We investigated the photoresponses of diadinoxanthin (DD) and diatoxanthin (DT) using Isochrysis galbana cultures that had been acclimated to four different levels of photon flux density (PFD). DD can play a role as a photosynthetic light-harvesting pigment, whereas DT can play a role as a photoprotective pigment. The PFD ranged from limited light conditions (LL) to saturated light conditions (SL) for growth. The photoresponses of these pigments during light–dark transitions at each acclimation PFD were consistent. The photoresponses of these xanthophyll pigments, as normalized to either cell density or chlorophyll a (Chl a) concentration, to light–dark transitions varied with acclimation PFD. The stoichiometric variations between the DD or DT : Chl a ratio observed at the two higher acclimation PFDs suggested that these xanthophyll dynamics are controlled by xanthophyll cycling processes in addition to de novo processes and interconversion between DD and fucoxnathin under SL conditions. Significant linear relationships were observed between the levels of DD and/or DT normalized to either the cell density or Chl a concentration. Based on the cellular pigment content and the xanthophyll : Chl a ratio, the slopes of the linear relationships normalized to either the cell density or Chl a concentration were significantly related to the acclimation PFD linearly and sigmoidally, respectively. Similarly, the acclimation PFD at the interaction of the sigmoidal change and the transition point from the LL to SL condition suggests that these xanthophyll pigment dynamics can be characterized by acclimation to the light conditions. An examination of the slope may provide information leading to a better understanding of photoacclimation in the algal photosynthetic apparatus.

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Tintinnid ciliates have been traditionally classified according to morphological and morphometric features of the lorica. To examine if the morphological characteristics of loricae reflect the phylogenetic relationships, we collected 23 tintinnid morphospecies (11 genera) and reconstructed phylogenetic trees based on 84 partial sequences of nuclear small subunit rRNA (SSrRNA) gene fragments of 44 morphospecies from a total of eight families (55 sequences from this study, and 29 sequences from the literature). We found that tintinnid ciliates could be classified into six clades consisting of five major clades. While one clade consisted of one family, each of the other five clades consisted of more than two families. Although two families were found only in one clade, the remaining six families were found in more than two clades. In addition, each of six morphospecies was found in more than two major clades, indicating that they are polyphyletic. Principal coordinate analysis showed that morphology of the lorica overlapped substantially between the clades. These results imply that most of the morphological and morphometric traits of marine tintinnid species do not reflect SSrRNA genetic distances even at family levels. It is necessary to identify new characteristics that reflect phylogenetic relationships robustly.

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Transmission electron microscopy (TEM) is a basic technique used to study the ultrastructures of phytoplankton. However, the use of chemical fixation to prepare phytoplankton samples for TEM is laborious and time-consuming. Furthermore, chemical fixation often causes artificial structural changes (artifacts) in phytoplankton cells. In this study, we induced and optimized a rapid freezing and freeze substitution (RFS) method, accompanied by a filter dehydration system, to prepare TEM samples of marine microalgae, including Heterosigma akashiwo, Chattonella antiqua (Raphidophyceae), Heterocapsa circularisquama (Dinophyceae), Chaetoceros tenuissimus (Bacillariophyceae), and Teleaulax amphioxeia (Cryptophyceae). The whole procedure was much more efficient than the traditional method. In addition, the fine structure of each microalgal cell was well preserved. For example, the fine structures of the nuclear membrane, nuclear pores, and several vesicles were clearly observed, which were comparatively difficult to observe using the chemical fixation method. In addition, this new technique was applicable to the natural blooming cells of H. akashiwo, and high-resolution ultrastructural images were successfully observed. This is the first report on the application of the modified-RFS method for natural phytoplankton assemblages. This method can be used for diverse phytoplankters in natural waters, and the high-quality observations are expected to provide useful information toward understanding phytoplankter physiology and ecology.

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Variation in the δ¹³C of a cultured marine cyanobacterium (Agmenellum quadruplicatum, strain PR-6) is described with an emphasis on the relationships between growth rate, pCO₂, and δ¹³C. An average nitrogen isotope fractionation of 1.0017 was obtained during N₂ fixation by cultured marine cyanobacteria. This result suggests that nitrogen supply via N₂ fixation may be characterized by a low δ¹⁵N, down to ca. -2‰ in marine environments, where the average δ¹⁵N is higher (at ca. 6‰) for major inorganic nitrogenous compounds such as nitrate. The natural abundances of nitrogen and carbon isotope ratios for cyanobacteria collected from the McMurdo Dry Valleys, Antarctica, were characterized by extremely low δ¹⁵N and widely ranging δ¹³C. Summarizing these and other data obtained, an isotopic map of cyanobacteria and other plankton in aquatic ecosystems was constructed and the ecological implications are discussed. Our results suggest that the δ¹³C of marine phytoplankton is positively correlated with sea surface temperature and can be a useful parameter for estimating in situ growth rates in the open ocean.

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The genetic diversity and population structure of a vestimentiferan annelid, Lamellibrachia satsuma, were analyzed based on the nucleotide sequences of mitochondrial DNA obtained from specimens from Kagoshima Bay in Kyushu, Southwestern Japan, which is the shallowest habitat of this species, and Nikko and Daikoku Seamounts in the northern Mariana Arc. The Kagoshima and northern Mariana populations showed significant genetic differentiation, with the genetic diversity of the former being lower than that of the latter. The estimated ages of deviation between the populations in these sea areas as well as expansion of the Kagoshima population suggest the presence of undiscovered habitat(s) of this species in the northwestern Pacific.

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Six novel microsatellite markers were developed in Corbicula japonica and used (along with an additional locus that was previously reported for another species) for population analysis of this species in Japan. The samples were collected from 4 local populations: Lake Abashiri and Lake Oikamanai in Hokkaido, Lake Ogawara in Aomori Prefecture, and Lake Shinji in Shimane Prefecture. The genetic diversity of the Shinji population was lower than that of the other 3 populations. Pair-wise F_ST analysis revealed significant genetic differentiation among all of the local populations and suggested two groups: northern (Abashiri, Oikamanai, and Ogawara) and southern (Shinji). Bayesian clustering analysis also supported this grouping, and analysis of molecular variance showed a hierarchical population structure for the two groups when genetic differentiation was calculated using R_ST. It is concluded that these local populations are genetically different from each other and that there is marked differentiation between geographically distant populations. These results demonstrate that these new microsatellite markers are useful for estimating genetic diversity and population structure, and should aid future research on effective aquaculture systems and sustainable fisheries for this species.

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