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Niveau: Supérieur, Doctorat, Bac+8
1562 Notes Limnol. Oceanogr., 47(5), 2002, 1562–1567 q 2002, by the American Society of Limnology and Oceanography, Inc. Does competition for nanomolar phosphate supply explain the predominance of the cyanobacterium Synechococcus? Abstract—Experimental work during a cruise along a W–E transect in the Mediterranean Sea suggests that (1) orthophos- phate concentrations in the upper photic zone show a decreas- ing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6–2-mm size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (,5 nM) as well as at relatively high (5–25 nM) concentrations could explain both general Synechococcus abundance in P-depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur. Recent work has shown that dissolved mineral phosphate concentrations are well below the classical colorimetric de- tection limit of 30 nM in several oligotrophic oceanic prov- inces (Karl et al. 1997; Wu et al. 2000; Moutin and Raim- bault 2002). It has been proposed that primary production (Karl et al. 1997; Wu et al. 2000; San˜udo-Wilhelmy et al.

  • limiting marine

  • ch oc

  • po4 uptake

  • po4

  • mediterranean sea

  • carbon per cell

  • between light

  • phosphate

  • than


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1562
Limnol. Oceanogr.,47(5), 2002, 1562±1567 q2002, by the American Society of Limnology and Oceanography, Inc.
Notes
Does competition for nanomolar phosphate supply explain the predominance of the cyanobacteriumSynechococcus?
AbstractÐExperimental work during a cruise along a W±E transect in the Mediterranean Sea suggests that (1) orthophos-phate concentrations in the upper photic zone show a decreas-ing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6±2-mm size fraction, probablySynechococcus,have, in addition to high af®nity for orthophosphate, signi®cantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These speci®c advantages concerning orthophosphate uptake at low (,5 nM) as well as at relatively high (5±25 nM) concentrations could explain both generalSynechococcusabundance in P-depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur.
Recent work has shown that dissolved mineral phosphate concentrations are well below the classical colorimetric de-tection limit of 30 nM in several oligotrophic oceanic prov-inces (Karl et al. 1997; Wu et al. 2000; Moutin and Raim-bault 2002). It has been proposed that primary production (Karl et al. 1997; Wu et al. 2000; SanÄudo-Wilhelmy et al. 2001) as well as bacterial production (Thingstad and Ras-soulzadegan 1999; Van Wambeke et al. 2002) are controlled by the availability of phosphate. There is thus need for a better understanding of the photic P cycle in marine systems. Most primary production in oligotrophic environments is re-alized by picoplanktonic (,2mm) unicellular cyanobacteria. Synechococcusis virtually ubiquitous in all marine environ-ments (Partensky et al. 1999) and was found to be the most abundant part of the phytoplankton in surface waters of the Mediterranean Sea during summer (Vaulot et al. 1996). Tran-sient blooms ofSynechococcushave also been observed in the open ocean (Glover et al. 1988; Morel 1997). The causes of such cyanobacterial predominance remain to be elucidated (Morel 1997). The Mediterranean Sea presents an interesting gradient of nutrient distribution toward its eastern part (Krom et al. 1991; Moutin and Raimbault 2002) because of the exchange of Atlantic and Mediterranean water at the strait of Gibraltar. This particular feature led us to study phosphate availability in the upper surface water as well as phosphate uptake ki-netics from planktonic organisms living under oligotrophic and ultraoligotrophic conditions. This work was conducted during the PROSOPE (PROd-uctiviteÂdesSystÁemesOcÂeaniquesPElagiques)cruise(Fig. 1) in the Mediterranean Sea (September 1999). Samples were taken 10±15 m deep using 12-liter Niskin bottles to initiate measurements of (1) primary production (PP) and (2) bacterial production (BP) rates (seeMoutin and Raim-14 bault [2002] for detailed protocols of theC method and 3 Van Wambeke et al. [2002] for theH-leucine method) and (3) orthophosphate turnover time (days), which corresponds
to the ratio between concentration (nM) and uptake (nM 21 d ). ate (T) wa Turnover time for bioavailable orthophosphPO4s measured twice in 10-ml samples incubated with 18.5 kBq 33 (0.5mPOCi) carrier-free4(Amersham BF1003) in poly-carbonate vials using an on-deck incubator. Incubations (15± 30 min) were stopped by a 100-ml addition of 10 mM non-radioactive KH2PO4(cold chase). Filtrations were performed in less than 1 h on 0.2-mm (25-mm diameter) Poretics poly-carbonate ®lters. Radioactivity on ®lters (cpm) was mea-sured by scintillation liquid counting andTwas calculated PO 4 from the equation R)/R] TPO5 2t/ln[12(Rf2b t 4 whereRf,Rb, andRtare the radioactivity of the ®lter, the 21 blank (®xed with ca. 100mHgCll of 2 g L2), and the total tracer added to the sample, respectively. Turnover times measured every 3 h during 24 h at Sta. 7 indicated no sig-ni®cant differences between light and dark measurements. Isotope dilution curves were determined by adding 2.5, 5, 10, and 25 nM cold orthophosphate to additional subsamples and measuring uptake as above on 0.2-, 0.6-, and 2-mm poly-carbonate ®lters. Linear regression ofTversus added con-PO 4 centration of cold orthophosphate allowed the estimation for each size fraction (0.2±0.6mm, 0.6±2mm and.2mm) of the terms Ks1[PO4] andVmax(Thingstad et al. 1993), where Ksis the half saturation constant for uptake, [PO4] is the natural concentration of biologically available orthophos-phate, andVmaxis the maximum uptake rate. The mean co-ef®cient of determination was 0.964 (SD50.037,n521). In addition,Tat natural phosphate concentration was es-PO 4 timated from they-axis intercept (no orthophosphate addi-tion). Biologically available orthophosphate [PO4] was estimat-ed fromTO4(d). Total PO4uptake was derived from car Pbon 2121 PP (nM d) and carbon BP (nM d) taking a C: Pratio of 106 and 50, respectively. 21 V(nM d)5PP/1061BP/50 [PO4] (nM) is then determined from the turnover time. [PO4]5TPO[PP/1061BP/50] 4 Chlorophylla(Chla) and particulate phosphate were de-termined by serial ®ltration of 1-liter samples through 47-mm Poretics polycarbonate ®lters (0.2, 0.6, 2mm) and using Sartorius polyester separators. Chlaconcentration was de-termined with a Turner Design 10-AU-005-CE ¯uorometer with optical con®gurations optimized to produce maximum sensitivity for Chla(Welschmeyer 1994) and using metha-nol extraction. Particulate phosphate was determined using a persulfate wet-oxidation method (Pujo-Pay and Raimbault 1994). Polyester separators were cleaned by preliminary wet-oxidation followed by a rinse with milliQ water.