Stoichiometric relationships among producers,consumers and nutrient cycling in pelagic ecosystems

Most ecosystem models consolidate members of food-webs, e.g. species, into a small number of functional components. Each of these is then described by a single state variable such as biomass. When a multivariate approach incorporating multiple substances within components is substituted for this univariate one, a stoichiometric model is formed. Here we show that the Nitrogen:Phosphorus ratio within zooplankton herbivores varies substantially intraspecifically but not intraspecifically. By using stoichiometric theory and recent measurements of the N:P ratio within different zooplankton taxa, we calculate large differences in ratios of nutrients recycled by different zooplankton species. Finally, we demonstrate that N:P stoichiometry can successfully account for shifts in N- and P-limitation previously observed in whole-lake experiments. Species stoichiometry merges food-web dynamics with biogeochemical cycles to yield new insights.Abbreviations b N:P in zooplankton biomass - f N:P in algal biomass - L maximum accumulation eficiency - N:P ratio of nitrogen to phosphorus (moles:moles) - s N:P supply ratio from grazers - TN Total nitrogen = seston N + dissolved N (µmoles/liter) - TP Total phosphorus = seston P + dissolved P (µmoles/liter)     

Bacteria as a source of phosphorus for zooplankton

The utilization of bacterial phosphorus in zooplankton metabolism was investigated using radio-phosphorus labelled natural bacteria as food source for zooplankton in feeding experiments. Incorporation of labelled bacteria was clearly related to the species' ability to graze on bacteria, with the cladoceran Daphnia reaching the highest biomass-specific activity and the copepod Acanthodiaptomus the lowest. Within Daphnia, juveniles had a higher biomass-specific uptake of phosphorus than adults. This was presumably caused by higher growth rates of the juveniles rather than age-specific differences in the ability to feed on bacteria, supported by the observation that the juveniles had the highest specific P-content. Retention of ingested ³²P from labelled particles exceeded 80%, indicating higher assimilation efficiencies on phosphorus compared with carbon. In the investigated humic lake, approximately 75% of the phosphorus in grazable particles was bound in bacterial cells, making bacteria the most important source of P to the bacterivore zooplankton species.     

The response of planktonic and microbenthic algal assemblages to nutrient enrichment in shallow coastal waters,southwest Sweden

Field and laboratory nutrient (nitrogen and phosphorus) enrichment experiments were performed using natural phytoplankton and microphytobenthic assemblages from the brackish water Öresund, S.W. Sweden. The response of algae from a low-nutrient area (Falsterbo Canal) was compared to that of algae from a polluted, nutrient-rich area (Lomma Bay).The biomass (measured as chlorophyll a) of both phytoplankton and microphytobenthos from the Falsterbo Canal increased after the addition of nitrogen. Phytoplankton growth was stimulated by the addition of phosphorus to the nitrogen-rich water of the polluted Lomma Bay. Sediment chlorophyll a showed no significant increase after the addition of nutrients in the Lomma Bay. In containers without sediment, phytoplankton uptake was calculated to account for ≈ 90% of the disappearance of inorganic fixed nitrogen from the water. In the sediment containers the microphytobenthos was estimated to account for ≈20% of the nitrogen uptake. The rest was presumably lost mainly through denitrification.When containers with microphytobenthos from Lomma Bay were kept in the dark, phosphorus was released at a rate of up to ≈ 180 μM · m^?2 · day^?1. We suggest that by producing oxygen microbenthic algae keep the sediment surface oxygenated thereby decreasing phosphorus transport from the sediment to the overlying water.     

Enclosures study of trophic level interactions in the mesotrophic part of Lake Balaton

Enclosures, open to the bottom sediments and to the atmosphere, containing about 17 m³ of lake water in the mesotrophic area of Lake Balaton, were used to elucidate the role of the benthivorous fish bream (Ambramis brama L.) in the lake during 1984–1986.Throughout the whole period water was less transparent in the enclosure containing fish, which strongly influenced the concentrations of suspended solids and chlorophyll a.Both phytoplankton biomass and production readily responded to nutrient increase in the enclosure with fish. In 1985 diatoms were replaced by cyanobacteria whereas in 1986, at a lower fish stocking, a shift in algal structure towards chlorophytes was observed.Egested organic substances and the resuspension of sediment particles by fish increased bacterial production.     

Regeneration of inorganic phosphorus and nitrogen from decomposition of seston in a freshwater sediment

The mineralization of phosphorus and nitrogen from seston was studied in consolidated sediment from the shallow Lake Arreskov (July and November) and in suspensions without sediment (July). In the suspension experiment, phosphorus and nitrogen were mineralized in the same proportions as they occurred in the seston. During the 30 days suspension experiment, 47 and 43% of the particulate phosphorus and nitrogen, respectively, was mineralized with constant rates.Addition of seston to the sediment had an immediate enhancing effect on oxygen uptake, phosphate and ammonia release, whereas nitrate release decreased due to denitrification. The enhanced rates lasted for 2–5 weeks, while the decrease in nitrate release persisted throughout the experiment. The increase in oxygen uptake (equivalent to 21% of the seston carbon) was, however, only observed in the July experiment. The release of phosphorus and nitrogen from seston decomposing on the sediment surface differed from the suspension experiments. Thus, between 91 and 111% of the phosphorus in the seston was released during the experiments. Due to opposite directed effects on ammonium and nitrate release, the resulting net release of nitrogen was relatively low.A comparison of C/N/P ratios in seston, sediment and flux rates indicated that nitrogen was mineralized faster than phosphorus and carbon. Some of this nitrogen was lost through denitrification and therefore not measurable in the flux of inorganic nitrogen ions. This investigation also suggests that decomposition of newly settled organic matter in sediments have indirect effects on sediment-water exchanges (e.g. by changing of redox potentials and stimulation of denitrification) that modifies the release of mineralized phosphate and nitrogen from the sediment.     

Planktonic phosphorus pool sizes and cycling efficiency in coastal and interior British Columbia lakes

1. Limnologists have long acknowledged the importance of phosphorus (P) in determining the organism biomass and productivity of lake ecosystems. Despite a relatively large number of studies that have examined P cycling in lake ecosystems, there remain several substantial methodological issues that have impeded our understanding of P cycling in limnetic plankton communities. Two critical issues confronting ecologists are (1) a lack of precise measurements of the dissolved inorganic phosphorus (PO) and (2) accurate or complete measurements of dissolved P regeneration rates by plankton communities. 2. Here, we examine patterns of epilimnetic planktonic P pool sizes and turnover rates in eight lakes in British Columbia, Canada over a 2‐year period. We determine the concentrations and turnover times of P in various planktonic compartments (dissolved and various planktonic size fractions), using recently developed methods for estimating phosphate concentration and planktonic regeneration rates. 3. The pico‐ and nanoplankton size fraction (0.2–20 μm) played a central role in planktonic P cycling in lakes examined by this study. On average across lakes, pico‐ and nanoplankton contained >60% of the planktonic P, accounted for >90% PO uptake, and contributed 50% of the plankton community dissolved P regeneration rate. 4. PO concentrations determined by steady state bioassays (ssPO) were extremely low (87–611 pmol L⁻¹) and were 2–3 orders of magnitude less than simultaneously measured colorimetric soluble reactive phosphorus estimates. Lake ssPO concentrations increased linearly with total phosphorus (TP), and the slope of this relationship was approximately 1, indicating that PO remained a consistent proportion of the TP pool across a range of TP concentrations. 5. Turnover rates of the total planktonic P pool and the <20 μm pool became more rapid with increasing lake TP, indicating that, according to this metric, planktonic P cycling efficiency increased with TP concentrations. We also detected a significant relationship between particulate phosphorus (PP) <20 μm turnover time and seston N : P ratios, with PP <20 μm turnover times becoming slower with increasing seston N : P. These findings suggest that long‐standing conceptual models of nutrient cycling that predict slower cycling rates and decreasing cycling efficiency with increasing TP concentrations require further empirical examination. We postulate that patterns in lake P turnover and cycling efficiency are a result of complex interactions between plankton biomass and composition, and the ratios of multiple nutrients (C, N, P), rather than solely a function of the TP pool.     

Heterogeneity in the nutrient limitation of different bacterioplankton groups in the Eastern Mediterranean Sea

The heterotrophic bacterial community of the Eastern Mediterranean Sea is believed to be limited by phosphorus (P) availability. This observation assumes that all bacterial groups are equally limited, something that has not been hitherto examined. To test this hypothesis, we performed nutrient addition experiments and investigated the response of probe-identified groups using microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization. Our results show contrasting responses between the bacterial groups, with Gammaproteobacteria being the group more affected by P availability. The Roseobacter clade was likely colimited by P and nitrogen (N), whereas Bacteroidetes by P, N and organic carbon (C). In contrast, SAR11 cells were active regardless of the nutrient concentration. These results indicate that there is high heterogeneity in the nutrient limitation of the different components of the bacterioplankton community.     

Decomposition responses to phosphorus enrichment in an Everglades (USA) slough

The effects of phosphorus (P) enrichment ondecomposition rates were measured in a Ploading experiment conducted in an oligotrophicmarsh in the northern Everglades, USA. In thisstudy, eighteen 2.5 m² enclosures(mesocosms) were placed in a pristineopen-water (slough) wetland and subjectedweekly to 6 inorganic P loads; 0, 0.2, 0.4,0.8, 1.6 and 3.2 g·m^–2g·yr^–1. Phosphorus accumulated rapidly in the benthicperiphyton and unconsolidated detrital (benthicfloc) layer and significantly higher Pconcentrations were recorded after 1 yr of Paddition. In contrast, a significant increasein surface soil (0–3 cm) TP concentrations wasmeasured in the surface soil layer only after 3yr of loading at the highest dose. Plantlitter and benthic floc/soil decompositionrates were measured using litter bags,containing sawgrass (Cladium jamaicenseCrantz) leaves, and cotton (cellulose) strips,respectively. Litter bag weight losses weresimilar among treatments and averaged 30% atthe end of the 3 yr study period. Litter Nconcentrations increased over time by anaverage of 80% at P loads < 1.6g·m^–2·yr^–1, and by > 120% at Ploads 1.6 g·m^–2·yr^–1.In contrast,litter P concentrations declined up to 50% inthe first 6 months in all P loads and onlysubsequently increased in the two highestP-loaded mesocosms. Cotton strip decaydemonstrated that benthic floc and soilmicrobial activity increased within 5 mo of Paddition with more significant treatmenteffects in the benthic than the soil layer. The influence of soil microbial transformationswas shown in porewater chemistry changes. While porewater P levels remained close tobackground concentrations throughout the study,porewater NH₄ ⁺ and Ca²⁺increased in response to P enrichment,suggesting that one significant effect of Penrichment in this oligotrophic peat system isenhanced nutrient regeneration.     

柄杆菌对固氮蓝藻生物量及色素的影响

柄杆菌对固氮蓝藻生物量和色素的影响研究结果表明:多态柄杆菌(Caulobacter polymorphus)017-41或新月柄杆菌(Caulobacter creseentus)CB_2的活菌、死菌及破碎细胞悬液分别与鱼腥藻(Anabaena)、念珠藻(Nostoc)不同藻珠混合培养时,试验组生长量均优于对照组;对衰老黄化的藻培养物的生长刺激作用尤为显著;试验组藻培养物的藻蓝或藻红素含量亦明显高于对照组。其作用机理尚待进一步阐明。     

The production of clonal and axenic cultures of microalgae using fluorescence-activated cell sorting

Unialgal cultures of the flagellate algae Cyanophora paradoxa, Haematococcus lacustris, Monomastix sp., Scherffelia dubia and Spermatozopsis similis which contained bacteria were sorted by flow cytometry to obtain axenic clonal cultures. The variables used for fluorescence-activated cell sorting (FACS) were chlorophyll autofluorescence, forward scatter and side scatter of the laser beam. To produce clonal cultures, a single cell was sorted into each culture flask. Depending on the species, about 20–30% of the sorted cultures grew successfully and at least 20% of these were axenic even if the numerical ratio betweeen bacteria and algae in the original cultures was as high as 300:1. FACS represents an effective and rapid method for the preparation of clonal and axenic cultures of microalgae.     

Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

Release rates and biological availability of phosphorus released from sediments receiving aquaculture wastes

Aquaculture is an increasingly significant user of freshwater resources in Scotland. In 1989, the total fish biomass produced in Scottish freshwater amounted to 7000 t. 50% of this total was reared in floating cage systems situated in lochs (lakes). Both solid (mainly in the form of uneaten feeds and faecal matter) and dissolved byproducts of the production cycle enter the limnetic environment untreated. Much solid waste material accumulates directly on the sediments beneath the cage systems. This leads to a localised enrichment in nutrient elements of the sedimentary environment. The experiments served to quantify rates of total phosphorus (TP) and dissolved reactive phosphorus (DRP) release from undercage and control sites, and to relate such releases to the biological availability of the released P. Results indicate significantly higher levels of NH₄Cl-extractable P in sediments affected by waste deposition from fish cages. TP and DRP release, and greater growth of Chla are obtained from undercage cores compared with control sites. No link between extractable-P content of sediments, or release rate and Chla production was established.     

A radiative transfer modeling approach for accurate interpretation of PAM fluorometry experiments in suspended algal cultures

The results of a numerical study on the simulation of pulse amplitude modulated (PAM) fluorometry within dense suspensions of photosynthetic microorganisms are presented. The Monte Carlo method was used to solve the radiative transfer equation in an algae‐filled cuvette, taking into account absorption, anisotropic scattering, and fluorescence, as well as Fresnel reflections at interfaces. This method was used to simulate the transport of excitation and fluorescence light in a common laboratory fluorometer. In this fluorometer, detected fluorescence originates from a multitude of locations within the algal suspension, which can be exposed to very different fluence rates. The fluorescence‐weighted fluence rate is reported, which is the local fluence rate of actinic light, averaged over all locations from which detected fluorescence originated. A methodology is reported for recovering the fluorescence‐weighted fluence rate as a function of the transmittance of measuring light and actinic light through the sample, which are easily measured with common laboratory fluorometers. The fluorescence‐weighted fluence rate can in turn be used as a correction factor for recovering intrinsic physiological parameters, such as the functional cross section of Photosystem II, from apparent (experimental) values. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1601–1615, 2016     

Microbial to reef scale interactions between the reef-building coral Montastraea annularis and benthic algae

Competition between reef-building corals and benthic algae is of key importance for reef dynamics. These interactions occur on many spatial scales, ranging from chemical to regional. Using microprobes, 16S rDNA pyrosequencing and underwater surveys, we examined the interactions between the reef-building coral Montastraea annularis and four types of benthic algae. The macroalgae Dictyota bartayresiana and Halimeda opuntia, as well as a mixed consortium of turf algae, caused hypoxia on the adjacent coral tissue. Turf algae were also associated with major shifts in the bacterial communities at the interaction zones, including more pathogens and virulence genes. In contrast to turf algae, interactions with crustose coralline algae (CCA) and M. annularis did not appear to be antagonistic at any scale. These zones were not hypoxic, the microbes were not pathogen-like and the abundance of coral-CCA interactions was positively correlated with per cent coral cover. We propose a model in which fleshy algae (i.e. some species of turf and fleshy macroalgae) alter benthic competition dynamics by stimulating bacterial respiration and promoting invasion of virulent bacteria on corals. This gives fleshy algae a competitive advantage over corals when human activities, such as overfishing and eutrophication, remove controls on algal abundance. Together, these results demonstrate the intricate connections and mechanisms that structure coral reefs.     

Establishment of suspension cultures from seeds of plains bluestem [Bothriochloa ischaemum (L.) Keng.] and regeneration of plants via somatic embryogenesis

Summary Mature seeds of plains Old World bluestem [Bothriochloa ischaemum (L.) Keng.] were used to initiate suspension cultures. The medium contained the major and minor minerals of Murashige and Skoog, Gamborg's B-5 vitamins, 30 g/liter sucrose, and 3 mg/liter 2,4-dichlorophenoxyacetic acid with or without 12 mM proline at pH 5.5. Cultures contained both embryogenic and nonembryogenic (NE) cells. Suspensions that had been filtered through a 40-mesh sieve were plated out on medium with 6 g/liter agar. Two-to-three weeks later, clumps that formed in suspension cultures that had been filtered previously were removed by filtration through a 40-mesh sieve and plated out on agar medium. Colonies were rated on the basis of surface area. of the total area of colonies formed from plated suspensions 70.9% were embryogenic, 19.8% were NE, and 9.3% were mixed colonies. Of the total area from plated clumps, 57.1% were E, 12.9% were NE, and 30% were mixed colonies. Embryoid maturation and germination was accomplished by transferring E or mixed colonies to MS medium with 1 mg/liter zeatin (mixted isomers). Rooting was completed on half-strength basal MS medium. Over 90% of plantlets survived transfer to the greenhouse and 95% of them survived transfer to the field. Seeds were provided by Dr. Charles Taliaferro, Agronomy Department, Oklahoma State University.     

Plant regeneration from protoplasts of durum wheat (Triticum durum Desf. cv. D6962)

Suspension cultures of durum wheat were established from embryogenic callus maintained in liquid medium for 30 months. Protoplasts were readily isolated from the suspension cultures with yields as high as 3 X 10⁷ protoplasts per g fresh weight suspension cells. When incubated in a modified MS medium containing half strength of the macroelements, 5 M 2,4-D (2,4-dichlorophenoxyacetic acid) and 0.6 M glucose, protoplast-derived cells divided at frequencies ranging from 1.4 to 10.0 %. After transfer to a solid subculture medium, the protoplast-derived colonies formed embryogenic protuberances, from which green plants have been regenerated.