Dilution Incubation Research Articles

Phytoplankton growth and grazing dynamics during anomalous heat wave and suppressed upwelling conditions in the southern California Current

We investigated phytoplankton dynamics in the southern California Current System (SCCS) in August 2014 during the early phase of the 2014-15 marine heat wave (MHW). Multi-day experiments were conducted at three inshore and two offshore sites, with daily depth profiles of dilution incubations on a drifting array to determine growth and grazing rates and shipboard assessments of nutrient effects. Picophytoplankton populations were analyzed by flow cytometry and eukaryotic phytoplankton by 18 S sequencing. Mixed-layer nutrients were low across the region, but inshore sites had substantial nitrate concentrations and prominent Chla maxima in the lower euphotic zone. Shoreward transport of warm-stratified waters from the offshore suppressed coastal upwelling and shifted picophytoplankton distributions toward increased onshore abundance of Prochlorococcus and decreased Synechococcus and picoeukaryotes. These trends were reinforced by higher-than-average growth of Prochlorococcus at inshore sites and higher grazing of Synechococcus and picoeukaryotes. Prasinophytes (Chlorophyceae) were notably important among eukaryotic taxa, and pennates replaced centric taxa as the dominant diatoms in inshore waters compared to normal upwelling. Despite substantial spatial variability in community composition, offshore and inshore experimental locations both showed growth-grazing balances, with microzooplankton consuming similar percentages (80%) of primary production. We thus confirm expectations that the 2014-15 MHW resulted in greater trophic flow through the microbial food web at the expense of reduced direct phytoplankton (Chla) consumption by mesozooplankton. However, impacts on mesozooplankton energy budgets were partially offset by increased trophic flow through protistan microzooplankton and higher phytoplankton C:Chla.

Methane flux from transplanted soil monoliths depends on moisture, but not origin

Soils both produce and consume methane (CH4), a potent greenhouse gas that contributes to climate change. In coastal forests, upland soils are shifting from being CH4 sinks to sources as sea levels rise, increasingly flooding soils with little prior inundation history. Ecosystem CH4 budgets are highly uncertain due in part to the difficulty in separating fluxes measured at the soil surface into individual production and consumption processes which are likely to have different responses to future environmental conditions. We measured growing season CH4 fluxes from soil monoliths transplanted four years prior along an inundation and salinity gradient to determine how changes in abiotic conditions control CH4 flux rates. To parse net fluxes measured at the soil surface into their component gross rates, we paired field measurements with a stable isotope pool dilution incubation of surface soils. Throughout the growing season, net soil surface CH4 flux was positively correlated with soil moisture (p < 0.01), with lowland-located soils tending towards CH4 sources (mean 0.349 ± 1.11 mg CH4–C m−2 h−1, error is standard deviation) and upland-located soils tending towards CH4 sinks (−0.003 ± 0.003 mg CH4–C m−2 h−1). Transplanted soils’ fluxes were statistically identical to their native neighbors once microtopography-driven differences in soil moisture were controlled for. The pool dilution experiment revealed that production and consumption rates were similar in upland and lowland surface soils (2.82 ± 3.29 μmol CH4 g−1 dry soil d−1 production, 3.47 ± 2.03 μmol CH4 g−1 dry soil d−1 consumption), indicating the majority of production likely occurs at depth in lowland soils. Both gross and net fluxes from transplanted soils showed no effect of soil origin after four years, suggesting low resistance of CH4 cycling to global change drivers. Our results indicate the strength of the coastal forest CH4 sink is likely to decrease in proportion to sea-level rise.

Alkaline Dilution Alters Sperm Motility in Dairy Goat by Affecting sAC/cAMP/PKA Pathway Activity.

In dairy goat farming, increasing the female kid rate is beneficial to milk production and is, therefore, economically beneficial to farms. Our previous study demonstrated that alkaline incubation enriched the concentration of X-chromosome-bearing sperm; however, the mechanism by which pH affects the motility of X-chromosome-bearing sperm remains unclear. In this study, we explored this mechanism by incubating dairy goat sperm in alkaline dilutions, examining the pattern of changes in sperm internal pH and Ca2+ concentrations and investigating the role of the sAC/cAMP/PKA pathway in influencing sperm motility. The results showed that adding a calcium channel inhibitor during incubation resulted in a concentration-dependent decrease in the proportion of spermatozoa with forward motility, and the sperm sAC protein activity was positively correlated with the calcium ion concentration (r = 0.9972). The total motility activity, proportion of forward motility, and proportion of X-chromosome-bearing sperm decreased (p < 0.05) when cAMP/PKA protease activity was inhibited. Meanwhile, the enrichment of X-chromosome-bearing sperm by pH did not affect the sperm capacitation state. These results indicate that alkaline dilution incubation reduces Ca2+ entry into X-sperm and the motility was slowed down through the sAC/cAMP/PKA signaling pathway, providing a theoretical foundation for further optimization of the sex control method.

Nafamostat Mesylate in Combination with the Mouse Amino-Terminal Fragment of Urokinase-Human Serum Albumin Improves the Treatment Outcome of Triple-Negative Breast Cancer Therapy.

Triple-negative breast cancer (TNBC) is highly aggressive and causes a higher proportion of metastatic cases. However, therapies directed to specific molecular targets have rarely achieved clinically meaningful improvements in the outcome of TNBC therapy. A urokinase-type plasminogen activator (uPA), one of the best-validated biomarkers of breast cancer, is an extracellular proteolytic serine protease involved in many pathological and physiological processes, including tumor cell invasion and metastasis. Nafamostat mesylate (NM) is a synthetic compound that inhibits various serine proteases and has been used as a therapeutic agent for the treatment of TNBC. Nevertheless, NM has poor specificity for serine proteases and is easy be hydrolyzed; moreover, the inhibitory mechanism of TNBC therapy is unclear. In this study, we combine NM with a macromolecular drug delivery vehicle, mouse amino-terminal fragment of urokinase-human serum albumin (mATF-HSA), to form a complex (mATF-HSA:NM) using the dilution-incubation-purification method. mATF specifically targets uPAR overexpressed on the surface of TNBC cells; moreover, HSA prevents NM from being hydrolyzed by numerous serine proteases. mATF-HSA:NM showed stronger inhibitory effects on the proliferation and metastasis of TNBC in vitro and in vivo without significant cytotoxicity on normal cells and tissues. In addition, we demonstrated that NM mediates metastasis of TNBC cells through inhibition of uPA using a stable uPA knockdown cell line (MDA-MB231 shuPA). Overall, we have developed a macromolecular complex targeted to treat high uPAR-expressing tumor types, and mATF-HSA can potentially be used to load other types of drugs with tumor-targeting specificity for mouse tumor models and is a promising tool to study tumor biology in mouse tumor models.

Microbial community biomass, production and grazing along 110°E in the eastern Indian Ocean

We investigated plankton biomass structure, production and grazing rates from temperate to tropical waters (39.5–11.5°S) along the historic 110°E transect in the eastern Indian Ocean (IO) during May–June 2019. The timing captures the seasonal transition from moderate productivity in the subtropical sector to seasonally high primary production in tropical waters as described in IIOE (International Indian Ocean Expedition) studies of the 1960s. Carbon-based estimates of phytoplankton production and microzooplankton grazing were determined from depth profiles of dilution incubations analyzed by flow cytometry and pigments; mesozooplankton biomass and grazing were determined from net sampling and gut fluorescence for the integrated euphotic zone. Phytoplankton biomass varied from 860 to 1740 mg C m−2, averaging 1187 mg C m−2 with no latitudinal trend. Mixed-layer C:Chla ranged from 20 to 40 in the nitrogen-rich subtropical front to 100–180 in tropical waters. Prochlorococcus increased from 141 to 915 mg C m−2 between 39.5°S and 20°S and averaged 700 mg C m−2 at lower latitudes. Synechococcus and photosynthetic eukaryotes contributed least to biomass (3.6 and 30.5%, respectively) at mid-transect locations (15.5–27.5°S). Dinoflagellates and diatoms were typically rare (28 and 6 mg C m−2, respectively). Among heterotrophs, bacteria averaged 476 mg C m−2, with a subtropical front maximum but no latitudinal trend; ciliates averaged 112 mg C m−2, and mesozooplankton increased significantly south-to-north (131–488 mg C m−2). Phytoplankton production and grazing averaged 466 and 461 mg C m−2 d−1, respectively, based on the sums for flow-cytometry measured populations, and 618 and 604 mg C m−2 d−1, respectively, based on Chla-determined rates. Our results highlight key relationships that link stocks and process rates across oceanographic provinces of the eastern IO. Production and grazing increased 6–8 fold from south to north. Prochlorococcus dominated productivity, and microzooplankton accounted for 85–89% of grazing. Production and grazing were strongly coupled and balanced on average. Over the transect, increasing growth conditions (light and temperature) mainly manifested as more rapid biomass turnover and mesozooplankton biomass accumulation.

Low temperature sensitivity of picophytoplankton P : B ratios and growth rates across a natural 10°C temperature gradient in the oligotrophic Indian Ocean

AbstractWe investigated temperature sensitivities of picophytoplankton growth along a natural 10°C (18–28°C) temperature gradient in the eastern Indian Ocean characterized by deep mixing and consistently low dissolved nitrogen. Population biomass (B), cell carbon, and chlorophyll were measured by flow cytometry. Instantaneous growth (μ) and production (P) were calculated from dilution incubations at four light levels. Contrary to most empirical and theoretical predictions, Prochlorococcus, the biomass dominant, showed insignificant temperature sensitivity, with nominal Q10 values of 1.06 and 1.18 for P : B and μ, respectively, and activation energies (Ea) of 0.05 and 0.12 eV. Q10 and Ea values for Synechococcus (1.36–1.42 and 0.23–0.27 eV) were also below prediction, and picoeukaryotes showed high variability, including negative rates suggesting lytic cycles, at high temperature. We emphasize the importance of using adapted communities in natural environmental gradients to test climate predictions and hypothesize that mortality defenses are a significant selection criterion in balanced oligotrophic systems.

Diel Variation of Viral Production in a Coastal Subtropical Marine System

Viral production (VP) and bacterial mortality by viral lysis critically influence the production and mortality of aquatic bacteria. Although bacterial production, mortality by viral lysis, and viral density have been found to exhibit diel variations, the diel change in viral production has rarely been investigated. In this study, we conducted two diel dilution incubation experiments in a semi-enclosed, nutrient-rich coastal region in northeastern Taiwan to estimate the diel viral production and the mortality by viral lysis. We also compared two methods (linear regression between viral density and time versus arithmetic mean of VP during incubation) of estimating viral production. We found that viral production estimated by linear regression and bacterial mortality by viral lysis were higher during the daytime than during the nighttime. A possible explanation for the high viral production at daytime is that the bacterial community was composed of cell types with higher burst sizes at daytime. We further argued that the classical linear regression method can be used only when viral density significantly linearly increases with time, which does not always occur in dilution incubations. This study offered observations of diel variation in viral dynamics and discussed the methods estimating viral production in a marine environment.

Grazing by microzooplankton and copepods on the microbial food web in spring in the southern Yellow Sea, China

Assessment of microzooplankton and copepods grazing pressure on picoplankton is a key requirement for resolving the microbial food web efficiency. Although microzooplankton grazing on picoplankton has been extensively studied, the impact of microzooplankton on different groups of picoplankton, i.e., heterotrophic bacteria, Synechococcus and picoeukaryotes have rarely been compared. Furthermore, in the very few existing studies there is no consistent evidence of an enhancing or restraining effect of copepods on picoplankton. More studies are needed to improve our understanding of the influence of microzooplankton and copepod on picoplankton. Dilution incubations and copepod addition incubations were performed during a cruise to the southern Yellow Sea on May 16–29, 2007. The bulk grazing of microzooplankton and the calanoid copepod Calanus sinicus on phytoplankton, flagellates and picoplankton was estimated. Stations were divided into either eutrophic or oligotrophic according to the nutrient and biological parameters. Picoplankton comprised a large part of the diet of microzooplankton in the central oligotrophic area, while phytoplankton was the main food of microzooplankton in the coastal eutrophic area. In the central oligotrophic area, microzooplankton preferred grazing on Synechococcus. After copepod addition, ciliate abundance decreased while Synechococcus abundance increased (382%, 64% and 64% at three experimental stations, respectively), indicating strong grazing pressure of microzooplankton on Synechococcus. Our results suggest that Synechococcus might be an essential carbon source the planktonic food web in the oligotrophic waters of southern Yellow Sea.

Structural changes of filling creams after in vitro digestion. Application of hydrocolloid based emulsions as fat source

The objective of this study was to evaluate the application of hydrocolloid-based o/w emulsions as fat source in filling creams. Emulsions containing sunflower oil stabilized by different hydrocolloids (hydroxypropyl methylcellulose (HHPMC), methylcellulose (MC) and xanthan gum (XG)), were employed. The changes in the structure of the systems before and after in vitro digestion were studied by textural analysis and confocal and light microscopy, and they were related to lipid digestibility after intestinal digestion. The presence of XG led to a significant increase in consistency in the stomach digestion phase compared to stomach water dilution incubation (6.32 N s and 4.72 N s respectively). After the intestinal phase, an increase in globule size was observed in all creams (MC: from 8.4 (fresh cream) to 15.4 μm (after intestinal step); HHPMC: from 12.8 to 17.6 μm; XG: from 27.5 to 32.0 μm), as well as the appearance of coalescence phenomena due to changes in fat stability, although fat globules are still visualized. Filling creams formulated with hydrocolloid-based o/w emulsion were more resistant to in vitro digestion than the control cream, exhibiting a significant decrease in fat digestibility (MC 240 g/Kg, HHPMC 230 g/Kg and XG 490 g/Kg reduction of fatty acids released).

Experimental study using the dilution incubation method to assess water biostability

Water biostability is of particular concern to water supply as a major limiting factor for heterotrophic bacterial growth in water distribution systems. This study focused on bacterial growth dynamics in the series dilution of water samples with TOC (total organic carbon) values determined beforehand. The results showed that the specific growth rate of Pseudomonas fluorescens P17 varied dramatically and irregularly with TOC value when TOC concentrations were low enough during the initial periods of incubation under given conditions. According to this relationship between bacterial growth rate and TOC, a dilution incubation method was designed for the study of water biostability. With the method under a given condition, a turning-point TOC value was found at a relatively fixed point in the curve between bacterial growth rate and TOC of water sample, and the variation of growth rate had different characteristics below the turning-point TOC value relative to that over this value. A turning-point TOC value similarly existed in all experiments not only with tap water, but also with acetate and mixed solutions. And in the dilution incubation method study, the affections were analyzed by condition factors such as inoculum amount, incubation time and nature of the organic carbon source. In very low organic carbon water environments, the variation characteristics of bacterial growth rate will be useful to further understand the meaning of water biostability.

Temperature dependence of gross N transformation rates in two Chinese paddy soils under aerobic condition

Temperature is an important factor that affects nitrogen (N) transformation in soils. In this study, variations in the gross and net N transformation rates between two Chinese paddy soils, namely, hydragric anthrosol (HA) and anthraquic cambisol (AC) in response to temperature (5, 15, 25, and 35 °C) were investigated by conducting an aerobic 15N dilution incubation experiment. The short-term (11 days) dynamics of gross N transformation rates was estimated using the numerical model FLUAZ. The results showed that increasing incubation temperature caused an increased gross and net transformation in the tested soils. However, the sensitivity to temperature fluctuation varied among the N transformation processes. Nitrogen mineralization was less sensitive than immobilization and nitrification at low temperatures (<15 °C). The optimum temperature for gross N nitrification in the studied paddy soils was approximately 25 °C. Higher gross N mineralization and gross N nitrification were observed in soil AC than in soil HA and were attributed to its higher pH (pH = 8.3), but further confirmation is needed. The ammonium (NH4 +) in studied paddy soils was mostly consumed through the nitrification process compared with that in the immobilization process, especially in alkaline soil AC. High consumption of NH4 + in alkaline soil AC might lead to a high risk of N loss through leaching and denitrification in case the generated nitrate (NO3 −) was not absorbed by crops in time.

北部湾北部海域浮游动物若干生态学研究V：夏季微型浮游动物对浮游植物的摄食压力

PDF HTML阅读 XML下载 导出引用 引用提醒 北部湾北部海域夏季微型浮游动物对浮游植物的摄食压力 DOI: 10.5846/stxb201207201034 作者: 作者单位: 厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院,厦门大学海洋与地球学院 作者简介: 通讯作者: 中图分类号: 基金项目: 海湾公益项目（201005012） An ecological study on zooplankton in the northern Beibu Gulf V: the effects of microzooplankton grazing on phytoplankton in summer Author: Affiliation: College of ocean,College of ocean,,,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:2011年8月份于北部湾北部海域5个观测站位获得的分层水样，分析了表层叶绿素a含量和表层微型浮游动物丰度以及类群组成；同时于现场采用稀释培养法研究了该海域浮游植物生长率（μ）和微型浮游动物的摄食率（g）。分析和测定结果表明：调查海区的微型浮游动物丰度400-1167 个/L，类群组成以无壳纤毛虫为主；浮游植物的生长率为-1.50-1.13 d-1，微型浮游动物摄食率为0.33-1.08 d-1；推算微型浮游动物对浮游植物现存量以及初级生产力的摄食压力分别为28.1%-66.0%和-7.4%-438.4%。相对于中国其他海区，8月份北部湾北部海域微型浮游动物摄食速率处于中等水平。调查期间，广西沿海高生产力海区，浮游植物生长率大于微型浮游动物动物的摄食率，浮游植物生物量处于积累期；涠洲岛以南海域，浮游植物生产力较低，微型浮游动物摄食作用是控制浮游植物生长的重要因素。 Abstract:In August 2011, we quantified the impact of microzooplankton grazing on phytoplankton production in the surface waters of the northern Beibu Gulf, Vietnam. Shipboard dilution incubation experiments were carried out at five stations to calculate phytoplankton specific growth rates and the specific rates of grazing losses to microzooplankton. Dilution experiments using chlorophyll a (Chl a) as a tracer were used to estimate daily rates in two size fractions; image-analyzed microscopy provided quantitative estimates of microzooplankton standing stock (including ciliate and vertebrate larvae). PFW (particle-free water) was used to dilute seawater to five target dilutions of 0%, 20%, 40%, 60%, and 80%. Microzooplankton grazing and phytoplankton growth rates were estimated by the linear regression of AGR (apparent growth rate) versus dilution factor. We estimated the grazing impact of microzooplankton on phytoplankton by calculating the percentage of phytoplankton standing stock and potential primary production ingested. Average total chlorophyll a concentration was (0.67±0.58) μg/L. Small-celled phytoplankton dominated this particular community. A notable characteristic of the phytoplankton community structure was that nano-and picoplankton made up a significant portion. Aloricate ciliates dominated the microzooplankton samples, in particular Strombidium, which accounted for 27.2% of total aloricate ciliate abundance. In addition, loricate ciliates and copepod nauplii were recorded. Microzooplankton density varied from 400 to 1167 ind/L. The horizontal distribution of microzooplankton was greater in nearshore than in offshore waters. Abundance did not differ significantly with chlorophyll a concentration in this study; however, there was a positive correlation between microzooplankton abundance and temperature. The phytoplankton growth rate (<200 μm) ranged between -1.50 and 1.13 d-1, with the highest values being associated with nearshore waters. Meanwhile phytoplankton mortality due to microzooplankton grazing (0.33-1.08 d-1) was highest at the same nearshore sites; however, growth rates were low at these sites. Consequently, microzooplankton consumed 28.1%-66.0% of chlorophyll a standing stocks and -8.1%-438.4% of the primary production per day. However, the grazing rate of microzooplankton on nanoplankton (2-20 μm) varied from 1.87 to 0.52 d-1, with an average of (0.94±0.57) d-1. Given that the rates of grazing on phytoplankton of <200 μm were lower than those on nanoplankton, we conclude that microzooplankton prefer to graze on nanoplankton. There was a positive correlation between phytoplankton growth rate and grazing rate; in addition, significant differences between grazing rate and chlorophyll a were found. This suggests that microzooplankton grazing activity is the main factor regulating the community structure and restraining the growth of phytoplankton. During the investigation period, microzooplankton grazing effectively controlled the growth of phytoplankton, consuming 176.2% of primary production on average. When grazing rates are far greater than phytoplankton growth rates, energy is transferred from lower to higher trophic levels more effectively. In the surface waters of the northern Beibu Gulf, microzooplankton largely control the growth of phytoplankton by grazing offshore rather than in nearshore waters where primary production is high. The growth and grazing rates combined promote planktonic ecosystem stability in this area. These results indicate that grazing by microzooplankton is a key process controlling the growth of phytoplankton in this area. 参考文献 相似文献 引证文献

Effects of the nitrification inhibitor dicyandiamide (DCD) on gross N transformation rates and mitigating N2O emission in paddy soils

The nitrification inhibitor dicyandiamide (DCD, C2H4N4) is effective in suppressing nitrification and N2O emission in many agro-ecosystems. However, whether DCD affects other nitrogen (N) transformation processes, and the mechanisms of its inhibitory effects on N2O emission, are not fully understood. In the present study, we conducted a 15N dilution incubation experiment to investigate the effects of DCD on gross N transformation rates and N2O emissions in two paddy soils under an intensive irrigated rice-upland crop rotation system. The duration of microbial inhibition by DCD was also evaluated. We used the numerical model FLUAZ, based on the 15N tracing technique, to calculate gross N transformation rates. The results showed that DCD could effectively inhibit gross N nitrification in the tested soils; however, no influence on N mineralization and immobilization was detected. Lower N2O emissions from both soils were observed in the presence of DCD, due to the dual functions of DCD on both nitrification rates and the fraction of N2O in nitrification products. While, further study to confirm our results by direct measurement of DCD in reducing the fraction of N2O in nitrification products should be conducted. The inhibitory effect of DCD on gross N nitrification and N2O emission decreased over time, and was not statistically significant four weeks after the addition of this inhibitor to the soils.

Top-down control of spring surface phytoplankton blooms by microzooplankton in the Central Yellow Sea, China

Phytoplankton growth and microzooplankton grazing were studied during the 2007 spring bloom in Central Yellow Sea. The surveyed stations were divided to pre-bloom phase (Chl a concentration less than 2μgL−1), and bloom phase (Chl a concentration greater than 2μgL−1). Shipboard dilution incubation experiments were carried out at 19 stations to determine the phytoplankton specific growth rates and the specific grazing rates of microzooplankton on phytoplankton. Diatoms dominated in the phytoplankton community in surface waters at most stations. For microzooplankton, Myrionecta rubra and tintinnids were dominant, and heterotrophic dinoflagellate was also important in the community. Phytoplankton-specific growth rates, with an average of 0.60±0.19d−1, were higher at pre-bloom stations (average 0.62±0.17d−1), and lower at the bloom stations (average 0.59±0.21d−1), but the difference of growth rates between bloom and pre-bloom stations was not statistically significant (t test, p=0.77). The phytoplankton mortality rate by microzooplankton grazing averaged 0.41±0.23d−1 at pre-bloom stations, and 0.58±0.31d−1 during the blooms. In contrast to the growth rates, the statistic difference of grazing rates between bloom and pre-bloom stations was significant (after removal of outliers, t test, p=0.04), indicating the importance of the top-down control in the phytoplankton bloom processes. Average potential grazing efficiency on primary productivity was 66% at pre-bloom stations and 98% at bloom stations, respectively. Based on our results, the biomass maximum phase (bloom phase) was not the maximum growth rate phase. Both phytoplankton specific growth rate and net growth rate were higher in the pre-bloom phase than during the bloom phase. Microzooplankton grazing mortality rate was positively correlated with phytoplankton growth rate during both phases, but growth and grazing were highly coupled during the booming phase. There was no correlation between phytoplankton growth rate and cell size during the blooms, but they were positive correlated during the pre-bloom phase. Our results indicate that microzooplankton grazing is an important process controlling the growth of phytoplankton in spring bloom period in the Central Yellow Sea, particularly in the “blooming” phase.

Effect of post-mortem time on post-thaw characteristics of Spanish ibex ( Capra pyrenaica) spermatozoa

Viable epididymal sperm can be obtained in the Spanish ibex during 24 h after death, but it has been observed a significant effect of the post-mortem time on fertility success, so only goats inseminated with semen recovery during the first 8 h became pregnant. The aim of this study was to determine the effect of post-mortem time on epididymal semen samples from of Spanish ibex. For this purpose, sperm samples from 36 males were collected at different post-mortem times, from 2 to 24 h, and cryopreserved. Thawed samples were incubated for 2 h at 37 °C without dilution or after dilution in a modified Tyrode medium, in order to study the sperm resistance to dilution. Moreover, flow cytometry was used to assess the sperm viability (PI), phospolipid disorder of the plasma membrane (M540), mitochondrial membrane potential (Mitotracker Deep Red), indirect apoptosis markers (YOPRO-1) and sperm chromatin stability (SCSA ®). Sperm motility was evaluated by computer-assisted sperm analysis (CASA). Our results have shown that post-mortem time caused a reduction in mitochondrial membrane potential. In this regard, the loss of energy could be responsible for the loss of maintenance of the membrane with a consequent increase in permeability leading to a decrease in sperm viability and motility, losing linearity and speed. Moreover, the loss of maintenance of the membrane influence the extent to which sperm will survive the cryopreservation process, as it shows the results obtained from the dilution–incubation resistance test. Finally, one important finding of this study is the demonstration of no effect of post-mortem time on post-thaw DNA integrity, giving us the possibility of using sperm samples from valuable males, even if it was not possible to process during the first 8 h.

Impact of microzooplankton and Calanus sinicus (Brodsky, 1962) on phytoplankton in the Yellow Sea during early summer

Dilution incubations and Calanus sinicus addition incubations were simultaneously conducted at five stations in the Yellow Sea in June of 2004 to evaluate the impact of microzooplankton and Calanus sinicus on phytoplankton based on the Chlorophyll a (Chl-a) levels. The Chl-a growth rates (k) ranged from 0.60-1.67 d(-1), while microzooplankton grazed the Chl-a at rates (g) of 0.29-0.62 d(t-1). The addition of C. sinicus enhanced the Chl-a growth rate (Z) by 0.004-0.037 d(-1) ind.(-1) L. C. sinicus abundance ranged from 84.1-160.9 ind. m(-3), which occupied 90.7%-99.1% of the copepod (> 500 mu m) population. The in-situ increase in phytoplankton by C. sinicus community was estimated to be 0.000 4-0.005 9 d(-1). These results showed that microzooplankton were the main grazers of phytoplankton, while C. sinicus induced a slight increase in the levels of phytoplankton.

Lagrangian studies of phytoplankton growth and grazing relationships in a coastal upwelling ecosystem off Southern California

Experimental studies of phytoplankton growth and grazing processes were conducted in the coastal upwelling system off Point Conception, California to test the hypothesis that phytoplankton growth and grazing losses determine, to first order, the local dynamics of phytoplankton in the upwelling circulation. Eight experiments of 3–5 days each were conducted over the course of two cruises in May–June 2006 and April 2007 following the trajectories of satellite-tracked drifters. Rates of phytoplankton growth and microzooplankton grazing were determined by daily in situ dilution incubations at 8 depths spanning the euphotic zone. Mesozooplankton grazing was assessed by gut fluorescence analysis of animals collected from net tows through the euphotic zone. We compared directly the net rates of change observed for the ambient phytoplankton community to the net growth rates predicted from experimental determinations of each process rate. The resulting relationship accounted for 91% of the variability observed, providing strong support for the growth-grazing hypothesis. In addition, grazing by mesozooplankton was unexpectedly high and variable, driving a substantial positive to negative shift in phytoplankton net rate of change between years despite comparable environmental conditions and similar high growth rates and suggesting strong top-down control potential. The demonstrated agreement between net ambient and experimental community changes is an important point of validation for using field data to parameterize models. Data sets of this type may provide an important source of new information and rate constraints for developing better coupled biological–physical models of upwelling system dynamics.

MARINE PHYTOPLANKTON SPECIFIC GROWTH RATE

The concept of marine phytoplankton community specific growth rate (μ) and its related concepts were discussed. The four basic methods for estimating μ, frequency of dividing cells, biochemical indices, model approaches, and dilution incubations without grazers were reviewed, among them the dilution method was recommend as standard method for preliminary investigations of μ in China seas. After preliminary comparing and analyzing the μ around the world, we found that (1) μ depends on taxa composition of target phytoplankton community, (2) μ is relatively higher in offshore waters than in open seas and oligotrophic waters, (3) μ is even higher in mesotrophic areas than in eutrophic areas in offshore waters, and (4) μ is higher in small-cell-dominated community than in big-cell-dominated community. Many things still poorly understand and more studies on μ are needed in China.

Evidence of Trichodesmium viral lysis and potential significance for biogeochemical cycling in the oligotrophic ocean

The planktonic cyanobacterium Trichodesmium spp. is a globally important diazotroph, fixing at least 80 Tg N yr -1 in tropical waters. Despite its biogeochemical importance, the mechanisms of Trichodesmium mortality, and the means by which its fixed N enters upper levels of the food web, are poorly understood. Potential virus-like particle (VLP) production by Trichodesmium spp. was observed in both culture (IMS101) and field samples from the tropical North Pacific Ocean, in oceanic waters around the Hawaiian Islands. VLP observed by TEM in IMS101 lysate were approximately 56 nm wide and untailed, and VLP with similar morphology were observed in tissue treated with mit- omycin C. A most-probable number cultivation technique (utilizing bacterized cultures of Tricho- desmium) detected moderate abundances of Trichodesmium-infecting cyanophage (605 to 9750 ml -1 infecting only 1 cultured strain) in 0.2 µm-filtered seawater samples from surface, subsurface and deep chlorophyll maximum samples. Estimation of mortality from virus production was not possible due to rapid VLP release in the first 6 h of dilution incubations. Rather, an indirect approach using burst size (determined from mitomycin C treatment of washed trichomes resuspended in virus-free seawater), decay rate of VLP (from latter part of virus production incubations) and average cyanophage titer was used to estimate mortality. These conservative calculations suggested that 0.3 to 6.5% d -1 (mean = 1.65 ± 0.99% d -1 ) of trichomes could potentially be lysed by viruses, represent- ing the release of approximately 3 to 64% fixed N d -1 . These estimates are based upon a steady-state maintenance of observed VLP abundance, which in nature could be from lysogeny, pseudolysogeny, carrier state, or successive lytic infection. Viral lysis therefore may represent a significant novel mechanism of N release from Trichodesmium spp., even in non-bloom conditions.

Coupling between primary production and pelagic consumption in temperate ocean margin pelagic ecosystems

Abstract Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon. As part of both US DOE- and NSF-sponsored studies on the Cape Hatteras and South Atlantic Bight (SAB) shelves, the role of microzooplankton in these processes was determined by measuring phytoplankton production and its consumption by microzooplankton. Grazing and growth rates were measured during 46 dilution incubation experiments using chlorophyll a (chl a ) as a proxy for phytoplankton (prey) biomass. Chl a production and grazing were determined for the 14 CO 3 − incubations during two Lagrangian drifter studies lasting several days in March and July 1996. From similar measurements during cross-shelf transects over larger spatial scales, primary production was also calculated for the Hatteras study area using a wavelength-resolved bio-optical model. Primary production during the Lagrangian studies was generally 0.5–1.0 gC/m 2 /d in March and 0.5–2.0 gC/m 2 /d in July. Modeled estimates of primary production for the larger Hatteras study region in March and July averaged 1.8 gC/m 2 /d. Typically, a and its grazing by microzooplankton. In the dilution experiments conducted throughout the SAB and Hatteras shelves, microzooplankton grazed 65% of a production, and 81% of a production. These relationships were temperature-dependent: losses of chl a production in both size fractions to microzooplankton herbivory increased with increasing temperature. Higher grazing rates were found in the 14 CO 3 − -derived estimates of primary production in Cape Hatteras shelf waters. Integrated water column grazing removed 40% and 58% of