Bacterioplankton dynamics in surface waters of the north-eastern (sub-)tropical Atlantic Ocean affected by Aeolian dust

The microbial community dominates biogeochemical cycling of the ocean, affecting global climate. The impact of physical disturbance of near surface microbial populations was studied in the northeastern tropical and subtropical Atlantic Ocean. This region lies beneath easterly trade winds, resulting in strong perturbations in terms of wind driven mixing and Aeolian dust deposition. Firstly, the region’s surface water bacterioplankton community was compared with adjacent regions in terms of metabolic activity, by measuring the uptake rates of radioactively labelled amino acids (3H-leucine and 35S-methionine) as a proxy for bacterial production. Remarkably, there was little variation in uptake rates between the two Atlantic (sub-)tropical gyres. Rates reflected regional photosynthetic biomass, except in the study region. The bacterioplankton community of this region was less metabolically active than that of the oligotrophic north Atlantic gyre, despite ocean colour data identifying the region as productive. The region’s uniqueness is probably related to the episodic Saharan dust inputs experienced. To test whether dust deposition controls microbial community structure, surface communities were compared, using flow cytometry and fluorescence in situ hybridisation, between two winter periods when either wind-driven mixing or dust deposition occurred. Wind-driven mixing was associated with domination by the ubiquitous SAR11 clade of Alphaproteobacteria, whereas key primary producers, Prochlorococcus cyanobacteria, numerically dominated during calmer conditions. Phytoplankton-associated Bacteroidetes and Synechococcus cyanobacteria were most abundant during turbulent conditions. Gammaproteobacteria, encompassing opportunistic species, were the only group to benefit from dust inputs; thus dust deposition seems to have a minor influence on the region’s bacterioplankton community compared to wind mixing, suggesting community change following dust storm events may be linked to nutrients delivered by wind mixing, as much as from dust. To test further whether changes in the SAR11 and Prochlorococcus populations varied between years due to wind- or dust-related perturbation, a method based on 35S-methionine uptake was developed for measuring the metabolic response of these groups to Aeolian dust, whilst excluding wind impacts. Subsurface seawater samples were treated with freshly collected dust, added directly or indirectly as a “leachate” after its rapid dissolution in deionised water. Prochlorococcus and SAR11 cells were sorted by flow cytometry to determine their group-specific responses. Both Prochlorococcus and SAR11 were metabolically impaired by the addition of dust, which may explain the low metabolic activity observed in the region (mentioned above). Although SAR11 showed minor positive responses to dust leachate additions, leachate proved detrimental to Prochlorococcus. Thus dust dissolution in situ appears to be more deleterious to Prochlorococcus than SAR11 and hence could initiate a compositional shift in the indigenous bacterioplankton, suggesting the observed switch from SAR11- to Prochlorococcus-domination following dust deposition (mentioned above) was indeed a result of an alternative stimulus, most likely wind stress. In conclusion, whereas dust deposition may prove beneficial to bacterioplankton species with high nutrient demands, such as some Gammaproteobacteria, it does not appear to affect the ambient dominant bacterioplankton groups of the northeast (sub-)tropical Atlantic to the same degree as wind-driven perturbations. Furthermore, large dust deposition events may prove detrimental to ambient populations, resulting in low community metabolic activity.