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• W1974920674 abstract "[1] In the work of Vallina et al. [2007] we showed that the seasonality of the solar radiation dose received in the upper mixed layer (or SRD) was highly positively correlated with dimethylsulfide (DMS) concentration seasonality over most of the global ocean. The underlying mechanism was suggested to be an increase in phytoplankton (direct) DMS production through algal cleavage of dimethylsulfoniopropionate (DMSP) in response to ultraviolet radiation (UVR) driven cell stress [Sunda et al., 2002]. In this regard, Larsen [2008] gives an alternative explanation for the observed coupling between DMS and the SRD. Briefly, he suggests that the driving forces behind DMS production are in fact sea surface temperature (SST) and PAR, mainly through their effect on phytoplankton growth rate. [2] Although in a theoretical frame the arguments exposed by Larsen [2008] are elegant, some of his assumptions are currently known to be inaccurate. Besides a reduction of growth rates, a temperature decrease has been shown to be associated with an increase in the amount of DMSP per cell, and DMSP per cell volume, in batch culture experiments with Emiliania huxleyi [van Rijssel and Gieskes, 2002]. Therefore, in terms of DMSP production, the effect of temperature on growth rate is canceled out by its effect on intracellular DMSP. [3] Regarding light effects on DMSP, during the same experiments no effect of light on intracellular DMSP content was found [van Rijssel and Gieskes, 2002]. However, when studying the effect of low and high irradiance without the temperature effect (by using a light-limited turbidostat culture at 15°C in steady state) there was an increase of ≈75% in DMSP per cell and of ≈20% in DMSP per cell volume. Regarding the response of DMSP to UVR, there are contradictory results in the literature. While van Rijssel and Buma [2002] reported no effects of UVR stress on DMSP synthesis in Emiliania huxleyi, Sunda et al. [2002] found an increase in the DMSP per cell volume by 5–100% for the same species. Slezak and Herndl [2003] also found small (10–25%) although systematic increases in the amount of DMSP per cell in Emiliania huxleyi in UVR treatments as compared to PAR-only treatments. [4] For DMS, the effect of UVR has been shown to be much more dramatic, with reported increases in the DMS per cell volume of 140–3500% for Emiliania huxleyi [Sunda et al., 2002]. In the field, DMS has been observed to vary seasonally with UVR or SRD by up to 1000–2000% (from ≈0.5 to ≈5–10 μmolS m−3) [Toole and Siegel, 2004; Vallina and Simó, 2007]. Globally, the correlation between monthly zonally averaged SRD and DMS was ρ = 0.56. On the other hand, the correlation between monthly zonally averaged SST and DMS was much weaker (ρ = 0.16) [Vallina and Simó, 2007]. In this regard, recently it has been demonstrated that Phaeocystis globosa, a high-DMSP producer able to convert it enzymatically into DMS plus acrylate [Stefels and Dijkhuizen, 1996], exudes huge amounts of polycarbonate polymers by active exocytosis under short-wave visible radiation (i.e., blue light, 450–490 nm) [Chin et al., 2004]. Although no attempts have been made to determine if DMS is present in these actively exuded polymers, it has been previously reported that Phaeocystis colony mucus contains sulfated polymers [van Boekel, 1992] and very high concentrations of acrylate (one of the products of DMSP degradation) [Noordkamp et al., 1998]. Finally, the work of Hefu and Kirst [1997] with cultures of Phaeocystis antarctica under different light levels shows a higher-conversion DMSP to DMS under most UVR+PAR treatments compared to the PAR-alone treatment. Therefore the highest final DMS concentrations are found for the 335 nm + PAR treatment, almost double than for the PAR treatment. The ratio DMS/DMSP is also highest for the 335 nm + PAR treatment (about 4 times the one obtained for the PAR-alone treatment). [5] Regarding the work of Toole and Siegel [2004], their main point is not the positive correlation UVR versus DMS “net biological production” (DMSb) but the correlation (ρ = 0.88) between UVR and DMS “concentrations”. This correlation is free of the autocorrelation artefact arising when correlating UVR and DMSb, and therefore it supports a mechanistic link between UVR and DMS. [6] As Larsen [2008] also points out, phytoplankton specific growth rate (d−1) is not simply driven by PAR and SST. In many regions of the global (low to middle latitudes) ocean, phytoplankton growth is mainly limited by nutrients. This is specially true during summertime, when DMS shows its annual maximum [Vallina et al., 2007]. As an exercise, phytoplankton specific growth rate (μP, units of d−1) can be estimated using real data from Sargasso Sea (≈32°N) of primary production (PP; in units of mmolN m−3 d−1) and phytoplankton biomass (PHYTO; in units of mmolN m−3) by doing μP = . In situ PP data (mgC m−3 d−1; see Figure 1a) were converted into mmolN m−3 d−1 assuming a Redfield ratio of 6.625 mmolC mmolN−1 (so, the conversion factor is 1/(12*6.625) = 0.0126 mmolN per mgC). There is no available data of phytoplankton biomass, so that the only way to estimate PHYTO is by using chlorophyll-a (CHL; in mg m−3, see Figure 1b) measurements and applying a CHL/C ratio (and then a C/N Redfield ratio). CHL is known for being a very poor direct estimate of phytoplankton biomass because of the very high variability of CHL/C ratios [Goericke and Welschmeyer, 1998], which are driven mainly by light intensity. A variable CHL/C (see Figure 1c) ratio can be calculated as a function of light levels [Lefèvre et al., 2002] on the basis of seasonal depth-resolved measurements of the CHL/C ratio [Goericke and Welschmeyer, 1998]. [7] From there, PHYTO is estimated (see Figure 1d) and μP is calculated (see Figure 1f). We can observe how μP displays a summer minimum due to nutrient depletion, even though SST (as well as PAR) displays its annual maximum during this time (Figure 1e). These results suggest that SST is affecting phytoplankton growth (and therefore DMSP or DMS production) in a secondary way. In other words, neither DMSP nor DMS net production seem to be driven by SST, at least not through the mechanism suggested by Larsen [2008] (its effect on phytoplankton growth). [8] Another problem in Larsen's [2008] argumentation is that he refers to “DMSP/DMS net production” as if they where always fully coupled. For example, when he states that as SST affects the phytoplankton growth rate it will also affect the DMSP/DMS net production, he implicitly is assuming that an increase of net DMSP production has to be followed by a concomitant increase of net DMS production. However, it is known that net DMSP production and net DMS production are not always coupled (e.g., oligotrophic regions like the Sargasso Sea). If they were, the ratio DMS/DMSP should be constant throughout the year; however, the ratio DMS/DMSP in the Sargasso Sea shows a clear increase in late summer (see Figure 2c). [10] The results are shown in Figures 2e and 2f. We observe that net DMSP production (Figure 2e) peaks in May displaying a similar seasonality than DMSP concentrations (Figure 2a), and that it is mainly driven by the phytoplankton internal S:N quota (Qint; see Figure 2d). Similarly, net DMS production (Figure 2f) displays the same seasonality as DMS concentrations (Figure 2b). There is a lag of about 2 months between the peak in net DMSP production and the peak in net DMS production. [11] Regarding the effect of SST on ocean stratification, Larsen [2008] says that the reduction of MLD and nutrient depletion favors the development of DMSP-rich species like the coccolithophorids. Again, although this may happen in some regions, it should not be taken as a dogma. In the Sargasso Sea the annual maximum of coccolithophorids is not in summer but in spring [Haidar and Thierstein, 2001]. Also, coccolithophorid abundance does not correlate with SST or PAR in this region [see Haidar and Thierstein, 2001, Table 3]. Therefore, changes in the phytoplankton S:N internal quota (either due to changes in phytoplankton physiological state or species composition, or both) are not enough to explain the observed summer increase of DMS [Toole and Siegel, 2004; Vallina et al., 2008] (see also Figure 2). The summer increase in the DMS/DMSP ratio (Figure 2c) suggests that another mechanism (like phytoplankton direct exudation of DMS) may occur. [12] The effect of SST on DMS production by bacteria is highly uncertain, as Larsen [2008] also points out. The metabolism of DMSP and DMS have opposite effects on DMS concentrations: while bacterial uptake of DMSP is a “source” of DMS, bacterial uptake of DMS is a “sink” of DMS. Since SST would affect both pathways, the “net” effect of SST on DMS concentrations through bacterial metabolism is unknown but probably small. [13] Similar problems exist with the effect of UVR on bacterial DMS production since UVR reduces the overall bacterial metabolism [Herndl et al., 1993]. Larsen [2008] cites the work of Slezak et al. [2007] to say that “bacterial net” DMS production rates decreased between 43% and 64% under the UVR + PAR treatment. One of Slezak et al.'s [2007] main conclusions is that of the “total net” DMS production only 20% to 75% came from bacterial conversion of DMSP. So a substantial fraction of DMS comes from non-DMSP sources [Slezak et al., 2007]. UVR will also decrease bacterial uptake of DMS [Slezak et al., 2001], what could cause DMS to accumulate. Again, the net effect still is uncertain but is likely to be close to zero [Toole and Siegel, 2004; Vallina et al., 2008]. [14] As explained before, the summer peak in DMS production does not seem to be related to phytoplankton growth (PAR and SST effect) as proposed by Larsen [2008], and it can hardly result from bacterial metabolism alone [Slezak et al., 2007]. The data available do not support the theoretical “tug of war” between PAR/SST and UVR suggested by Larsen [2008] to explain the relationship found between SRD and DMS in the global ocean [Vallina et al., 2007; Vallina and Simó, 2007]. Further, current mechanistic models of DMSP/DMS dynamics (e.g., PlankTOM5.1, PISCES, HaMOCC, NODEM, DMOS, etc.) are unable to properly simulate (e.g., reproduce the DMS summer paradox) just on the basis of PAR and SST effects on phytoplankton growth alone [Vallina et al., 2008; A. Vézina et al., A first appraisal of ocean DMS models and prospects for their use in climate models (SOLAS-CODIM), submitted to Geophysical Research Letters, 2008]. Only when forced by solar radiation does the model successfully reproduce DMS seasonality [Lefèvre et al., 2002; Le Clainche et al., 2004; Vallina et al., 2008; M. Vogt et al., Simulating the seasonal cycle of dimethylsulfide with the global multiplankton functional types biogeochemistry model PlankTOM5, manuscript in preparation for Deep Sea Research]. Since a significant fraction of DMS seems to come from non-DMSP sources [Slezak et al., 2007], the most likely explanation is that phytoplankton cells are under some kind of physiological stress that cause them to produce and exude high amounts of DMS [Toole et al., 2008]. From the existing literature [Hefu and Kirst, 1997; Sunda et al., 2002; Slezak and Herndl, 2003; Chin et al., 2004; Toole and Siegel, 2004; Toole et al., 2006], UVR is the most likely candidate. [15] My special thanks to R. Simó for his useful comments and suggestions during the preparation of this manuscript." @default.
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• W1974920674 date "2008-07-16" @default.
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• W1974920674 title "Reply to comment by S. H. Larsen on “Analysis of a potential “solar radiation dose-dimethylsulfide-cloud condensation nuclei” link from globally mapped seasonal correlations”" @default.
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• W1974920674 doi "https://doi.org/10.1029/2007gb003099" @default.
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