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

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 deionised water. Samples were incubated in ambient temperature and light for up to 24 h and microbial metabolic responses assessed by 35S-methionine 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 fluorescence in situ hybridisation. 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-methionine uptake compared to controls, respectively). However, LNA bacterioplankton showed minor positive responses to dust leachate additions (7 ± 4% increase in 35S-methionine uptake) while 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.