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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Hill, Polly Georgiana