Algal Carbon Research Articles

Identifi cation of an Algal Carbon Fixation-Enhancing Factor Extracted from Paramecium bursaria

The green ciliate Paramecium bursaria contains several hundred symbiotic Chlorella species. We previously reported that symbiotic algal carbon fixation is enhanced by P. bursaria extracts and that the enhancing factor is a heat-stable, low-molecular-weight, water-soluble compound. To identify the factor, further experiments were carried out. The enhancing activity remained even when organic compounds in the extract were completely combusted at 700 degrees C, suggesting that the factor is an inorganic substance. Measurement of the major cations, K+, Ca2+, and Mg2+, by an electrode and titration of the extract resulted in concentrations of 0.90 mM, 0.55 mM, and 0.21 mM, respectively. To evaluate the effect of these cations, a mixture of the cations at the measured concentrations was prepared, and symbiotic algal carbon fixation was measured in the solution. The results demonstrated that the fixation was enhanced to the same extent as with the P. bursaria extract, and thus this mixture of K+, Ca2+, and Mg2+ was concluded to be the carbon fixation-enhancing factor. There was no effect of the cation mixture on free-living C. vulgaris. Comparison of the cation concentrations of nonsymbiotic and symbiotic Paramecium extracts revealed that the concentrations of K+ and Mg2+ in nonsymbiotic Paramecium extracts were too low to enhance symbiotic algal carbon fixation, suggesting that symbiotic P. bursaria provide suitable cation conditions for photosynthesis to its symbiotic Chlorella.

Functional effects of parasites on food web properties during the spring diatom bloom in Lake Pavin: a linear inverse modeling analysis.

This study is the first assessment of the quantitative impact of parasitic chytrids on a planktonic food web. We used a carbon-based food web model of Lake Pavin (Massif Central, France) to investigate the effects of chytrids during the spring diatom bloom by developing models with and without chytrids. Linear inverse modelling procedures were employed to estimate undetermined flows in the lake. The Monte Carlo Markov chain linear inverse modelling procedure provided estimates of the ranges of model-derived fluxes. Model results support recent theories on the probable impact of parasites on food web function. In the lake, during spring, when ‘inedible’ algae (unexploited by planktonic herbivores) were the dominant primary producers, the epidemic growth of chytrids significantly reduced the sedimentation loss of algal carbon to the detritus pool through the production of grazer-exploitable zoospores. We also review some theories about the potential influence of parasites on ecological network properties and argue that parasitism contributes to longer carbon path lengths, higher levels of activity and specialization, and lower recycling. Considering the “structural asymmetry” hypothesis as a stabilizing pattern, chytrids should contribute to the stability of aquatic food webs.

Oxygen minimum seafloor ecological (mal) functioning

Although organic matter (OM) settling on the seafloor is generally rapidly recycled, a key ecological process, large scale burial events manifest itself in the marine sedimentary record as organic carbon (Corg)-rich layers. Presently, this prevails under certain oceanic settings such as the oxygen minimum zones (OMZ) where OM accumulates in underlying sediments. A basic question that remains is as to what extent this Corg accumulation reflects ecological “malfunctioning” or a shunting of ecological processes? Experimenting with eastern Arabian Sea OMZ sediment we found no evidence that Corg accumulation here is not due to trophic satiation or to low tolerance of biota to severe oxygen depletion. However, we found direct evidence that suggests that the OMZ sediment Corg has very low bioavailability that probably impairs biological transformation.In the first set of experiments, the impact of oxygenation on the benthic ecological functioning was examined by following the fate of fresh, highly degradable OM (13C-labelled diatoms) in intact sediment cores incubated for 7days under normoxic versus suboxic bottom water conditions. Tracer organic matter assimilation (by bacteria and fauna) and respiration was evident and similar under both treatments and demonstrates that the benthic response was not hindered by severe oxygen depletion. Furthermore, relatively low biomass standing stock of fauna and bacteria, in spite of sediment high Corg content, together with this clear uptake of fresh tracer OM suggest that the benthic community was not food saturated.In a second set of experiments, the bioavailability of in situ OMZ organic matter was determined directly through CO2 production rate measurements in bottle sediment–water slurry incubations. In sharp contrast to fresh tracer algal carbon which had a half-life 0.07years, the OMZ surficial sediment OM half-life was ~67years already in very early diagenesis. Clearly, a distinct difference in functioning and indicates that a large fraction of OMZ sediment organic matter is evidently excluded from immediate first-hand biotic transformation but on its own represents a link between the “fast (biological)” and the “slow (geological)” carbon cycle along the continuum of OM recycling. Furthermore, while this rapid shift out of the “fast” biological cycle may be common or characteristic of large scale OM accumulation events, a comparison with an ancient Corg-deposit suggest that the trigger mechanisms may not be uniform.

Identification of an Algal Carbon Fixation-Enhancing Factor Extracted from Paramecium bursaria

Identification of an Algal Carbon Fixation-Enhancing Factor Extracted from Paramecium bursaria

Longitudinal trends in river functioning: Patterns of nutrient and carbon processing in three Australian rivers

AbstractUnderstanding longitudinal trends in the processing of carbon in rivers represents a much conceptualised, but infrequently tested, issue in aquatic ecology. In this study, we conducted concurrent longitudinal examinations of three very different rivers in eastern Australia to determine whether general principles in river functioning exist across broad geographic and hydrologic scales. Specifically, we examined trends in ambient basic water chemistry, nutrient concentrations, dissolved organic carbon (DOC), extracellular enzymes and food web structure and functioning and conducted bioassays to examine the degree to which DOC and nutrients limit heterotrophic bacterial respiration. These parameters revealed striking similarities across all sites. For metazoan communities, stable isotope analysis showed that algal carbon was the dominant basal resource utilised by consumers in all three rivers, suggesting that in‐stream primary producers strongly underpin trophic pathways regardless of the position within a catchment or catchment condition. Analyses of extracellular enzymes revealed that microbial communities are actively utilising DOC at all sites. In fact, heterotrophic microbial respiration was strongly limited by DOC at all sites, with nutrient additions resulting in only relatively minor increases in respiration. Ultimately, this study demonstrates that DOC and algal carbon are critically important drivers of ecosystem processes in Australian riverine ecosystems. Furthermore, across all of our sites and rivers, ambient nutrient concentrations did not influence carbon processing. The consistent longitudinal trends in river function identified in this study provide useful insights for catchment managers and modellers with respect to identifiying key principles that underpin ecosystem functioning in Australian rivers. Copyright © 2009 John Wiley & Sons, Ltd.

Temporal variability of benthic algal δ13C signatures influences assessments of carbon flows in stream food webs

Stream food web function is often assessed using carbon stable isotope assessments of the relative contribution of autochthonous and allochthonous sources of organic matter to consumer diets. As a result, variability in source signatures can strongly influence the assessment of carbon flows. To examine the implications of temporal source variability on food web interpretations, benthic algal δ13C signatures were measured over 8 weeks in five streams in subtropical Queensland, Australia. All food webs were largely driven by benthic algal carbon; however, substantial week-to-week variation in benthic algal δ13C signatures modified the calculated contributions of algae to consumer diets, with differences in autochthonous contributions of up to 11% between weeks. In addition, variable algal signatures led to many occasions in which the δ13C signatures of some consumers was beyond the range of available sources, meaning the mixing model analyses did not have a valid solution. Together, these findings suggest that temporal variability in algal δ13C signatures can strongly influence the interpretation of carbon flows in stream food webs. Future food web studies should assess the temporal variability of sources prior to sampling consumers, in order to characterise end member signatures and their relevance to consumers at the time of collection.

Microbial carbon oxidation rates and pathways in sediments of two Tanzanian mangrove forests

Temporal and spatial variations in benthic metabolism and anaerobic carbon oxidation pathways were assessed in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. The objectives were (1) to evaluate how benthic metabolism is affected by organic carbon availability; (2) to determine the validity of diffusive release of CO2 as a measure benthic carbon oxidation; and (3) to assess the partitioning of anaerobic carbon pathways and factors controlling the availability of electron acceptors (e.g. oxidized iron). Microbial carbon oxidation measured as diffusive exchange of O2 and CO2 (32–67 and 28–115 mmol m−2 day−1, respectively) showed no specific temporal patterns. Low intertidal sediments at Mtoni fed by labile algal carbon of anthropogenic origin had higher diffusive CO2 release than high intertidal sediments that primarily received less reactive mangrove detritus. Diffusive release of CO2 apparently underestimated total sediment carbon oxidation due to CO2 loss from deep sediments via emission through biogenic structures (i.e. crab burrows and pneumatophores) and porewater seepage into creeks. We propose that diffusive fluxes in the present mangrove sediments are roughly equivalent to depth-integrated reactions occurring in the upper 12 cm. Anaerobic carbon oxidation was dominated by FeR irrespective of anthropogenic influence in sediments where the oxidizing effects of biogenic structures increased the Fe(III) level. More than 80% of the anaerobic carbon oxidation in Mtoni and Ras Dege sediments was due to FeR when reactive Fe(III) exceeded 30 μmol cm−3. The anthropogenic influence at Mtoni was primarily noted as up to one order of magnitude higher denitrification than at Ras Dege, but this process always accounted for less than 1% of total carbon oxidation. It is noteworthy that organic and nutrient enrichment of anthropogenic origin in Mtoni has no measurable effect on microbial processes, other than carbon oxidation in the low intertidal area and denitrification throughout the forest, and indicates a strong resilience of mangrove environments towards disturbances.

Initial impacts of Microcystis aeruginosa blooms on the aquatic food web in the San Francisco Estuary

The impact of the toxic cyanobacterium Microcystis aeruginosa on estuarine food web pro- duction in San Francisco Estuary is unknown. It is hypothesized that Microcystis contributed to a recent decline in pelagic organisms directly through its toxicity or indirectly through its impact on the food web after 1999. In order to evaluate this hypothesis, phytoplankton, cyanobacteria, zooplankton, and fish were collected biweekly at stations throughout the estuary in 2005. Concentrations of the tumor-promot- ing Microcystis toxin, microcystin, were measured in water, plankton, zooplankton, and fish by a protein phosphatase inhibition assay, and fish health was assessed by histopathology. Microcystis abundance was elevated in the surface layer of the western and central delta and reached a maximum of 32 9 10 9 cells l -1 at Old River in August. Its distribution across the estuary was correlated with a suite of phytoplankton and cyanobacteria species in the surface layer and 1 m depth including Aphanizomenon spp., Aulacoseira granulata, Bacillaria paradoxa, Rhodomonas spp., and Cryptomonas spp. Shifts in the phytoplankton community composition coincided with a decrease in the percentage of diatom and green algal carbon and increase in the percentage of cryptophyte carbon at 1 m depth. Maximum calanoid and cyclopoid cope- pod carbon coincided with elevated Microcystis abundance, but it was accompanied by a low clado- cera to calanoid copepod ratio. Total microcystins were present at all levels of the food web and the greater total microcystins concentration in striped bass than their prey suggested toxins accumulated at higher trophic levels. Histopathology of fish liver tissue suggested the health of two common fish in the estuary, striped bass (Morone saxatilis), and Missis- sippi silversides (Menidia audens), was impacted by tumor-promoting substances, particularly at stations where total microcystins concentration was elevated. This study suggests that even at low abundance, Microcystis may impact estuarine fishery production through toxic and food web impacts at multiple trophic levels.

Seafloor ecosystem functioning: the importance of organic matter priming

Organic matter (OM) remineralization may be considered a key function of the benthic compartment of marine ecosystems and in this study we investigated if the input of labile organic carbon alters mineralization of indigenous sediment OM (OM priming). Using 13C-enriched diatoms as labile tracer carbon, we examined shallow-water sediments (surface and subsurface layers) containing organic carbon of different reactivity under oxic versus anoxic conditions. The background OM decomposition rates of the sediment used ranged from 0.08 to 0.44 μmol C mlws−1 day−1. Algal OM additions induced enhanced levels of background remineralization (priming) up to 31% and these measured excess fluxes were similar to mineralization of the added highly degradable tracer algal carbon. This suggests that OM priming may be important in marine sediments.

Primary Production and Carbon Dioxide Metabolic Balance of a Lake-Rich Arctic River Floodplain: Partitioning of Phytoplankton, Epipelon, Macrophyte, and Epiphyton Production Among Lakes on the Mackenzie Delta

The Mackenzie River Delta, a floodplain system in the western Canadian Arctic contains approximately 45,000 lakes used by resident and migratory fish, aquatic birds, and mammal populations, which are critical resources for aboriginal peoples. Our results show the Mackenzie Delta is a remarkably productive aquatic ecosystem, not out of place relative to other large river floodplains and unusually productive for its Arctic latitude. Along with other such deltas of north flowing rivers to the Arctic Ocean, it represents a critical habitat of high productivity to support dependent consumers. Our results also showed a consistent under-saturation of CO2 in lake waters of the delta, co-occurring even with high concentrations of dissolved organic carbon, and indicating P:R greater than 1 during the open water period. In less turbid lakes, abundant macrophytes provide a considerable surface area for supporting production of epiphyton, but epiphyton production is constrained strongly by macrophyte shading, when macrophyte biomass is high. Despite this, epiphyton represented a greater supply of non-macrophyte algal carbon than phytoplankton, and thus explains why benthic algae may be a more important food source for primary consumers than phytoplankton, except in the most turbid systems. Most importantly, the high autotrophic production in the Delta lakes relative to nearby lakes on the arctic tundra suggests the extended ice-free season of the floodplain lakes and their landscape setting on, and replenishment by, nutrient-rich river sediments, is the strongest influence on aquatic production levels.

Oligochaetes as a possible entry route for terrigenous organic carbon into estuarine benthic food webs

Estuaries receive large quantities of terrestrially derived organic material, yet the cur- rent view is that such terrestrial carbon is unimportant for temperate estuarine benthic trophic dynamics beyond microbial processing. This consensus, however, may derive from a lack of consid- eration in estuarine food web studies of taxa with evolutionary affinities to freshwater systems where these taxa process terrestrial detritus. Here, we used a multiple stable isotope approach (carbon, nitrogen and sulphur) in 3 similar estuarine systems to test whether taxa with high (oligochaetes), medium (amphipods) and low (marine polychaetes and other crustacean taxa) evolutionary associa- tions with freshwater systems differed in their assimilation of carbon derived from contrasting detri- tal sources. Oligochaetes had isotopic signatures significantly different to those of other organisms, yet not significantly different from tree and ground plant signatures, demonstrating that they assim- ilate terrestrial carbon. In contrast, amphipods and marine taxa had isotope signatures that indicated a reliance on marine algal carbon, independent of where in the estuary they were sampled, suggest- ing that, unlike oligochaetes, these taxa do not have an inherent physiological ability to successfully assimilate terrestrial material. These findings indicate that terrestrial carbon can play a significant role in estuarine systems, with oligochaetes providing the metazoan entry route for this carbon source into food webs, and that evolutionary detritivore-detritus associations may influence present-day trophic dynamics within estuaries.

Ice algal assemblages and vertical export of organic matter from sea ice in the Barents Sea and Nansen Basin (Arctic Ocean)

Algal communities and export of organic matter from sea ice were studied in the offshore marginal ice zone (MIZ) of the northern Barents Sea and Nansen Basin of the Arctic Ocean north of Svalbard by means of ice cores and short-term deployed sediment traps. The observations cover a total of ten stations within the drifting pack ice, visited over a period of 3 years during the period of ice melt in May and July. Maximum flux of particulate organic carbon and chlorophyll a from the ice at 1 m depth (1,537 mg C m−2 per day and 20 mg Chl a m−2 per day) exceeded the flux at 30 m by a factor of 2 during spring, a pattern that was reversed later in the season. Although diatoms dominated the ice-associated algal biomass, flagellates at times revealed similarly high biomass and typically dominated the exported algal carbon. Importance of flagellates to the vertical flux increased as melting progressed, whereas diatoms made the highest contribution during the early melting stage. High export of ice-derived organic matter and phytoplankton took place simultaneously in the offshore MIZ, likely as a consequence of ice drift dynamics and the mosaic structure of ice-covered and open water characteristic of this region.

Organic matter input and processing in two contrasting North Sea sediments: insights from stable isotope and biomass data

Organic matter input and processing was studied in 2 contrasting sediments (Stn 115FINE and Stn 330COARSE) in the southern North Sea. The sediments are subjected to similar hydrodynamic conditions, but Stn 115FINE underlies a high turbidity zone, making it a fine, low-permeability sedi- ment. Monthly data on chlorophyll a (chl a), δ 13 C and δ 15 N of particulate organic matter in the water column and sediment showed that the algal spring bloom deposition created a strong vertical gradi- ent of sedimentary chl a at Stn 115FINE. Macrobenthic biomass (78 ± 60 g C m -2 , mean ± SD) was dom- inated by suspension feeders, suggesting biological mediation of the organic matter input. In con- trast, the offshore Stn 330COARSE is a coarse, high-permeability sediment in which chl a penetrated centimeters deep due to physically mediated input. The macrobenthic community, low in biomass (3.8 ± 2.4 g C m -2 ), was dominated by mobile polychaetes and epibenthic amphipods, which is char- acteristic of physically disturbed sediments. Overall, sediment characteristics played an important but indirect role in the organic matter input and processing. At Stn 115FINE, a large macrobenthic com- munity developed that mediated the input of organic matter to the sediment through herbivore and predatory pathways. At Stn 330COARSE, in contrast, organic matter input seemed to be dominated by physical processes. Overall, the fraction of algal carbon degraded in the sediment was higher at Stn 115FINE than at Stn 330COARSE, indicating that the physical input at Stn 330COARSE was less efficient than the biological input at Stn 115FINE.

Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 375:41-52 (2009) - DOI: https://doi.org/10.3354/meps07735 Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status Antonio Pusceddu1,*, Antonio Dell’Anno1, Mauro Fabiano2, Roberto Danovaro1 1Department of Marine Sciences, Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy 2Department for the Study of the Territory and its Resources, University of Genoa, Corso Europa, 26, 16100 Genova, Italy *Email: a.pusceddu@univpm.it ABSTRACT: Tools used for assessing marine trophic status are generally based on water column characteristics, which, however, may provide unreliable classification of the benthic trophic status. Here, we provide evidence from the literature that quantity and bioavailability of sediment organic matter are reliable proxies to assess benthic marine trophic status. We compiled data on the protein, carbohydrate and lipid concentration of sediments from different oceanic and coastal regions and varying water depths. The concentration of these 3 components as a whole (biopolymeric carbon) was found to be significantly correlated (r = 0.84) with the total organic carbon concentration, suggesting that the biopolymeric fraction is representative of the total organic carbon pool. However, the systematic variation of the biopolymeric fraction was higher than that of total organic carbon concentrations, suggesting that biopolymeric carbon is a more sensitive proxy of benthic trophic status than is the total carbon pool. Furthermore, biopolymeric carbon was significantly correlated to the amount of phytopigments, indicating that biopolymeric carbon accumulation in the sediment is related to inputs of algal carbon. Biopolymeric carbon concentrations were also positively correlated to the sediment community oxygen consumption, suggesting that the progressive accumulation of biopolymeric carbon could be an additional co-factor potentially responsible for hypoxic or anoxic events. The enzymatically digestible and algal fractions of biopolymeric carbon decreased in sediments with increasing biopolymeric carbon content (i.e. eutrophic systems), suggesting that organic carbon in eutrophic sediments is mostly refractory in nature. We propose that a biopolymeric carbon concentration in the sediment of >2.5 mg C g–1, being associated with a bioavailable fraction of <10%, can be considered as a threshold level at which benthic consumers may experience mostly refractory organic carbon. KEY WORDS: Marine sediments · Organic matter · Trophic status Full text in pdf format PreviousNextCite this article as: Pusceddu A, Dell’Anno A, Fabiano M, Danovaro R (2009) Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status. Mar Ecol Prog Ser 375:41-52. https://doi.org/10.3354/meps07735 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 375. Online publication date: January 26, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Nutritional Role of Two Algal Symbionts in the Temperate Sea AnemoneAnthopleura elegantissimaBrandt

The intertidal sea anemone Anthopleura elegantissima in the Pacific Northwest may host a single type of algal symbiont or two different algal symbionts simultaneously: zooxanthellae (Symbiodinium muscatinei) and zoochlorellae (green algae; Trebouxiophyceae, Chlorophyta). A seasonal comparison of zooxanthellate and zoochlorellate anemones showed stable symbiont population densities in summer and winter, with densities of zoochlorellae about 4 times those of zooxanthellae. Photosynthesis-irradiance curves of freshly isolated symbionts show that the productivity (P(max) cell) of freshly isolated zooxanthellae was about 2.5 times that of zoochlorellae during July; comparable rates were obtained in other months. Models of algal carbon flux show that zoochlorellae may supply the host with more photosynthetic carbon per unit anemone biomass than zooxanthellae supply. Zooxanthellate anemone tissue was 2 per thousand ((13)C) and 5 per thousand ((15)N) enriched and zoochlorellate anemone tissue was 6 per thousand ((13)C) and 8 per thousand ((15)N) enriched over their respective symbionts, suggesting that zoochlorellate anemones receive less nutrition from their symbionts than do zooxanthellate individuals. The disparity between predicted contributions from the algal carbon budgets and the stable isotopic composition suggests that short-term measures of algal contributions may not reflect actual nutritional inputs to the host. Isotopic data support the hypothesis of substantial reliance on external food sources. This additional nutrition may allow both algae to persist in this temperate intertidal anemone in spite of differences in seasonal photosynthetic carbon contributions.

EVIDENCE SUPPORTING THE IMPORTANCE OF TERRESTRIAL CARBON IN A LARGE-RIVER FOOD WEB

Algal carbon has been increasingly recognized as the primary carbon source supporting large-river food webs; however, many of the studies that support this contention have focused on lotic main channels during low-flow periods. The flow variability and habitat-heterogeneity characteristic of these systems has the potential to significantly influence food web structure and must be integrated into models of large-river webs. We used stable-isotope analysis and IsoSource software to model terrestrial and algal sources of organic carbon supporting consumer taxa in the main channel and oxbow lakes of the Brazos River, Texas, USA, during a period of frequent hydrologic connectivity between these habitat types. Standardized sampling was conducted monthly to collect production sources and consumer species used in isotopic analysis. Predictability of hydrologic connections between habitat types was based on the previous 30 years of flow data. IsoSource mixing models identified terrestrial C3 macrophytes (riparian origin) as the primary carbon source supporting virtually all consumers in the main channel and most consumers in oxbow lakes. Small-bodied consumers (<100 mm) in oxbow lakes assimilated large fractions of algal carbon whereas this pattern was not apparent in the main channel. Estimates of detritivore trophic positions based on delta15N values indicated that terrestrial material was likely assimilated via invertebrates rather than directly from detritus. High flows in the river channel influenced algal standing stock, and differences in the importance of terrestrial and algal production sources among consumers in channel vs. oxbow habitats were associated with patterns of flooding. The importance of terrestrial material contradicts the findings of recent studies of large-river food webs that have emphasized the importance of algal carbon and indicates that there can be significant spatial, temporal, and taxonomic variation in carbon sources supporting consumers in large rivers.

TRANSIENT DYNAMICS OF PELAGIC PRODUCER–GRAZER SYSTEMS IN A GRADIENT OF NUTRIENTS AND MIXING DEPTHS

Phytoplankton-grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton-grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon:nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light-nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the "paradox of energy enrichment" (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.

Quantitative significance of n-3 essential fatty acid contribution by heterotrophic protists in marine pelagic food webs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 354:85-95 (2008) - DOI: https://doi.org/10.3354/meps07215 Quantitative significance of n-3 essential fatty acid contribution by heterotrophic protists in marine pelagic food webs Fu-Lin E. Chu*, Eric D. Lund, Jennifer A. Podbesek Virginia Institute of Marine Science, College of William and Mary, PO Box 1346, Gloucester Point, Virginia 23062, USA *Email: chu@vims.edu ABSTRACT: To assess the contribution of n-3 essential lipids by heterotrophic protists in the pelagic food webs, we examined the kinetics and efficiency of long-chain n-3 essential fatty acid (LCn-3EFA) production of 2 common heterotrophic protists, Oxyrrhis marina and Gyrodinium dominans, fed an alga (Dunaliella tertiolecta) deficient in both eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). D. tertiolecta was rapidly ingested and consumed by the heterotrophic protists. Growth rates ranged from 0.77 to 0.82 and 0.78 to 0.92 d–1 in O. marina and G. dominans, respectively. LCn-3EFA production in the 2 heterotrophic protists reached the highest levels at the highest protist cell density in the feeding experiments, equivalent to a production of 1.9 ± 0.3 µg EPA and 22.3 ± 6.4 µg DHA by O. marina and 4.7 ± 0.5 µg EPA and 16.5 ± 1.1 µg DHA by G. dominans per mg algal carbon consumed. Both protists contained much higher levels of DHA, a nutrient critical for neural and visual development for marine organisms at higher trophic levels, compared to the good food quality alga Rhodomonas salina. This suggests that the LCn-3EFA contribution by heterotrophic protists to the pelagic food webs is quantitatively significant and may be crucial for the production and recruitment of species at higher trophic levels, particularly at times of blooms dominated by algal species deficient in LCn-3EFAs or when the primary producer standing stock is dominated by pico- and nanoplankton during non-bloom periods. KEY WORDS: Heterotrophic protists · Algae · Essential fatty acids · Nutrient upgrading · Docosahexaenoic acid · Eicosapentaenoic acid · Phytoplankton-heterotrophic protist interface Full text in pdf format PreviousNextCite this article as: Chu FLE, Lund ED, Podbesek JA (2008) Quantitative significance of n-3 essential fatty acid contribution by heterotrophic protists in marine pelagic food webs. Mar Ecol Prog Ser 354:85-95. https://doi.org/10.3354/meps07215 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 354. Online publication date: February 07, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Comparison of methods to determine algal δ13C in freshwater

To accurately assess the flux of mass and energy to higher trophic levels in a food web using stable isotopes, the isotopic signature of basal sources is required. When studying aquatic food webs, it is difficult to obtain a signature for algae because of challenges associated with isolating small organisms from a bulk sample. In this study, we compared freshwater algal δ13C values obtained using five approaches from the literature. Results indicated that the signatures derived from a primary consumer such as Daphnia sp., from particulate organic carbon with a correction for algal biomass, and from isolated algal samples were comparable. By contrast, algal δ13C values based on the signature of carbon dioxide and algal carbon fractionation were significantly lower than those of the other approaches. The inconsistent values produced by this method were likely due to problems in determining fractionation values based on current models and were potentially related to bicarbonate uptake by algae.

Influence of biochemical, mineral and morphological features of natural food on tropical cladocerans

Five laboratory experiments were performed to evaluate the effect of supplements of fatty acids and a green alga on the individual growth and reproduction of three species of tropical cladocerans Ceriodaphnia richardi, Daphnia ambigua, and D. gessneri feeding on natural seston from the Brazilian Lake Monte Alegre. Cohorts of newborns from cultivated females were submitted to one of the following treatments: (1) Natural seston, (2) Natural seston + microcapsules of EPA and DHA or linoleic and linolenic fatty acids, (3) Natural seston + oil-free microcapsules, and (4) Natural seston + green alga Scenedesmus spinosus (1 mg C l−1). Particulate organic carbon, algal carbon, C:P ratios of seston and green alga, polyunsaturated fatty acids (PUFA) content of seston and cladocerans, as well as phytoplankton composition, size, and shape were measured. The addition of fatty acids to seston did not significantly enhance growth and reproduction of the cladocerans, suggesting that sestonic PUFA content is sufficient for promoting cladoceran development, even in the cool–dry season when the fatty acids used in the experiments were 5–10 times lower in the seston than in the warm–wet season. Despite high C:P molar ratios in most experiments, there was only one indication of growth limitation by P. Reproduction was more affected than individual growth on some occasions by food quantity (energy) caused apparently by algal size, morphology, and digestion resistance. Energy availability, which is affected by algae morphological characteristics, seems to prevail over PUFA and P in controlling growth and reproduction of cladocerans in tropical Lake Monte Alegre.