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1. Gibson et al. 1990. Original data published as DMS concentrations, but due to the use of HgCl2, most probably an estimation of total DMS + DMSP (see also Curran et al. 1998)

2. Yang et al. (1994): data as for Gibson et al. (1990)

3. DiTullio and Smith 1995. Data from samples with 19'-hexanoyloxyfucoxanthin as the dominant pigment. Calculated from DMSPp-data plus 11% of the DMS-data; the latter was estimated to be the methodological bias due to diluting and not-filtering the samples

4. Crocker et al. 1995. Data as for Gibson et al. (1990), but only dissolved DMS + DMSP

5. Turner et al. 1995

6. Barnard et al. 1984. Mean of samples in which Phaeocystis makes up >25% of cell density

7. Matrai and Vernet 1997. Mean of samples in which Phaeocystis makes up 43% of phytoplankton-C

8. Data from April 1997 field campaign of the EU-funded ESCAPE project: Belviso et al. 2006 (DMS and DMSP data) and Stefels (POC and chlorophyll data). Ratios are from the Phaeocystis maximum on April 25

9. Malin et al. 1993

10. Holligan et al. 1987

11. Liss et al. 1994 and Turner et al. 1996

12. Unpublished data from April 1998 field campaign of the EU-funded ESCAPE-project: Belviso (DMS and DMSP data) and Stefels (POC and chlorophyll data). Ratio's are derived from regression coefficients of the respective parameters. Carbon represents phytoplankton carbon as derived from the regression of POC versus chlorophyll-a

13. Kwint and Kramer 1996

14. van Duyl et al. 1998

15. Turner et al. 1988. Samples with >20% coccolithophores; winter and summer 1985

16. Malin et al. 1993. June-July 1987. Samples with >50% of total carbon biomass as coccolithophores

17. Holligan et al. 1993. E. huxleyi bloom, June 1991

18. Jickells et al. unpublished data from ACSOE cruise in the NE Atlantic, June 1998 (eddy—Lagrangian), with E. huxleyi dominating

19. Archer et al. 2002. DISCO Lagrangian experiment, June 1999; E. huxleyi contributed 16% of the DMSPp standing stock in surface waters

20. Matrai and Keller 1993. Centre of an E. huxleyi bloom, July 1990; data range in top 10 m of one depth profile

* Estimated from published figures

# Surface concentrations along a transect at 20°W

higher plants (Trossat et al. 1996), but there is no conclusive evidence for this in marine algae, which use a different biochemical pathway for DMSP production (Gage et al. 1997; Summers et al. 1998). Another complicating factor is that with the common techniques for DMS(P) analysis it is impossible to measure the fluxes through this cascade of compounds. Sunda and co-workers suggested the anti-oxidant hypothesis on the basis of elevated concentrations of intracellular DMSP under stress conditions. In the process of radical scavenging, however, DMSP would be converted into one of its breakdown products. Therefore, a loss of DMSP would be expected, unless the stress reaction results in increased de novo synthesis (up-regulation) of DMSP. Only in those cases, a subsequent overshoot production may lead to increased intracellular concentrations of DMSP and/or one of the downstream products. A method for the measurement of de novo synthesis of DMSP is clearly warranted.

Since unbalanced growth and the production of ROS often co-occur under high irradiance and/or nutrient-limited conditions, it is difficult to test the two hypotheses individually without detailed investigation of the physiological condition of the cells and fluxes through the relevant biochemical pathways. Moreover, the two hypotheses do not necessarily need to be mutually exclusive, since a function in oxidative stress management does not exclude additional functions in cell metabolism. However, for a better understanding, one should be aware of the fundamental differences in operating principles and value the published data accordingly. Here, we present only an update of the current knowledge and quantify the relationships whenever possible.

Salinity

There is little doubt that an increase in salinity will result in an increase of the equilibrium concentration of intracellular DMSP, but an active up- or down regulation of its concentration upon short-term salinity changes (minutes to hours) has not been observed (reviewed by Stefels 2000). Indeed, phyto-plankton from high-salinity environments such as

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PAR (pmol photons m-2 sec-1)

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