Coral-crab association: a compact domain of a multilevel trophic system

Colonies of the Red Sea branching coral Stylophora pistillata were incubated in situ with radioactive carbon and Trapezia cymodoce crabs were introduced to the colonies, for one month each, up to 7 months after coral labelling. Zooxanthellar photosynthetic products were translocated to the crabs via host coral tissue. Based upon crab/coral tissue conversion from 53 crab samples, crabs possessed radioactive material equal to that recorded in 320–770 mm² of coral tissue (up to 2257 mm²). This material was translocated mainly by direct grazing on coral tissue rather than mucus collection. Ovigerous female crabs (39 specimens) accumulated significantly more labelled carbon than males, and up to 53 % of their radioactivity was concentrated in their reproductive organs. A pair of crabs dwelling in a coral colony consumed ca 130 cm² of host tissue per month (40–45 cm length of coral branches). This system represents a compact, obligatory multilevel trophic domain which also radiates horizontally by allocation of energy derived from algal photosynthesis into planula and zooea larva production, permitting the development of long trophic chains and a complex food web.     

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO₂) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (?0.2±0.9) Pg C yr^?1 (1 Pg=10¹⁵ g) in the 1960s to a sink of (1.9±1.1) Pg C yr^?1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20–0.25 Pg C yr^?1 in China during the past decades, and predict it to be 0.15–0.52 Pg C yr^?1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO₂ concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO₂ concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

     

Effect of light and ontogenetic stage on sink strength in bean leaves

Light (about 3,000 foot-candles) neither increased nor decreased the sink strength of young, rapidly expanding leaves of Phaseolus vulgaris L. cv. Black Valentine, as measured by the comparative rates of import of ¹⁴C-labeled photosynthates by sink leaves in the light versus dark in short term experiments. Although irradiated sink leaves accumulated more ¹⁴C activity, the difference was fully accounted for by photosynthetic reabsorption of respiratory CO₂ derived from substrates translocated to the sink leaves.

Maximum sink strength was attained when the sink leaf reached 7 to 8 cm² in area (9 to 10% of its fully expanded size). Thereafter sink strength declined rapidly and asymptotically to a near zero value at about 45% final area. During this period, however, the rapid decline in translocation was offset by a rapid rise in the photosynthetic rate of the sink leaf, maintaining a near constant relative rate of dry weight increase until the sink leaf had expanded to about 17% of its final area. Although the increasing photosynthetic capacity was associated with a decreasing import capacity, suggesting that the rate of translocation to the sink leaf was controlled by the developing capacity of the sink leaf for photosynthesis, it was not possible to vary the total (true) translocation rate to the sink leaf by varying the photosynthetic rate of the sink leaf in short term light-dark experiments. Despite a high ratio of source to sink in these experiments, no evidence accrued that translocation into young bean leaves was ever sink-limited.

     

Spatial variability and major controlling factors of CO2 sink strength in Asian terrestrial ecosystems: evidence from eddy covariance data

Asian terrestrial ecosystems cover an extensive area characterized by a large variety in climates and ecosystem properties. The observations of ecosystem CO₂ flux in this area are increasing both in duration and spatial density, but no synthesis has yet been conducted. We surveyed CO₂ flux observation data obtained by eddy covariance methods at 49 sites in terrestrial Asia. The measurements at most sites (44 of 49) began after 2000. The net ecosystem uptake of CO₂ (NEE) varied greatly among sites and years and averaged −132.6±73.7, −250.1±206.1, and −180.1±361.7 g C m⁻² yr⁻¹, in boreal, temperate, and tropical Asia, respectively, and the coefficient of variation among sites increased from boreal to tropical Asia. The site-averaged annual NEE was correlated linearly with the mean annual temperature (T_air) and also correlated logarithmically with the precipitation. Multiple regression analysis and stepwise analysis indicated that photosynthetic active radiation (PAR) and T_air were the most significant predictors of the annual NEE. The study results suggest that Asian terrestrial ecosystems are currently significant net CO₂ sinks and that the sink strength is largely controlled by temperature, moisture, and light conditions.     

Reef-building corals act as long-term sink for microplastic

The pollution of the marine environment with microplastics is pervasive. However, microplastic concentrations in the seawater are lower than the number of particles entering the oceans, suggesting that plastic particles accumulate in environmental sinks. Yet, the exact long-term sinks related to the “missing plastic” phenomenon are barely explored. Sediments in nearshore biogenic habitats are known to trap large amounts of microplastics, but also the three-dimensional structures of coral reefs might serve as unique, living long-term sinks. The main framework builders, reef-building corals, have been shown to ingest and overgrow microplastics, potentially leading to a deposition of particles in reef structures. However, little is known about the number of deposited particles and the underlying processes determining the permanent deposition in the coral skeletons. To test whether corals may act as living long-term sink for microplastic, we exposed four reef-building coral species to polyethylene microplastics (200 particles L^?1) in an 18-month laboratory experiment. We found microplastics in all treatment specimens, with low numbers of particles trapped in the coral tissue (up to 2 particles per cm²) and much higher numbers in the skeleton (up to 84 particles per cm³). The numbers of particles accumulated in the coral skeletons were mainly related to coral growth (i.e., skeletal growth in volume), suggesting that deposition is a regularly occurring stochastic process. We estimate that reef-building corals may remove 0.09%–2.82% of the bioavailable microplastics from tropical shallow-reef waters per year. Our study shows for the first time that microplastic particles accumulate permanently in a biological sink, helping to explain the “missing plastic” phenomenon. This highlights the importance of coral reefs for the ecological balance of the oceans and reinforces the need to protect them, not only to mitigate the effects of climate change but also to preserve their ecosystem services as long-term sink for microplastic.     

C and N Mobilization from Stalk and Leaves during Kernel Filling by C and N Tracing in Zea mays L

The sink capacity of the stalk in Zea mays L. (cv DEA) during the elongation period was previously investigated with ¹³C and ¹⁵N tracing. The chase experiment described here demonstrates the different behavior of intermediary reserves for C and N remobilization until full maturity of the kernels. Carbon incorporated during stalk elongation participated mainly in cellulose formation in vegetative organs appearing after the labeling period; the remobilization to kernels was low (0.5%). Soluble carbohydrates and proteins were the main intermediary sink compounds, starch being little remobilized. N first incorporated in roots, sheaths, stalk, blades was translocated to the kernel; 42% of the labeled N were recovered in kernels where they represented 8% of the total N. Cob, husk, and shank acted first as N sinks and then as N sources during ear development. It appeared that aminoacids used for synthesis of kernel proteins have a common origin, except for glutelin G₃.     

Oriented translocation of energy in grafted reef corals

Colonies of the Red Sea reef coral Stylophora pistillata were grafted with alien branches (alografts), which had been labelled by NaH¹⁴CO₃ in the light. The “cold” host-colonies translocated the ¹⁴C-containing photosynthetic metabolites in an oriented pathway from the grafted branches into their own tissues. The highest accumulations of ¹⁴C products were detected in specific branch-tips of the host, away from the contact zones. The “recipient” colonies utilize these energy-rich materials for their metabolic requirements. The ¹⁴CO₂ produced through respiration is consequently detected in the skeletal-carbonate of the tips as Ca¹⁴CO₃. The purple morph of S. pistillata is found to be superior to the yellow morph.     

The effect of irradiance on long-term skeletal growth and net photosynthesis in Galaxea fascicularis under four light conditions

The relation between irradiance, skeletal growth and net photosynthesis was studied for the scleractinian coral Galaxea fascicularis to provide experimental evidence for mediation of light-enhanced calcification through photosynthesis. The hypothesis was tested that skeletal growth and photosynthesis are linearly correlated.A long-term experiment was performed in a closed-circuit aquarium system, in which four series of nine nubbins (single polyp clones of a coral colony) of Galaxea fascicularis were exposed to four light treatments (10L:14D): 144 W T8 fluorescent lighting providing an irradiance of 68 µE/m²/s and 70, 250 and 400 W Metal Halide lighting providing an irradiance of 38 µE/m²/s, 166 µE/m²/s and 410 µE/m²/s, respectively. Growth of these nubbins was measured as buoyant weight at different time intervals in a 294 day experiment. A light-saturation curve for photosynthesis was measured in a respirometric flow cell using a 54 week Galaxea fascicularis colony grown at 60 µE/m²/s.No saturation of net photosynthesis of Galaxea fascicularis was found at the irradiances tested. The specific growth rate (µ, in day^- 1) of the coral nubbins increased with irradiance. Whereas irradiance varied 11-fold (38 to 410 µE/m²/s), buoyant weight (increase after 294 days) increased 5.7 times (2243 to 12374 mg), specific growth rate (1-294 days) increased 1.6 times (0.0103 to 0.0161 day^- 1), while net photosynthetic rate increased 8.9 times (0.009 µmol O₂/min/cm² to 0.077 µmol O₂/min/cm²). The increase of specific growth rate with irradiance was less than expected based on the increase in net photosynthetic rate with irradiance. This discrepancy between potential energy produced in photosynthesis and energy used for skeletal growth indicates that skeletal growth is not limited by photosynthetic potential at high irradiance levels.     

Partial Mortality in Porites Corals: Variation among Philippine Reefs

Partial mortality or tissue necrosis was quantified in the massive scleractinian coral Porites at three sites in The Philippines (Bolinao, NW Luzon; Puerto Galera, Mindoro; and El Nido, N Palawan). Overall, 15 ± 1 (mean ± 1 standard error, 642 replicates) percent of colony area was dead, mean colony area was 1135 plusmn; 127 cm², and lesion density was 1.7 ± 0.1 dm^—2. Total live coral cover varied between 20 and 63% in belt transects, and Porites and Acropora cover were inversely correlated. ANOVA models incorporating effects of site, colony size, sedimentation rates, wave exposure and depth were highly significant but explained only a small proportion of the variation observed in lesion density and percent dead area (respectively 8 and 2%). Lesion density was found to vary significantly with site (contributed 29% to this explained variance), decrease with increasing colony area (33%), and increase with increasing sedimentation (23%) and wave exposure (14%). Colony size was significantly explained by the factor site (contributing 61% to the total 29% explained variance) and depth (34%), with the smallest colonies being observed in Bolinao and the largest in El Nido. Densities of lesions were highest in Bolinao, intermediate in Puerto Galera, and lowest in El Nido. This pattern is parallel to intensity of human reef exploitation and opposite to that in colony size, live coral cover and Acropora cover. Since only a small part of the observed variance in partial mortality estimators was explained by the ANOVAs, other factors not quantified here must have been more important (e.g. disease incidence, predation, human exploitation).     

Temporal patterns in effective quantum yield of individual zooxanthellae expelled during bleaching

Bleaching is a worldwide phenomenon affecting coral reefs. During elevated temperature and light conditions (bleaching), expelled zooxanthellae show distinct patterns in photosynthetic health. An innovative new device was used to collect individual expelled zooxanthellae, when a coral was exposed to bleaching conditions. This has provided new insight into the photosynthetic condition and abundance of expelled zooxanthellae. It has been assumed that expelled zooxanthellae were dead or moribund; however, we have found individual cells can have healthy effective quantum yields (?_PSII) >0.65 after 8 h of bleaching conditions (500 μmol photons m⁻² s⁻¹, 33 °C). The population of expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis showed distinct patterns in the frequency distribution of ?_PSII over time and between locations (sun versus shade) within a colony. During the first 4 h of exposure to bleaching conditions, only 5% of expelled individual cells from P. damicornis were photosynthetically inactive (?_PSII<0.05), whereas for C. serailia, this was 30%. The overall photosynthetic health of expelled zooxanthellae from C. serailia was better than P. damicornis (0.53±0.13 and 0.38±0.13 after 8 h, respectively). This was generally reflected by the in hospite measurement of the coral, yet, the in hospite cells always had a higher ?_PSII than expelled cells, suggesting that host tissue provided added photoprotection for the zooxanthellae.     

Small-scale environmental variation influences whether coral-dwelling fish promote or impede coral growth

Mutualistic symbioses are ubiquitous in nature and facilitate high biodiversity and productivity of ecosystems by enhancing the efficiency of energy and nutrient use within ecological communities. For example, small groups of fish that inhabit coral colonies in reef ecosystems potentially enhance coral growth through defense from coral predators, aeration of coral tissue and nutrient provisioning. This study examines whether the prevalence and consequences of fish-coral interactions vary among sites with different environmental conditions in a coral reef lagoon, using the humbug damselfish Dascyllus aruanus and its preferred coral host Pocillopora damicornis as a study system. Using a field experiment, we tested the site-specific effects of D. aruanus on coral growth, and show that the cost-benefit ratio for corals hosting fish varies with local environmental variation. Results of this study also demonstrate that fish prefer to inhabit coral colonies with particular branch-spacing characteristics, and that the local abundance of D. aruanus influences the proportion of coral colonies within a site that are occupied by fish rather than increasing the number of fish per colony. We also show that corals consistently benefit from hosting D. aruanus via defense from predation by corallivorous butterflyfish, regardless of local environmental conditions. These findings highlight the need to consider the potential for multiple scale- and state-dependent interaction effects when examining the ecology of fish-coral associations. We suggest that fluctuating cost-benefit ratios for species interactions may contribute to the maintenance of different colony phenotypes within coral populations.     

Opportunistic exploitation of dinosaur dung: fossil snails in coprolites from the Upper Cretaceous Two Medicine Formation of Montana

Multiple associations of fossil snails with dinosaur coprolites demonstrate that snails and dinosaurs not only shared ancient habitats but were trophically linked via dinosaur dung. Over 130 fossil snails representing at least seven different taxa have been found on or within herbivorous dinosaur coprolites from the Upper Cretaceous Two Medicine Formation of Montana. The terrestrial snail Megomphix is the most common taxon, but three other terrestrial taxa (Prograngerella, Hendersonia and Polygyrella) and three aquatic snails (Lioplacodes, ?Viviparus and a physid) also occur in coprolites. At least 46% of the shells in the faeces are whole or nearly so, indicating that most (if not all) of the snails were not ingested by dinosaurs, but were post‐depositional visitors to the dung pats. The sizeable, moist and microbially enriched dinosaur faeces would have provided both food and roosting sites for the ancient snails, and the large number of snail–coprolite associations reflect recurring, opportunistic exploitation of dung. The terrestrial taxa in the coprolites suggest that this Late Cretaceous locality included sufficiently moist detrital or vegetative cover for snails when dinosaur dung was not present. The aquatic snails probably entered the faeces during flood events. Dinosaur dung would have provided an abundant but patchy influx of resources that was probably seasonally available in the ancient environment.     

Effects of age and ontogeny on photosynthetic responses of a determinate annual plant to elevated CO2 concentrations

Plant responses to elevated CO₂ concentrations ([CO₂]) may be regulated by both accelerated ontogeny and allocational changes as plants grow. However, isolating ontogeny‐related effects from age‐related effects are difficult because these factors are often confounded. In this study, the roles of age and ontogeny in photosynthetic responses to elevated [CO₂] were examined on Xanthium strumarium L. grown at ambient (365 µmol mol^?1) and elevated (730 µmol mol^?1) [CO₂]. To examine age‐related effects, six cohorts were planted at 5‐day intervals. To examine ontogeny‐related effects, all plants were induced to flower at the same time; ontogeny in Xanthium is relatively unaffected by growth in elevated [CO₂]. Growth in elevated [CO₂] increased net photosynthetic rates by approximately 30% throughout vegetative growth (i.e. active carbohydrate sinks), approximately 10% during flowering (i.e. minimal sink activity), and approximately 20% during fruit production (i.e. active sinks). At the harvest, the ratio of source to sink tissue significantly decreased with increasing plant age and was correlated with leaf soluble sugar concentration. Leaf soluble sugar concentration was negatively correlated with the relative photosynthetic response to elevated [CO₂]. These results suggest that age and ontogeny independently affect photosynthetic responses to elevated [CO₂] and the effects are mediated by reversible changes in source : sink balance.     

Carbon Transfer and Partitioning between Vegetative and Reproductive Organs in Pisum sativum L

Assimilate partitioning was studied in the common pea (Pisum sativum L.) by feeding ¹⁴CO₂ to whole plants and measuring radioactivity in different organs 48 hours after labeling. Two experimental protocols were used. For the first, one reproductive node was darkened with an aluminum foil, to prevent photosynthesis during labeling. The aim was to study assimilate translocation among nodes. The second was carried out to assess any priority among sinks. Whole plants were shaded, during labeling, to reduce carbon assimilation. Various developmental stages between the onset of flowering and the final stage in seed abortion of the last pod were chosen for labeling. When all photosynthetic structures at the first reproductive node were darkened at any stage of development after the formation of the first flower, the first pod was supplied with assimilates from other nodes. In contrast, later developed pods, when photosynthetic structures at their node were darkened, received assimilates from other nodes only when they were beyond their final stage in seed abortion. Reducing illumination to 30% did not change distribution of assimilated carbon between vegetative and reproductive structures, nor among pods. It appears that the relative proportion of ¹⁴C allocated to any one pod, compared to other pods, depends on the dry weight of that pod as a proportion of the total reproductive dry weight. When the plant was growing actively, following the start of the reproductive phase until a few days before the end of flowering, the top of the plant (i.e., all the organs above the last opened flower) had a higher sink strength and a higher relative specific activity than pods, suggesting that it was a more competitive sink for assimilates. The pattern of assimilate distribution described here provides an explanation for pod and seed abortion.     

陆地生态系统碳汇评估方法研究进展

“碳中和”是我国作出的一项重大的国家战略决策,陆地生态系统碳汇作为碳增汇的重要组成部分,在碳中和目标实现的过程中发挥着重要的作用。但当前基于不同观测数据和方法的陆地碳汇计算仍有很大的不确定性,为了全面了解陆地生态系统碳汇分布特征,提高陆地生态系统碳汇评估的准确性,梳理了近年来关于陆地生态系统碳汇评估的国内外研究进展,从“自下而上”和“自上而下”两类途径阐述了陆地生态系统碳汇评估的主要方法(样地清查法、涡度相关法、模型模拟法和碳同化反演法)的主要原理和特征,优势和缺陷,及在不同尺度碳汇研究中的应用,并从土地利用/覆盖变化、气候因素(大气CO₂浓度、氮沉降)、环境因素(太阳辐射、温度、降水)等因素阐述了陆地系统碳汇主要驱动因子;分析了我国陆地生态系统碳汇的主要特征及时空变化趋势,并从人类活动(生态工程)和环境因素阐述了中国陆地生态系统碳汇的驱动因素;最后,展望了新的监测手段和评估方法在提升陆地生态系统碳汇评估精度中的作用,从而更好的服务于我国“碳中和”的长远目标。     

Effects of Chronic Internal beta-Radiation from Photoassimilated CO(2) on the Retention and Distribution of C in Young White Pine Plants

Eastern white pines (Pinus strobus L.) in their 3rd year of growth photoassimilated 35, 110, 220, or 400 microcuries of ¹⁴CO₂ on a single occasion in the spring when needle expansion was occurring. Once assimilated into organic products and translocated to various sinks, the retained ¹⁴C represented an internal chronic source of ionizing radiation to the plant. About 2.5 months later, the pines were harvested, and the distribution of ¹⁴C activity was determined. In addition, new needle lengths, fresh weights, and rates of processes involving CO₂ exchange were measured.     

An appraisal of ear, leaf and stem 14CO2 assimilation, 14C-assimilate distribution and growth in a reproductive seed crop of amenity Lolium perenne

In seed crops of Lolium perenne, yield may be reduced by competition for a limited assimilate supply from sinks other than the ear. This study was undertaken to evaluate the priorities for allocation of assimilate within the crop from all photosynthetic sites on the main reproductive tiller after anthesis. Ear, stem and leaves were fed with ¹⁴CO₂ on two occasions; the assimilatory efficiency of these sources and the magnitude and pattern of ¹⁴C-assimilate export from each was determined. The growth of each part of the main tiller and subtending tillers was also measured. Stem elongation apparently dominated the current assimilate resource and the ear did not become a net importer of assimilate until this process had ceased. Assimilate allocation to the tillers was high throughout. The nature of any competition between these sinks is discussed. When crop growth was contrasted with that in a previous year, environmental factors were implicated as determinants of priority for assimilate allocation to each sink. Sources of carbon for seed filling are also discussed as is the relevance of these findings to seed crop management.     

Carbon allocation during fruiting in Rubus chamaemorus

Background and Aims

Rubus chamaemorus (cloudberry) is a herbaceous clonal peatland plant that produces an extensive underground rhizome system with distant ramets. Most of these ramets are non-floral. The main objectives of this study were to determine: (a) if plant growth was source limited in cloudberry; (b) if the non-floral ramets translocated carbon (C) to the fruit; and (c) if there was competition between fruit, leaves and rhizomes for C during fruit development.

Methods

Floral and non-floral ramet activities were monitored during the period of flower and fruit development using three approaches: gas exchange measurements, ¹⁴CO₂ labelling and dry mass accumulation in the different organs. Source and sink activity were manipulated by eliminating leaves or flowers or by reducing rhizome length.

Key Results

Photosynthetic rates were lower in floral than in deflowered ramets. Autoradiographs and ¹⁴C labelling data clearly indicated that fruit is a very strong sink for the floral ramet, whereas non-floral ramets translocated C toward the rhizome but not toward floral ramets. Nevertheless, rhizomes received some C from the floral ramet throughout the fruiting period. Ramets with shorter rhizomes produced smaller leaves and smaller fruits, and defoliated ramets produced very small fruits.

Conclusions

Plant growth appears to be source-limited in cloudberry since a reduction in sink strength did not induce a reduction in photosynthetic activity. Non-floral ramets did not participate directly to fruit development. Developing leaves appear to compete with the developing fruit but the intensity of this competition could vary with the specific timing of the two organs. The rhizome appears to act both as a source but also potentially as a sink during fruit development. Further studies are needed to characterize better the complex role played by the rhizome in fruit C nutrition.Key words: Allocation pattern, ¹⁴C labelling, carbon translocation, carbon reserves, cloudberry, defoliation, fruit production, gas exchange, Rubus chamaemorus, source–sink relationship, flowering     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.