Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium(NH₄⁺) were studied along a salinity gradient (salinities 0.2–34.4)in the Mississippi River plume during an extreme drought inspring 2000. Chlorophyll concentrations up to 30 µg L^–1were highest in the low- and mid-salinity regions (salinities8.5–28.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.1–28.3 µg C L^–1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.4–4.0µg C L^–1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L^–1at salinity 8.5. Microbial NH₄⁺ regeneration rates were estimatedby ¹⁵NH₄⁺ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH₄⁺ regenerationrates (0.018–0.124 µmol N L^–1 h^–1) werelow relative to previous reports and peaked at salinity 28.Total NH₄⁺ regeneration rates were higher than those in the<2 µm size fraction at only four stations, suggestingthat bacterial mineralization was a significant component ofNH₄⁺ recycling in some parts of the river plume. Higher NH₄⁺regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations.     

Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

The abundance and biomass of the large heterotrophic dinoflagellateNoctiluca scintillans, together with the changes in its potentialprey items, were monitored in the Seto Inland Sea, Japan, duringsummer 1997 (17 July-11 August). Growth and grazing rates ofNscintillans fed natural plankton populations were also measuredeight and seven times, respectively, during the survey period.The abundance and biomass of N scintillans averaged over thewater column (19 m) were in the range 1–345 cells 1^–1(temporalaverage = 93 cell1^–1) and 0.1–49.6 µg C l^–1(temporalaverage = 13.8 µg C l^–1; three times higher thanthat of calanoid copepods during the same period). Noctilucascintillans populations followed the changes in phytoplankton:N.scintillans biomass was increasing during the period of diatomblooms and was at a plateau or decreasing during periods oflow chlorophyll a. The growth rates of N.scintillans (µ)were also consistent with the wax and wane of the N.scintillanspopulation: N.scintillans showed highest growth rates duringdiatom blooms. A simple relationship between µ and chlorophylla concentration was established, and the production of N.scintillanswas estimated using this relationship and the measured biomass.The estimated production averaged over the water column wasin the range >0.1–5.2 µg C l^–1 day^–1(temporalaverage = 1.4 µg C l^–1 day^–1; 64% of the productionof calanoid copepods during the same period). Diatom clearancerates by N.scintillans were in the range 0.10–0.35 mlcell^–1 day^–1, and the phytoplankton population clearanceby N.scintillans was >12% day^–1. Thus, although thefeeding pressure of N.scintillans on phytoplankton standingstock was low, N.scintillans was an important member of themesozooplank-ton in terms of biomass and production in the SetoInland Sea during summer.     

Microplanktonic respiration off northern Chile during El Nino 1997-1998

Microplanktonic respiration rates were estimated in waters offthe coast of northern Chile (Antofagasta, 23°S) during ElNiño and pre-El Niño conditions. Three cruiseswere conducted during pre-El Niño summer (January/February1997), El Niño winter (July 1997) and El Niñosummer (January 1998). Oxygen consumption was estimated by theWinkler method using a semi-automatic photometric end-pointdetector. The ranges of microplanktonic respiration rates foundwere 0.11–21.15, 0.03–6.25 and 0.06–9.01 µmolO₂ l^–1 day^–1 during pre-El Niño summer, ElNiño winter and El Niño summer, respectively.Significant differences were found between winter and summerrespiration rates (non-integrated and integrated). The meanintegrated respiration (mixed layer) for pre-El Niñosummer, El Niño winter and El Niño summer was95 ± 51 (SD) mmol O₂ m^–2 day^–1, 50 ±23 (SD) mmol O₂ m^–2 day^–1 and 63 ± 32 (SD)mmol O₂ m^–2 day^–1, respectively. The strong seasonalsignal detected in microplanktonic integrated respiration inthe area seems to be characteristic of the pre-El Niño/ElNiño 1997–98 period. The integrated respirationrates found off Antofagasta are similar to reported values forthe upwelling area off Peru despite methodological differences.A positive significant correlation was found between respirationand water temperature (r = 0.76, P     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

The contribution of ammonia excreted by zooplankton to phytoplankton production in Narragansett Bay

Ammonia excreted by mixed zooplankton populations over an annual(1972–1973) cycle in Narragansett Bay varied from 0.04to 3.21 µg at NH₃-N dry wt^–1 day^–1, exclusiveof two exceptional rates measured one year apart: 11.74 and18.39 µg at NH³-N mg dry wt^–1 day^–1. Grossphytoplankton production integrated over the year (1972–1973)averaged 151 mg C m^–3 day^–1 for an 8 m water column;peaks of 332 and 905 mg C m^–3 day^–1 occurred duringthe winter-spring and summer blooms, respectively. Excretedammonia, integrated seasonally and annually, contributed only0.2% and 4.9% of the nitrogen required for observed gross productionduring the winter-spring and summer blooms, respectively, and4.4% annually. However, excreted ammonia may be an importantsource of the nitrogen required by Skeletonema costatum, thedominant diatom in Narragansett Bay, during the post-bloom periodwhen 186% of the nitrogen required for its net production wasmet by ammonia excretion. A combination of zooplankton ammoniaexcretion and benthic ammonia flux contributed 22% of the nitrogenrequired for the annual gross production (440 g C m^–2)while 51% of the nitrogen required for the net production ofSkeletonema was accounted for by regenerated nitrogen. ¹This research was supported by NSF grant GA 31319X awardedto Dr.T.J.Smayda.     

Flexible Coupling of Phosphate Uptake in Dunaliella acidophila at Extremely Low pH Values

The acidophilic alga Dunaliella acidophila exhibits optimalgrowth at pH 1. We have investigated the regulation of phosphateuptake by this alga using tracer techniques and by performingintracellular phosphate measurements under different growthconditions including phosphate limitation. In batch culturewith 2·2 mol m^–3 phosphate in the medium the uptakeof phosphate at micromolar phosphate concentrations followeda linear time dependence in the range of minutes and rates werein the range of 1 µmol phosphate mg^–1 chl h^–1,only. However, under discontinuous phosphate-limited growthconditions, tracer influx revealed a biphasic pattern at micromolarphosphate concentrations: An initial burst phase resulted ina 10⁴-fold internal phosphate accumulation and levelled offafter about 10 s. A double reciprocal plot of the initial influxrates obtained for phosphate-limited and unlimited algae exhibitedMichaelis-Menten kinetics. Phosphate limitation caused a significantactivation of the maximum velocity of uptake, yielding V_maxup to 1 mmol mg^–1 chl h^–1 as compared to valuesin the order of 50 µmol phosphate mg^–1 chl h^–1for the second phase (this magnitude is also representativefor non-limited batch cultures). Concomitantly the Michaelisconstant was altered from 4 mmol m^–3 to 0·7 mmolm^–3. The rapid uptake of phosphate was inhibited by arsenateand FCCP and was not stimulated by Na⁺. The pH dependence oftracer accumulation and measurements of the intracellular phosphatepool under different growth conditions indicate that at lowpH and low external phosphate concentrations the high protongradient present under these conditions is utilized for a H₃PO₄uptake or a H⁺/H₂PO^–4 cotransport. However, when the externalphosphate concentration was increased to levels sufficientlyhigh for transport to be driven by the positive membrane potential(10 mol m^–3 phosphate), the pH dependence of phosphateuptake was more complex, but could be explained by the uptakeof H₃PO₄ or a H⁺/H₂PO^–₄-cotransport at low pH and a differenttype H₂PO^–4-transport (with unknown type of ion coupling)at high pH-values. It is suggested that this flexible couplingof phosphate transport is of essential importance for the acidresistance of Dunaliella acidophila. Key words: Acid resistance, Dunaliella acidophila, phosphate cotransport, phosphate limitation, plasma membrane, sodium     

Ammonium excretion and glutamate dehydrogenase activity of zooplankton in Great South Bay, New York

In Great South Bay, nanoplankton, (<20 sµm) accountedfor the largest fraction (56%) of zooplankton glutamate dehydrogenase(GDH) activity over a one year period. Microzooplankton (20–200µm) and macrozooplankton (>200 µm) accountedfor 20% and 24%, respectively. Total zooplankton ammonium regenerationin Great South Bay could account for 74% of the ammonium requirementby phytoplankton in winter, but in summer when phytoplanktondemand was greater, and zooplankton population was low, it suppliedless than 5%. This study suggests that the smallest zooplanktonfraction, less than 20 µm, can be the most important asregards nitrogen regeneration in estuarine environments. MacrozooplanktonGDH activity in Great South Bay ranged from 0.18 mg atoms NH⁺₄-Nm^–3 d^–1 in winter to 3.34 mg atoms NH⁺₄-N m^–3d^–1 in spring. Over an annual period, the averaged GDH/excretionratio was 20.4 3.5 (n = 10), and this ratio agrees well withobservations by other investigators. Observed macrozooplanktonexcretion rates showed a strong correlation with the excretionrates indirectly estimated from GDH activities. The GDH/excretionratio seems to vary depending on the internal physiologicalstates of zooplankton as well as food availability.     

Nitrogen dynamics in lower Narragansett Bay, Rhode Island. I. Uptake by size-fractionated phytoplankton populations

The contribution of nanoplankton (< 10 µm fraction)to winter – spring (1977 – 78) and summer (1978,1979) phytoplankton nitrogen dynamics in lower NarragansettBay was estimated from ammonium, nitrate and urea uptake ratesmeasured by ¹⁵N tracer methods. During the winter – spring,an average of 80% of chlorophyll a and nitrogen uptake was associatedwith phytoplankton retained by a 10 µm screen. In contrast,means of 51 – 58% of the summer chlorophyll a standingcrops and 64 – 70% of nitrogen uptake were associatedwith cells passing a 10 µm screen. Specific uptake ratesof winter – spring nanoplankton populations were consistentlylower than those of the total population. Specific uptake ratesof fractionated and unfractionated summer populations were notsignificantly different. Ammonium uptake averaged between 50and 67% of the total nitrogen uptake for both the total populationand the < 10µm fraction. The total population and the10 µm fraction displayed similar preferences for individualnitrogen species. Though composed of smaller cells, flagellatedominated nanoplankton assemblages may not necessarily takeup nitrogen at faster rates than diatom dominated assemblagesof larger phytoplankters in natural populations. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

The effects of Cu on the adenylate energy charge of open ocean phytoplankton

The effects of short-term, acute Cu exposure (6 h) on the adenylateenergy charge (EC_A) of open-ocean phytoplankton populations(northeastern equatorial Pacific) were investigated. Energycharge remained at {small tilde}0.77 over the range of Cu additions(0.025 – 5.µg l^–1), even though ¹⁴C uptakeand total adenylate levels (ATP + ADP + AMP) were reduced byas much as 60%. These findings suggest that EC_A alone is nota sensitive indicator of acute sublethal metal effects on phytoplankton. ¹This research was supported by the NSF Biological OceanographyProgram grant #OCE 81-17286.     

Growth and development of Metridia pacifica (Copepoda: Calanoida) in the northern Gulf of Alaska

Juvenile growth and development rates for Metridia pacifica,one of the dominant larger copepods in the subarctic Pacific,were investigated from March through October of 2001–2004in the northern Gulf of Alaska. The relationship between prosomelength (PL, µm) and dry weight (DW, µg) was determined:log₁₀ DW = 3.29 x log₁₀ PL – 8.75. The stage durationsof copepodites ranged from 3 to 52.5 days but were 8–15days under optimal condition. Seasonally, growth rates increasedfrom March to October and typically ranged between 0.004 and0.285 day^–1, averaging 0.114 ± 0.007 day^–1(mean ± SE). After standardization to 5°C (Q₁₀ of2.7), growth rates averaged 0.083 ± 0.005 day^–1and were significantly correlated to chlorophyll a, with saturatedgrowth rates of 0.149 day^–1 for C1–C3, 0.102 day^–1for C4–C5 and 0.136 day^–1 for all stages combined.Measured juvenile growth rates were comparable with specificegg production rates in this species. The comparisons of ourrates in this study with those predicted by the global modelsof copepod growth rates suggested that further refinement ofthese models is required.     

Carbon Fixation, Ammonium Uptake and Regeneration in an Equatorial Lake: Biological Versus Physical Control

Ammonium uptake and regeneration were measured in the euphoticzone of Petit Saut Lake, French Guyana, to examine nitrogencycling in this recently flooded equatorial forest environment.Am-monium regeneration rates were extremely high (mostly inthe range 1–6 µmol N l^–1 h^–1), and aredue to the very high grazing rates of the microzooplankton,which consumed between 56 and 95% of the phytoplankton productionin any given incubation. These regeneration rates were aboutan order of magnitude higher than the net ammonium uptake rates.This imbalance is probably due to dissolved organic nitrogenrelease during grazing. At the bottom of the euphotic zone (4–5m), photosynthetic bacteria are responsible for ammonium uptake.Diffusion-driven ammonium fluxes are an order of magnitude lowerthan biologically driven fluxes. Therefore, ammonium fluxesare dominated by biology rather than by physics in this lake.     

Microplankton and primary production in the Sea of Okhotsk in summer 1994

Phytoplankton composition, density, vertical distribution andprimary production were investigated in the Sea of Okhotsk andin the adjacent northern north Pacific in July–August1994, together with measurements of density and distributionof planktonic microheterotrophs: bacteria, nanoheterotrophsand ciliates. Different phases of phytoplankton seasonal successionwere encountered during the period of investigation in variousregions of this sea. Primary production measured at 144 stationswas found to be greatest (1.5–4 g C m^-2day^-1) in areasof spring-phase succession along the Sakhalin shelf and theKashevarov bank. Periodic relapses of the spring blooms of ‘heavy’diatoms during the whole growth season were recorded over thisbank. The summer phase of the phytoplankton minimum prevailedin the central and eastern parts of the sea, manifested by thedominance of nanoflagellates in terms of phytoplankton biomass.Primary production was 0.5–1 g C m^-2 day^-1. The earlyautumn phase of succession was typical of the Kurile straitarea and the adjacent north Pacific. Primary production therevaried from 0.7 to 2 g C m^-2 day^-1. The integrated phytoplanktonbiomass in the water column varied from 9–12 g m^-2 inzones supporting the summer minimum assemblage to 15–20g m^-2 in zones of early autumn recovery of phytoplankton growth,and up to 40–70 g m^-2 in areas of remnant or relapseddiatom blooms. The numerical density of bacterioplankton wasbetween 1 x 10⁶ and 3 x 10⁶ cells ml^-1 and its wet biomass wasbetween 100 and 370 mg m^-3. In deep waters it was 8–15mg m^-3. The integrated bacterioplankton biomass in the upperwater column varied from 6 to 29 g m^-2. The numerical densityof zooflagellates varied in the upper layer between 0.8 x 10⁶and 4 x 10⁶ l^-1 and their biomass was between 20 and 50 mg m^-3.In deep waters they were still present at a density of 0.05x 10⁶ to 0.2 x 10⁶ cells l^-1. The biomass of planktonic ciliatesvaried between stations from 20 to 100 mg m^-3. The joint biomassof planktonic protozoa in the water column was between 3 and12 g m^-3 at most of the stations.     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Comparative phosphorus nutrition of the marine cyanobacterium Synechococcus WH7803 and the marine diatom Thalassiosira weissflogii

Phosphate uptake kinetics of Synechococcus sp. WH7803 and Thalassiosiraweissflogii were studied in axenic batch culture. Phosphate-repleteSynechococcus sp. WH7803 cells have a lower affinity for inorganicphosphate (Pi) (K_s = 67 µmol l^–1) than Pi-starvedcells (K_s = 3.1 µmol l^–1). The K_s of Pi-starvedcells increased     

Separate entry pathways for phosphate and oxalate in rat brain microsomes

ATP-dependent ⁴⁵Ca uptake in rat brainmicrosomes was measured in intracellular-like media containingdifferent concentrations of PO₄ and oxalate. In the absenceof divalent anions, there was a transient ⁴⁵Caaccumulation, lasting only a few minutes. Addition of PO₄did not change the initial accumulation but added a second stage that increased with PO₄ concentration. Accumulation during thesecond stage was inhibited by the following anion transport inhibitors: niflumic acid (50 µM),4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS; 250 µM),and DIDS (3-5 µM); accumulation during the initial stage wasunaffected. Higher concentrations of DIDS (100 µM), however,inhibited the initial stage as well. Uptake was unaffected by 20 mM Na,an activator, or 1 mM arsenate, an inhibitor of Na-PO₄ cotransport. An oxalate-supported ⁴⁵Ca uptake was larger,less sensitive to DIDS, and enhanced by the catalytic subunit ofprotein kinase A (40 U/ml). Combinations of PO₄ and oxalatehad activating and inhibitory effects that could be explained byPO₄ inhibition of an oxalate-dependent pathway, but notvice versa. These results support the existence of separate transportpathways for oxalate and PO₄ in brain endoplasmic reticulum.

     

Fatty acid composition in photosynthetic products of natural phytoplankton population in Lake Biwa, Japan

Fatty acid composition of phytoplankton photosynthetic productswas determined by a ¹³C tracer and gas chromatography-mass spectrometry(¹³C-GC-MS) method from August 1985 to June 1986 in Lake Biwa,Japan. The total fatty acid production rate varied from 2.8to 10.9 µg C l^–1 day^–1 at the water surfaceand accounted for 9.1–30% of photosynthetic productionof particulate organic carbon. A high contribution of fattyacid to the particulate organic carbon production rate was noticedduring winter time, and an increase in the fatty acid contributionresulted in an increase in the C/N value in the photosyntheticproducts. The fatty acid composition varied throughout the year,mainly depending on the seasonal change in the dominant phytoplanktonspecies. The contribution of polyunsaturated fatty acids tototal fatty acids was low during the summer period, probablydue to nitrogen limitation of phytoplankton growth.     

Possible physiological mechanisms for production of hydrogen peroxide by the ichthyotoxic flagellate Heterosigma akashiwo

Blooms of the toxic red tide phytoplankton Heterosigma akashiwo(Raphidophyceae) are responsible for substantial losses withinthe aquaculture industry. The toxicological mechanisms of H.akashiwoblooms are complex and to date, heavily debated. One putativetype of ichthyotoxin includes the production of reactive oxygenspecies (ROS) that could alter gill structure and function,resulting in asphyxiation. In this study, we investigated thepotential of H.akashiwo to produce extracellular hydrogen peroxide,and have investigated which cellular processes are responsiblefor this production. Within all experiments, H.akashiwo producedsubstantial amounts of hydrogen peroxide (up to 7.6 pmol min^–110⁴ cells^–1), resulting in extracellular concentrationsof ~0.5 µmol l^–1 H₂O₂. Measured rates of hydrogenperoxide production were directly proportional to cell density,but at higher cell densities, accuracy of H₂O₂ detection wasreduced. Whereas light intensity did not alter H₂O₂ production,rates of production were stimulated when temperature was elevated.Hydrogen peroxide production was not only dependent on growthphase, but also was regulated by the availability of iron inthe medium. Reduction of total iron to 1 nmol l^–1 enhancedthe production of H₂O₂ relative to iron replete conditions (10µmol l^–1 iron). From this, we collectively concludethat production of extracellular H₂O₂ by H.akashiwo occurs througha metabolic pathway that is not directly linked to photosynthesis.