Showing papers by "Hongbin Liu published in 2012"

Does warming enhance the effect of microzooplankton grazing on marine phytoplankton in the ocean

Abstract: We evaluated a hypothesis derived from the metabolic theory of ecology (MTE) that the ratio of microzooplankton herbivory (m) to phytoplankton growth (m) will arise in a warming ocean because of the different temperature dependencies of autotrophic and heterotrophic organisms. Using community-level growth and grazing data from dilution experiments, generalized additive models (GAMs) were constructed to describe the effects of temperature and chlorophyll on m:m. At low chlorophyll levels, m:m decreases with increasing temperature, whereas at high chlorophyll levels, m:m increases initially with temperature before reaching a peak and then declines. These complex responses of m:m result from mixed effects of temperature and chlorophyll on microzooplankton biomass (Bz), biomass-specific microzooplankton grazing rate (m:Bz), and phytoplankton growth rate (m). Bz decreases with rising temperature and increases with rising chlorophyll. m:Bz increases with temperature and decreases with chlorophyll. Nutrient-enriched growth rate of phytoplankton (mn) and m increase with increasing temperature and chlorophyll. Holding chlorophyll constant, the calculated activation energies of m:Bz and mn are 0.67 6 0.05 and 0.36 6 0.05 eV, respectively, both consistent with previous MTE estimates for heterotrophs and autotrophs. Our study indicates that warming may enhance phytoplankton losses to microzooplankton herbivory in eutrophic but not in oligotrophic waters. The GAM analysis also provides important insights into underlying system relationships and reasons why community-level responses in natural systems may depart from theory based on laboratory data and individual species.

Effects of dietary essential fatty acids on reproduction rates of a subtropical calanoid copepod, Acartia erythraea

Abstract: University Grants Committee of Hong Kong's AoE [AoE/P-04/04]; Hong Kong Research Grants Council's (RGC) GRF [661809, 661610]; TUYF Charitable Trust [TUYF10SC08]; Xiamen University [2011121007]

Nitrogen Sources and Rates of Phytoplankton Uptake in Different Regions of Hong Kong Waters in Summer

Abstract: Phytoplankton uptake rates of ammonium (NH4+), nitrate (NO3−), and urea were measured at various depths (light levels) in Hong Kong waters during the summer of 2008 using 15N tracer techniques in order to determine which form of nitrogen (N) supported algal growth. Four regions were sampled, two differentially impacted by Pearl River discharge, one impacted by Hong Kong sewage discharge, and a site beyond these influences. Spatial differences in nutrient concentrations, ratios, and phytoplankton biomass were large. Dissolved nutrient ratios suggested phosphorus (P) limitation throughout the region, largely driven by high N loading from the Pearl River in summer. NH4+ and urea made up generally ≥50% of the total N taken up and the f ratio averaged 0.26. Even at the river-impacted site where concentrations of NO3− were >20 μM N, NH4+ comprised >60% of the total N uptake. Inhibition experiments demonstrated that NO3− uptake rates were reduced by 40% when NH4+ was >5 μM N. The relationship between the total specific uptake rates of N (sum of all measured substrates, V, per hour) and the chlorophyll a-specific rates (micromolars of N per microgram of Chl a per hour) varied spatially with phytoplankton biomass. Highest uptake rates and biomass were observed in southern waters, suggesting that P limitation and other factors (i.e., flushing rate) controlled production inshore and that the unincorporated N (mainly NO3−) was transported offshore. These results suggest that, at the beginning of summer, inshore algal blooms are fueled primarily by NH4+ and urea, rather than NO3−, from the Pearl River discharge. When NH4+ and urea are depleted, then NO3− is taken up and can increase the magnitude of the bloom.

Characterization of the Proteomic Profiles of the Brown Tide Alga Aureoumbra lagunensis under Phosphate- and Nitrogen-Limiting Conditions and of Its Phosphate Limitation-Specific Protein with Alkaline Phosphatase Activity

Abstract: The persistent bloom of the brown tide alga Aureoumbra lagunensis has been reported in coastal embayments along southern Texas, but the molecular mechanisms that sustain such algal bloom are unknown. We compared the proteome and physiological parameters of A. lagunensis grown in phosphate (P)-depleted, P- and nitrogen (N)-depleted, and nutrient-replete cultures. For the proteomic analysis, samples from three conditions were subjected to two-dimensional electrophoresis and tandem mass spectrometry analysis. Because of the paucity of genomic resources in this species, a de novo cross-species protein search was used to identify the differentially expressed proteins, which revealed their involvement in several key biological processes, such as chlorophyll synthesis, antioxidative protection, and protein degradation, suggesting that A. lagunensis may adopt intracellular nutrient compensation, extracellular organic nutrient regeneration, and damage protection to thrive in P-depleted environments. A highly abundant P limitation-specific protein, tentatively identified as a putative alkaline phosphatase, was further characterized by enzyme activity assay on nondenaturing gel and confocal microscopy, which confirmed that this protein has alkaline phosphatase activity, is a cytoplasmic protein, and is closely associated with the cell membrane. The abundance, location, and functional expression of this alkaline phosphatase all indicate the importance of organic P utilization for A. lagunensis under P limitation and the possible role of this alkaline phosphatase in regenerating phosphate from extra- or intracellular organic phosphorus.

Inorganic and Organic Nitrogen Uptake by Phytoplankton and Bacteria in Hong Kong Waters

Abstract: Measurements of uptake rates of inorganic (NO3- and NH4+) and organic (urea, glycine, and glutamic acid) N, and indirect estimates of total N uptake by bacteria, were made in four contrasting environments in sub-tropical Hong Kong waters in summer of 2008. In addition, the effects of several days of rain on N uptake rates were studied in eastern waters. Although ambient NO3- was the dominant form of N in Hong Kong waters, the dominant N form taken up by phytoplankton was usually NH4+ and organic N, including urea and amino acids, rather than NO3-. Hence, because of the low NO3- uptake, there was a long turnover time for NO3- (100 days), and most of the NO3- was apparently transported offshore into deeper shelf waters. In eastern waters where NH4+ was undetectable, NO3- uptake rates were positively correlated with phytoplankton cell size. In contrast, potential rates of glutamic acid uptake were negatively correlated with phytoplankton size. N uptake rates in the smaller size fraction (0.7-2.8 mu m) were less affected by the rain event, and smaller phytoplankton appeared to outcompete larger cells after several days of rain. The surface (PN)-specific N uptake rates in the >8-mu m fraction decreased from 0.02 to 0.0001 h(-1), while the smaller fraction only exhibited a one-to threefold decrease after the rainfall. In contrast, bacterial production and N uptake were not affected by the rain event, and bacteria N uptake accounted for 10-60% of the total N uptake by phytoplankton.

Phylogenetic composition of Prochlorococcus and Synechococcus in cold eddies of the South China Sea

Abstract: Two mesoscale cyclonic eddies (CE I and CE II) occurred along the Vietnam coast in the South China Sea during summer 2007; CE I was in the decaying stage and CE II was in the intensifying stage. CE II was also influenced by the Mekong River plume. The upwelling effect of the 2 eddies on the phylogenetic diversity of Prochlorococcus and Synechococcus was investi- gated by clone library construction based on sequences of the internal transcribed spacer (ITS) region, with station SEATS in the oligotrophic basin as a comparison. Phylogenetic analysis revealed the presence of sequences affiliated with 3 Prochlorococcus clades (HL I, HL II & LL IV) and 7 Synechococcus clades (II, III, VII, X, XV, XVI, and sub-cluster 5.2) from the various water bodies, with HL II Prochlorococcus and clade II Synechococcus being the dominant phylotypes. Multivariate analysis demonstrated that spatial distribution of the 2 genera was mostly explained by temperature, but also affected by salinity and chlorophyll a concentration. Upwelling of the nutrient-rich subsurface water at the center of the cold eddy resulted in a significant increase in the abundance of Synechococcus in the young and intensifying CE II, whereas Prochlorococcus dominated in the old and decaying CE I and the surrounding oligotrophic waters. It is intriguing that Synechococcus clades III, XV and XVI, which are known to contain chromatically adapting members, were most numerous in eddy and river plume influenced waters, where light quality changes might be expected, compared to the oligotrophic basin waters. The disparity in cyanobacterial phylogenetic composition between the 2 eddies clearly reflected the differing hydrographic dynamics and developmental stages of these eddy systems.

Dynamics of bacterial community composition during degradation of copepod fecal pellets

Abstract: The community of bacteria associated with the fecal pellets of planktonic copepods and those free living in surrounding seawater were investigated by denaturing gradient gel electrophoresis (DGGE) during a 10-day laboratory fecal pellet degradation experiment. Generally, fecal pellets containing bacteria were composed of different phylogenetic groups compared with those living in seawater. Bacteria in fecal pellets were dominated by g-Proteobacteria and Sulfitobacter (a-Proteobacteria), whereas bacteria in seawater had higher species richness and mainly consisted of a-Proteobacteria. Remarkable bacterial community shifts occurred in the first 2 days of the experiment along with the apparent increase in dissolved organic carbon and decrease of dissolved oxygen in the incubation bottles. Throughout the incubation, bacteria that were initially unique to fecal pellets were never found in surrounding seawater, while the communities of bacteria in fecal pellets and seawater became more similar, indicating colonization of bacteria from seawater to fecal pellets during the degradation process. These results suggest that the colonization of free-living bacteria took place rapidly and that they might contribute significantly to the degradation of planktonic copepod fecal pellets.

Contrasting Bacterial Dynamics in Subtropical Estuarine and Coastal Waters

Abstract: Seasonal succession and composition of both attached and free-living bacterial communities were studied in subtropical estuarine and coastal waters with contrasting hydrographic conditions. A higher abundance of attached bacteria was recovered in the estuarine waters containing high concentrations of dissolved organic carbon (DOC) resulting from the freshwater discharge in the adjacent Pearl River, and Proteobacteria, including α-, β-, and γ-groups, predominated the attached community at both stations. Free-living bacterial communities at both stations showed higher diversity and lower seasonality than their attached counterparts, and α-Proteobacteria accounted for the highest proportion at both stations. Redundancy analysis (RDA) demonstrated that, in addition to the obvious temperature effects, DOC and microphytoplankton (>20 μm Chl a) drive the temporal variation of attached bacteria at the estuarine and coastal stations, respectively. On the other hand, picophytoplankton ( 20 μm). These findings suggest that temperature and bottom–up effects play a more important role for the spatial–temporal variations of both attached and free-living bacterial communities in the subtropical estuarine and coastal waters.

The distribution of chlorophyll a in the tropical eastern Indian Ocean in austral summer

Abstract: To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a), an investigation was carried out in the tropical eastern Indian Ocean (80°-100°E along 7°S, and 7°-18°S along 80°E) in December 2010. The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations. The results show that surface Chl a concentration averaged at (0.168±0.095) mg/m3 s.d. (range: 0.034–0.475 mg/m3), concentrations appeared to be higher in the west for longitudinal variations, and higher in the north for latitudinal variations. Furthermore, the surface Chl a concentration was lower (0.034–0.066 mg/m3) in the region to the south of 16°S. There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline. The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO 4 3− , N(NO3-N+NO2-N) and temperature. Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton dominated, accounting for approximately 67.8% of total Chl a, followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%). The two larger fractions were sensitive to the limitation of P, while picophytoplankton was primarily affected by temperature.