We investigated the link between iron (Fe) limitation and intracellular dimethylsulfoniopropionate (DMSP) concentration in two oceanic phytoplankton species, the diatom Thalassiosira oceanica and the diazotrophic cyanobacterium Trichodesmium erythraeum. Dimethylsulfoxide (DMSO) concentrations were also measured in Fe-replete and Fe-limited T. oceanica. Fe limitation decreased the growth rates of T. oceanica and T. erythraeum by 33-fold and 3.5-fold, respectively and increased intracellular DMSP (DMSP p support for the role of these sulfur compounds as antioxidants. Under severe Fe limitation, the large increase in DMSP p : C and DMSP : chlorophyll a (Chl a) ratios for both T. oceanica (by 16-and 40-fold, respectively) and T. erythraeum (by 18-and 145-fold, respectively) places these species above the range of values generally attributed to diatoms and cyanophytes. Comparison of these values with in situ results, such as those from Fe fertilization experiments, suggests that the decrease in DMSP p : Chl a and DMSO p : Chl a that is generally observed with alleviation of Fe limitation may be partly related to decreases in DMSP p and DMSO p in individual species. The role of diatoms and diazotrophic cyanobacteria in the biogeochemical cycle of dimethylsulfide and associated sulfur compounds in Fe-limited oceanic environments should not be overlooked.The CLAW hypothesis, named after the initials of the authors (Charlson, Lovelock, Andreae, and Warren), postulates that atmospheric oxidation products of dimethylsulfide (DMS), a volatile sulfur compound produced by phytoplankton, are cloud condensation nuclei (CCN) that can increase cloud droplet formation and the planetary albedo (Charlson et al. 1987). It also proposes that increased albedo and resulting lower temperature may lower phytoplankton growth and decrease DMS production (Charlson et al. 1987). Such hypotheses, postulating biogeochemical feedbacks at the global scale, are very difficult to prove or even to test because of the complexity of the Earth system. Recently, the existence and significance of the CLAW hypothesis has been challenged by Quinn and Bates (2011), who provided evidence that sea salt and organics are better candidates for CCN production in the remote marine boundary layer than DMS. However, using 9 yr of global satellite data and ocean climatologies to derive parameterizations of the seasonal variability of sea salt particles, sulfur aerosols derived from DMS, and primary and secondary organic aerosols, Lana et al. (2012) concluded that both sulfur and organic aerosols are important in CCN production. This issue is still strongly debated.There are also many uncertainties in the various processes that control the biogeochemical cycle of DMS. Oceanic emissions of DMS to the atmosphere result from complex physical, biological, and chemical interactions that are not well constrained (Stefels et al. 2007). For example, the mechanisms that regulate phytoplankton production of dimethylsulfoniopropionate (DMSP, the DMS precursor) and its ...