Prochlorococcus is responsible for a significant part of CO 2 fixation in the ocean. Although it was long considered an autotrophic cyanobacterium, the uptake of organic compounds has been reported, assuming they were sources of limited biogenic elements. We have shown in laboratory experiments that Prochlorococcus can take up glucose. However, the mechanisms of glucose uptake and its occurrence in the ocean have not been shown. Here, we report that the gene Pro1404 confers capability for glucose uptake in Prochlorococcus marinus SS120. We used a cyanobacterium unable to take up glucose to engineer strains that express the Pro1404 gene. These recombinant strains were capable of specific glucose uptake over a wide range of glucose concentrations, showing multiphasic transport kinetics. The K s constant of the high affinity phase was in the nanomolar range, consistent with the average concentration of glucose in the ocean. Furthermore, we were able to observe glucose uptake by Prochlorococcus in the central Atlantic Ocean, where glucose concentrations were 0.5-2.7 nM. Our results suggest that Prochlorococcus are primary producers capable of tuning their metabolism to energetically benefit from environmental conditions, taking up not only organic compounds with key limiting elements in the ocean, but also molecules devoid of such elements, like glucose.high-affinity glucose transport | marine cyanobacteria | multiphasic uptake kinetics C yanobacteria is a phylum of the bacterial domain distinguishable by their unique capacity to perform oxygenic photosynthesis. This process, which relies on the existence of two photosystems and a chain of electron transporters, enables these organisms to use light energy and electrons from water to produce reductant molecules and fix atmospheric CO 2 to synthesize carbon compounds. Cyanobacteria probably arose on Earth billions of years ago (1), and prolonged evolutionary divergence has made them very diverse in terms of morphology, metabolism, and lifestyle.

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The metabolic responses of indigenous dominant bacterioplankton populations to additions of dust were examined in the tropical northeast Atlantic. Subsurface seawater samples were treated with dust, added directly or indirectly as a 'leachate' after its rapid dissolution in deionized water. Samples were incubated at ambient temperature and light for up to 24 h and microbial metabolic responses were assessed by (35)S-methionine ((35)S-Met) uptake. Prochlorococcus and low nucleic acid (LNA) cells were sorted by flow cytometry to determine their group-specific responses. Sorted cells were also phylogenetically affiliated using FISH. The high-light-adapted ecotype II dominated the Prochlorococcus group and 73+/-14% of LNA prokaryotes belonged to the SAR11 clade of Alphaproteobacteria. Both Prochlorococcus and LNA cells were metabolically impaired by the addition of dust (40+/-28% and 37+/-22% decrease in (35)S-Met uptake compared with controls, respectively). However, LNA bacterioplankton showed minor positive responses to dust leachate additions (7+/-4% increase in (35)S-Met uptake), while the metabolic activity of Prochlorococcus cells decreased in the presence of dust leachate by 16+/-11%. Thus, dust dissolution in situ appears to be more deleterious to Prochlorococcus than SAR11-dominated LNA bacterioplankton and hence could initiate a compositional shift in the indigenous bacterioplankton.