We investigated ontogenetic variations in dry mass and nucleic acid contents of the planktonic crustacean Artemia salina. Individual contents of DNA and RNA increased exponentially with incubation day and developmental stage, except for newly-hatched nauplii. High variability was found in the temporal and ontogenetic patterns of dry (DW) and ash-free dry weights (AFDW) due to the mixture of different ages and stages, and thus RNA/DNA ratios were not significantly associated with weight-specific growth rates. RNA/DNA and AFDW/DNA ratios decreased from newly-hatched naupliar to middle post-metanaupliar stages, but AFDW/DNA ratios decreased while RNA/DNA ratios increased from late post-metanaupliar stages. These results suggest that nucleic acid ratios are not the optimal indicator for evaluation of weight-specific growth rates when there is high variability in body mass, but that a combination of RNA/DNA and AFDW/DNA ratios can be indicative of important stages in ontogenetic development, such as indicating yolk consumption of newly hatched nauplii and gonad maturation in late life stages of A. salina.

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To understand the ecological relationships between diatoms and viruses in marine environments, extensive culture experiments under diverse environmental conditions are essential. Previous methods developed for enumeration of diatom cells and viruses in culture experiments, i.e., manual counting using a microscope and the most probable number (MPN) method, are time consuming and labour intensive. In this study, we developed a rapid enumeration method for cells of the planktonic diatom Chaetoceros tenuissimus and the genome copy numbers of the RNA virus CtenRNAV using image-based cytometry (IBC) and real-time quantitative polymerase chain reaction (qPCR), respectively. The live cell numbers counted using IBC were similar to those obtained by direct microscopic counts within the range from 10⁴ to 10⁶ mL⁻¹. The genome copy numbers of the viruses, as determined by qPCR, were almost 2–20 times higher than those determined by the MPN method owing to the presence of defective particles and virion aggregations in the samples. The dynamic patterns of virus abundances were similar for both methods; therefore, qPCR could be applied as an alternative method for enumeration of infectious virus units. The IBC and qPCR methods developed for diatom cells and viruses in this study were rapid and accurate and should therefore have applications as powerful tools for improving our understanding of diatom-virus ecological relationships.

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Zonation has been a major focus of intertidal ecology. In general, sandy beaches are thought to have three zones (supralittoral, littoral, and sublittoral zones). In the present study, 41 areas of sandy beaches were surveyed along the Japan Sea coast of Honshu (from Aomori to Yamaguchi Prefectures) characterized by a small tidal range, large seasonal fluctuations in the mean tide level, and harsh wind-driven waves in winter, and the applicability of the three-zone scheme was examined. Overall, a total of 316 beach stations were surveyed, 28 taxonomic units were identified, and the most abundant species was the amphipod Haustorioides japonicus. The taxonomic units were grouped using multivariate techniques into a total of 7 assemblages, and the relationship of these assemblages to zonation was examined. Investigation of the indicator species of the 7 assemblages and their vertical position on the beaches showed that one assemblage corresponded to the supralittoral zone (indicators were talitrid amphipods and the isopod Tylos granuliferus). Another assemblage corresponded to the sublittoral zone, and the other five assemblages to the littoral zone. The assemblage of the sublittoral zone tended to disappear on the northern beaches, while one of the littoral assemblages whose indicators included a mysid species, Archaeomysis kokuboi, tended to occur on the northern beaches. Thus, the three-zone scheme can be applied to sandy beaches of the Japan Sea, but there are large variations among the beaches.

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We investigated the dependence of alkaline phosphatase activity (APA) on temperature and light intensity in cultured strains of the diazotroph Trichodesmium erythraeum isolated from the Kuroshio Current in the East China Sea. In contrast to the low levels of APA observed under phosphorus (P)-replete conditions, cultures grown under P-limited conditions exhibited strong APA, and the levels steadily increased with increases in temperature and irradiance from 22–30°C and 40–260 µmol m⁻² s⁻¹, respectively. Based on the chlorophyll a and APA levels in actively growing T. erythraeum, the mean hydrolyzing time required to double the cellular P quota was estimated to be 0.68 and 9.7 times shorter than the reported values under P-replete and P-limited conditions, respectively. This strongly suggests that APA under P-limited conditions is excessively active compared with the cellular P requirement. Furthermore, under P limitation intense bacterial aggregations were observed with fluorescence microscopy around the T. erythraeum trichomes in the same location as the visualized APA, suggesting enhancement of bacterial growth by surplus phosphate due to the excess APA, as well as possible release of dissolved organic matter from T. erythraeum. The present study suggests an important role for T. erythraeum as a phosphate supplier to other organisms in the oligotrophic ocean.

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