Phosphorus (P) supplies from the sediment are important when evaluating the impacts of nutrient dynamics on the ecosystem. Many studies have reported P dynamics based on gradients of pore water (PW)-phosphate (PO43-) concentrations at the sediment–water interface and iron-related abiotic reactions in response to redox conditions. However, the aim of the present study is to evaluate the contributions of intracellular (IC)-P and marine organism-mediated reactions to P-dynamics under variable environmental conditions. The focus of this study is the intertidal mudflat habitat of the innermost part of Ariake Bay, Kyushu, Japan, during winter. The vertical distribution of several forms of P fraction [i.e. PW- and IC-dissolved inorganic/organic-P (DIP/DOP), and particulate inorganic/organic-P (PIP/POP)], the number of microalgae and bacteria, and chlorophyll a concentrations in the sediment were measured. The results obtained revealed that the IC-P pool, especially IC-DOP, accounted for 76% of the total dissolved P in the surface sediment layer, mainly associated with the microphytobenthos. Additionally, phytoplankton, which comprised more than 60% of the surface microalgae in tidal-flat sediments, are considered to carry river-derived-P to the intertidal flats via inner bay-specific physical movements and disturbances. Incubation experiments on mud sediment under oxic/anoxic conditions suggest that dissolved P fluxes between the IC pool and PW occur actively in response to redox conditions, according to the balance between microalgal uptake and bacterial degradation. These results indicate that evaluation of the IC-P pool is indispensable to understanding P cycling in intertidal mudflats, and that microalgae could play important roles not only as food sources, but also as P reservoirs and sources of PO43-.
The mass coral bleaching event in South East Asian countries in 2010 caused severe damage to reef dwellers on the coast of the Andaman Sea and the Gulf of Thailand. Bleaching is a process where the symbiosis between corals and zooxanthellae (dinoflagellates of the genus Symbiodinium) breaks down. The symbiosis between Symbiodinium and corals is essential to the productivity, survival and success of corals. Symbiodinium associated with coral hosts is classified into six genetically distinct clades based on nuclear ribosomal DNA and partial chloroplast large subunit rDNA. Each clade possesses unique environmental, ecological and geographic variations, influencing the resilience of corals to elevated temperatures and bleaching. This study was the first field observation of changes in Symbiodinium communities in the Gulf of Thailand over two years including a natural bleaching event from late April to June, 2010. Phylogenetic clades and diversity of Symbiodinium inhabiting soft corals of the genus Sinularia were investigated using the analysis of chloroplast gene sequence variation at the ribosomal large subunit 23S Domain V. A total of 69 Sinularia spp. samples were obtained from two sampling sites off Jorake and Juang Islands (Sattahip, Chonburi Province) during two sampling periods: before (November 2008–January 2010, n=45) and after (November 2010, n=24) the 2010 coral bleaching event. The results revealed a significant change in the Symbiodinium community after the bleaching event. Before bleaching, most of Symbiodinium clade D was detected from Jorake Island while clade C was dominant in the Juang Islands. In contrast, after bleaching, only clade D was identified from both sampling sites.
Both juveniles and adults of the venerid clam Meretrix lusoria secrete a transparent mucous cord and drift over tidal flats, using the mucous cord as a “sail.” To clarify the ecological roles of mucous-cord secretion, the conditions affecting secretion were examined in the laboratory using clams burrowing in sand. Darkness and sand exposure played important roles in mucous-cord secretion: Under a 12-h : 12-h Light/Dark cycle with continuous sand submergence, 84% of mucous-cord secretions occurred in the dark. Under a 6-h : 6-h Submergence/Exposure cycle in continuous darkness, 79% of mucous-cord secretions occurred within 2 h of the submergence. Since clams secreted mucous cords without being exposed to adverse conditions such as hypoxic waters, it appears that mucous-cord drifting in the field does not necessarily serve as an escape mechanism when the clam encounters unfavorable conditions. On the contrary, it is suggested that drifting is a risk-dispersion (bet-hedging) strategy to sustain the population under the varying and unpredictable conditions of tidal flats.
To elucidate what determines plankton community structures in subtropical and tropical oceans, cell size distributions of pico- and nanophytoplankton were extensively measured throughout the North and South Pacific Ocean. Mean cell sizes of three different populations, Synechococcus, nano-sized cyanobacteria, and eukaryotes, showed a clear diel cycle with a peak in the late afternoon. After subtracting the effect of the diel variation by fitting sine curves, the mean cell sizes of the three populations were regressed against temperature and nutrient concentrations. The multiple regression analyses revealed that a positive correlation between the cell size of eukaryotes and temperature was the only significant parameter. Size histograms demonstrated that in the warm waters, the contribution of pico-sized eukaryotes to total phytoplankton biomass was relatively small, which in combination with the numerical dominance of Prochlorococcus caused bipolarization of the size spectrum of the entire pico- and nanophytoplankton community. These results demonstrate that commonly used biogeochemical models require amendment to account for the fact that an increase in sea surface temperature will simply lead to a prevalence of smaller phytoplankton. To reveal the effect of temperature on the entire food web, extensive surveys of zooplankton community structures will be required in future studies.
Changes in bacterial community structure were followed in two outdoor experimental ponds. To compare the changes in bacterial community, Microcystis was inoculated into one of the ponds but not to the other one. In both ponds, Chlorophyceae algae from the genera Cosmarium and Scenedesmus were dominant in the first and last months of the study. During the middle period of the study, cyanobacteria were dominant. Microcystis and Aphanizomenon dominated in one pond, and Planktothrix dominated in the other. To investigate bacterial phylogenetic abundance and these compositions, we used catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) and denaturing gradient gel electrophoresis (DGGE). A significant relationship was observed between the number of α-proteobacterial operational taxonomic units (OTUs) and the abundance of Chlorophyceae algae (p < 0.001), although no significant relationship was identified between the abundances of the two groups. The sequencing analysis of DGGE bands detected microcystin-degrading bacteria belonging to the α-proteobacteria as one of the dominant bacterial phylogenetic groups when Microcystis was the dominant phytoplankton. To our knowledge, this is the first report demonstrating changes in the abundance and composition of bacterial groups during the wax and wane of dominant phytoplankton taxa, using two different molecular methods.
The size of dark spots on the body surface was evaluated quantitatively as a diagnostic character to distinguish two species of varunid crabs of similar morphology: Hemigrapsus penicillatus and H. takanoi, common estuarine species in eastern Asia. From 37 Japanese localities, 171 H. penicillatus (106 males, 65 females) and 185 H. takanoi (113 males, 72 females) were collected and identified to species level by a molecular method. Spot size (the longest diameter of the largest dark spot) on the ventral face of the cephalothorax and on the outer faces of chelae were measured, with carapace width as an index of body size. Although the spot sizes at both loci increased with increases in carapace width, the spot sizes were significantly larger in H. penicillatus than H. takanoi. In both sexes, the ranges of variation in spot size partially overlapped between the species but the extent of such interspecific overlap was smaller for spot sizes on the ventral face of the cephalothorax in both sexes. Based on the spot size on the ventral face of the cephalothorax and carapace width, 92% and 82% of males and females were identified correctly into species via quadratic discriminant analyses. The spot size measured from the ventral face of the cephalothorax was therefore judged to be a useful diagnostic character to distinguish H. penicillatus and H. takanoi.
To clarify the growth, reproduction and ontogenetic migration of Metapenaeus ensis, 96,898 individuals (6–175 mm in body length, BL) were collected from five areas in Osaka Bay. Larval settlement was estimated to occur from June or July to October in the Yodo River mouth. Juveniles grew there and migrated to the innermost area of the bay, and then to the deeper areas as they grew. Growth of females showed a good approximation to a seasonally fluctuating von Bertalanffy equation, with a size-frequency distribution suggesting the presence of 2 or 3 cohorts per year. Body length of females attained 35–125 mm in the first winter and 105–150 mm in the second, and their life span is estimated to be about two years. Their reproductive season was mainly from June or July to September in the second year (ca. 1 year old), and mainly July in the third year (ca. 2 years old). The broad variation in the size range of reproducible females (BL 76–168 mm) suggests indeterminate growth. Males were smaller than females with overlapping cohorts but their life cycle appears to be similar to that of females.
Spionid polychaetes, Prionospio and Paraprionospio, are abundant in organic enriched areas in view of their capacity to survive in low oxygen and high sulfide conditions of organic-rich sediments. The respiratory structures of Prionospio species were compared in two areas of Sriracha Bay: one where high organic conditions prevail from mussel raft culture installations and domestic waste effluents; and the other of low organic conditions, where coastal activities are minimal and far from mussel raft installations. Although the main morphological characters of Prionospio (Prionospio) membranacea, Prionospio (Minuspio) pulchra and Prionospio (Aquilaspio) sexoculata in both study areas were basically similar, the results showed morphological differences in their respiratory-related organs apparently related to levels of organic matters present. Branchial pairs were clearly longer and branchial pinnules more numerous in P. (P.) membranacea, branchial pairs tended to be longer in P. (M.) pulchra and P. (A.) sexoculata, and branchial pinnules tended to be more numerous in P. (A.) sexoculata from the area of high concentrations of organic matters, compared with those from the area of low organic concentrations. The respiratory structure of Paraprionospio inaequibranchia was also analyzed.
Short-term variation in the ammonium excretion rate by Calanus sinicus during the post-capture period was evaluated in 1-h incubations. Incubation water volume (13 mL vs. 100 mL) did not significantly affect the excretion rate, and the overall mean excretion rate of C. sinicus was similar to reported values from congeneric species. However, the 24-h variations in the excretion rate denoted that acclimation stress decreased the excretion rates 1–2 h after introduction to a no-food condition. The effect of presence/absence of food was not significant up to 24 h after capture.