Impact of iron limitation on primary production (dissolved and particulate) and secondary production in cultured Trichodesmium sp.

International audienceDiazotrophic cyanobacteria play an important role in biogeochemical cycles of carbon and nitrogen and, hence, in oceanic productivity in the tropical and subtropical regions of the ocean. Although many studies have examined the impact of iron (Fe) limitation on particulate primary production and dinitrogen (N2) fixation in the colonial cyanobacterium Trichodesmium, none have looked at the impact of Fe limitation on the percentage extracellular release (PER) and secondary production (SP) in Fe-limited cultures of this cyanobacterium. Here, we present the results of a series of culture experiments during which we examined the impact of 3 concentrations of dissolved iron (DFe) on total primary production (TPP = dissolved + particulate primary production, i.e. DPP + PPP), PER and on SP. Under severe Fe limitation (5 nM DFe), biomass, growth rates, TPP and N2 fixation were strongly reduced, while PER increased relative to the rates ob served at the highest Fe concentration. Moreover, reducing Fe concentration induced an increase in the percentage of photosynthetically fixed C used for algal growth, while the percentage of C used to support algal respiration decreased. Reduced Fe concentrations also induced a decrease in SP and in the SP:DPP ratio, indicating that the efficiency of transfer of fixed carbon from autotrophic to heterotrophic processes is reduced. This suggests that Fe, either directly through influencing cellular processes or indirectly through influencing organic matter structure or nitrogen availability, is controlling SP and, thus, microbial carbon utilization. These results suggest that the amount of carbon entering into the microbial loop may be reduced under Fe limitation, thus leading to an accumulation of dissolved organic carbon with potentially important impacts on microbial carbon cycling and, ultimately, on the biological carbon pum

Iron limitation and DMSP in diatoms and cyanobacteria

International audienc

Increased intracellular concentrations of DMSP and DMSO in iron-limited oceanic phytoplankton Thalassiosira oceanica and Trichodesmium erythraeum

International audienceWe 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 (DMSPp) concentrations by 12-fold (from 2.8 to 33.7 mmol Lcell−1) and by 45-fold (from 0.05 to 2.27 mmol Ltrichome−1), respectively. Intracellular dimethylsulfoxide (DMSOp) concentrations in T. oceanica increased by 5-fold under severe Fe limitation, from 0.78 mmol Lcell−1 in Fe-replete cells to 3.86 mmol Lcell−1. The increase in DMSPp and DMSOp under Fe limitation provides support for the role of these sulfur compounds as antioxidants. Under severe Fe limitation, the large increase in DMSPp : 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 DMSPp : Chl a and DMSOp : Chl a that is generally observed with alleviation of Fe limitation may be partly related to decreases in DMSPp and DMSOp 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