Differential responses of Prochlorococcus and SAR11-dominated bacterioplankton groups to atmospheric dust inputs in the tropical Northeast Atlantic Ocean

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 35S-methionine (35S-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 35S-Met uptake compared with controls, respectively). However, LNA bacterioplankton showed minor positive responses to dust leachate additions (7 +/- 4% increase in 35S-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.