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

Abstract

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.

More information

Identifiers

Catalogue record

Export record