.0 m beneath the sea surface). Light attenuation due to phytoplankton (chlorophyll-a; Chl-a) only accounted for approximately 20％ of total light attenuation (K_d) on average, suggesting that phytoplankton is not a major shading component in the averaged estimation. Meanwhile, because of the high coefficient of variation, not only non-algal particles, but also phytoplankton (Chl-a) contributed largely to the temporal variability in K_d. In the Seto Inland Sea, including its shallow areas such as Shido Bay, Secchi disk depth has increased gradually during the last three decades. Moreover, the biomass of seaweeds tends to increase recently, probably in response to the improvement of light environments in the overlying seawater. We suggest that even though phytoplankton is a relatively minor as a shading component, recent changes in underwater light climate in the Seto Inland Sea may be significantly related with changes in the phytoplankton biomass in relation to the reverse trend of eutrophication. If so, the abundance of benthic plants in the shallow area of the Seto Inland Sea should be indirectly controlled by phytoplankton through light shading.

Harmful algal blooms likely occur in enclosed coastal regions and sometimes cause the toxification and/or mortality of marine organisms such as fish and shellfish. To understand the bloom dynamics of harmful algae, it is important to clarify the effects of light intensity on the algal growth. This paper reviews the techniques of photoirradiation and cultivation for analyses of algal growth and discusses the growth responces (light-responsive growth) of harmful algae to various light intensities. As natural light enters the water column, the red color and infrared range of the light are absorbed near the surface of oceanic waters; thus, blue light accounts for a large proportion of PAR (photosynthesis available radiation) penetrating into water. High-brightness white LED (light-emitting diode) is capable of simulating closely such light spectrum of natural PAR in oceanic water. I, along with co-workers, evaluated the light-responsive growth of the harmful species such as Ostreopsis sp. 1 by using a newly developed photoirradiation-culture system which has white LEDs as light source. Our results showed that these tested harmful algae are able to grow at low light intensities of 10⁰~10¹ µmol photons m^-2 s^-1 and even under a strong light of 10³ µmol photons m^-2 s^-1. The optimal light intensities for algal growth seem to be different among the species. This suggests that harmful algae may have divergent growth strategies in response to light intensity. I think that high-brightness white LEDs and a photoirradiation culture system equipped with them can be powerful tools for the understanding of the bloom dynamics of harmful algae.

Most living organisms usually adapt to environmental conditions to survive in their natural habitats. To keep efficient photochemical reactions even in dramatically changing light, photosynthetic organisms optimize their photosynthetic apparatus including light-harvesting systems. Among the chlorophyll protein complexes that involved in photosynthetic electron transport, photosystem II (PSII), the oxygen-evolving chlorophyll protein complex, easily undergoes photodamage by a strong light, especially on the reaction center protein D1. The damaged D1 is rapidly replaced with newly synthesized one (Damage-repair cycle), and thus the net PSII activity is determined by the rate balance between damage and repair. To avoid highly exceeded damage on photosystems, several photoacclimation processes have been developed. NPQ (non-photochemical quenching), more precisely qE quenching (energy-dependent quenching), is one of the thermal dissipation mechanisms against excess light energy. On the other hand, frequent changes in light quality and intensity are also stressful for photosynthetic organisms. State transitions are considered as the acclimation processes against those kinds of conditions, in which light energy captured by light-harvesting proteins is re-distributed between two photosystems. The analyses using the mutant strains of a green alga Chlamydomonas reinhardtii and a model plant Arabidopsis thaliana have shown the molecular mechanisms and physiological roles of both NPQ and state transitions in laboratory conditions. However, actual behavior of these photoacclimation processes in natural environments is poorly understood.

This review describes the behaviors of photosynthetic apparatus and molecular mechanisms of NPQ and state transitions in laboratory conditions. Moreover, photobioreactor, an outdoor conditions-replaying system, is also introduced as one of the ways to connect the “lab” and “natural” photosynthesis.

In Hakozaki Harbor, Hakata Bay, Japan, diatoms bloom from June to August and decline from November to January, as typical cases of other temperate coastal waters. From November to January, nutrients tend to be sufficient levels for the diatom growth, although water temperature and photoperiod in this winter time decrease to the levels assumed to inhibit the growth and the abilities of germination/rejuvenation of diatoms. To clarify the effects of such lowering temperature and/or a short photoperiod in winter time, combinations of water temperature (25˚C, 15˚C)and photoperiods (14h L : 10h D, 11h L : 13h D) were examined on the growth of Skeletonema marinoi-dohrnii complex and germination/rejuvenation of several diatom species in the laboratory. The growth of S. marinoi-dohrnii complex under the low temperature and the short photoperiod condition (15˚C, 11h L : 13h D) was slightly lower than that under the high temperature and the long photoperiod condition (25˚C, 14h L : 10h D). On the contrary, germination/rejuvenation of the diatoms such as genus Skeletonema, Chaetoceros, and Thalassiosira were remarkably inhibited under low temperature and short photoperiod condition. These results suggest that the key mechanisms of diatoms decrease from November to January are most likely attributed to inhibition of germination/rejuvenation abilities under combination of low water temperature and a short photoperiod.

Primary action of photomovement in flagellate alga should be triggered by photoreceptor. Some algal photoreceptors are still unknown in the molecular level. Here I review the processes of finding for a couple of well-researched photoreceptors such as channelrhodopsins (ChR) in Chlamydomonas and photoactivated adenylyl cyclase (PAC) in Euglena. On the other hand, it has been reported that “light-antenna” structure plays a role in directional sensing of incident light. Despite the wide variety of flagellates, remarkable similarity can be found in the light-antenna structures. Here I also discuss the similarity of the light-antenna structures of Euglenophyta, Chlorophyta and Heterokonts.

Most noxious red-tide flagellates display diurnal vertical migration (DVM), in which they swim upward during the daytime and downward during the night. It has been suggested that the DVM cycle is produced by phototaxis and geotaxis controlled primarily by an endogenous clock. A DVM rhythm is entrained by shifting the light : dark cycle in most flagellates, but start times of upward and downward migrations under the light condition, photon flux density required for maintenance of the DVM cycle, and effects of day length on DVM cycle differ among the species. In natural environment, some dinoflagellates such as Alexandrium tamarense accumulate at subsurface layers during daytime, whereas a raphidophyte Chattonella antiqua frequently accumulates at surface. According to a recent laboratory work, C. antiqua cells display negative phototaxis to blue light, but mixing of slight green/orange light diminishes the effect of blue light.

The freshwater unicellular green alga Chlamydomonas reinhardtii is a model organism in various research fields such as flagellar motility, photosynthesis, and photomovements. Its photomovements (phototaxis and photophobic response) are remarkable cellular responses that include photoreception at the eyespot and following modulation of flagellar motility. Regulation of photomovements is important for cells to inhabit under suitable light conditions for photosynthesis; however, its molecular basis is poorly understood. We have studied how phototaxis and photophobic response are regulated, and found that cellular redox (reduction-oxidation) poise, which alters in response to changes in photosynthesis/respiratory activities, is a key signal. Here we summarize our recent study on redox regulation of Chlamydomonas photomovements.

Light is not only energy source but also important signal for photosynthetic organisms. Thus, photosynthetic organisms possess highly developed photoresponsive systems to acclimate to changing light environments. Cyanobacteria possess such systems and cyanobacteriochromes play central roles as a photoreceptor. In this review, we introduce cyanobacterial photoresponsive strategy regulated by the cyanobacteriochromes and our recent studies on photoresponsive systems of a unique cyanobacterium Acaryochloris marina.

The blue-light (BL) receptor, aureochrome was found and isolated from xanthophycean macro alga Vaucheria in 2007. Aureochrome is a BL receptor only in photosynthetic stramenopiles, such as diatoms, raphidophycean and brown algae. Aureochrome is composed of a bZIP (basic leucine Zipper) transcription factor domain at the N terminus and a LOV (Light-Oxygen-Voltage) domain at the C terminus. FMN (flavin mononucleotide) binds to LOV domain of aureochrome to function as a BL receptor. After BL irradiation, bZIP binds to cis elements (specific DNA binding sequence) with conformational change of LOV domain and regulates gene expression. These results suggest that various BL responses of photosynthetic stramenopiles are induced via the aureochrome. In this review, we introduce the function of aureochrome and future perspective.

Microscope technology has been used for observation of morphology and phenomena in tissues and cells. Recently, the technology has also been used for manipulation of living cells using the light, such as optogenetics which utilizes photo-dependent channels called channel rhodopsin-2 (ChR2) and photo-induced pump called halo rhodopsin (HR). We originally established another light induced cell manipulation technique which enables spatiotemporal in vivo expression of a target gene in a single-cell level. This method employs heat shock response of the target organisms to induce the target gene expression, and to heat the cell, a microscope technology using infrared irradiation is used. In this report, we introduce the method and examples of application that have been obtained by collaboration researches.