Degree of oligotrophy controls the response of microbial plankton to Saharan dust

Abstract

To determine the effects of Saharan dust on the abundance, biomass, community structure, and metabolic activity of oceanic microbial plankton, we conducted eight bioassay experiments between ca. 30uN and 30uS in the central Atlantic Ocean. We found that, although bulk abundance and biomass tended to remain unchanged, different groups of phytoplankton and bacterioplankton responded differently to Saharan dust addition. The predominant type of metabolic response depended on the ecosystem's degree of oligotrophy and was modulated by competition for nutrients between phytoplankton and heterotrophic bacteria. The relative increase in bacterial production, which was the dominant response to dust addition in ultraoligotrophic environments, became larger with increasing oligotrophy. In contrast, primary production, which was stimulated only in the least oligotrophic waters, became less responsive to dust as the ecosystem's degree of oligotrophy increased. Given the divergent consequences of a predominantly bacterial vs. phytoplanktonic response, dust inputs can, depending on the ecosystem's degree of oligotrophy, stimulate or weaken biological CO 2 drawdown. Thus, the biogeochemical implications of changing dust fluxes might not be universal, but variable through both space and time.

Saharan dust nutrients promote Vibrio bloom formation in marine surface waters

“…Additionally, studies that did assess community structure in response to dust typically focused on ≥48 h after dust addition, which could miss taxa that mount a more rapid response (31,36). Finally, much of the recent work in this area has examined response in experimental microcosms and mesocosms (15,31,36), with few studies able to capture in situ response to natural dust events. In this study, evidence from both experimental manipulations and in situ time course observations demonstrated that opportunistic heterotrophic genera like Vibrio mount a robust and rapid growth response to dust addition and potentially play an important, but currently largely unexplored, role in the cycling of Fe.…”

“…These recent investigations of heterotrophic response to dust-associated nutrients as well as the direct effect of Fe addition to marine waters generally measured bulk parameters (such as total bacterial abundance, bacterial respiration, or bacterial production), which could potentially fail to recognize community shifts and mask the emergence of functionally important taxa that can rapidly respond to nutrients (19,31,36). Additionally, studies that did assess community structure in response to dust typically focused on ≥48 h after dust addition, which could miss taxa that mount a more rapid response (31,36). Finally, much of the recent work in this area has examined response in experimental microcosms and mesocosms (15,31,36), with few studies able to capture in situ response to natural dust events.…”

“…These blooms, which were observed within 24 h of dust arrival, also likely preceded stimulation of autotrophic picoplankton and other phytoplankton, which have previously been shown to take ≥48 h to respond to dust or Fe inputs (e.g., ref. 31). This finding suggests that Vibrio blooms were not due to increased organic matter production from autotrophs, but were responding directly to the dust inputs, which is also supported by the microcosm Fig.…”

“…Although the response of marine autotrophs to dust deposition has been investigated (15,25), the full biological response to the episodic deposition of dust-Fe to ocean surface water, especially among microbial communities, has yet to be clearly elucidated. Evidence is growing that heterotrophic bacteria, especially among the class γ-proteobacteria, may play a role in processing deposited minerals and nutrients (31,32), but studies to date, which have largely focused on bulk bacterial response and longer timescales, have shown equivocal results (33,34). We hypothesized that specific opportunistic bacteria, like Vibrio, can respond quickly to newly available dust-associated Fe and suggest a role of dust-Fe as a driver of Vibrio population dynamics.…”

Evolving research directions in Surface Ocean - Lower Atmosphere (SOLAS) science

“…The effect of atmospheric deposition in the surface ocean may vary with the biogeochemical state of the receiving waters, [95] and result in fundamental differences in the response of the microbial community structure (e.g. stimulation of heterotrophy v. autotrophy [96] ), and so vertical carbon export and nutrient cycling. Particle deposition to the ocean can also provide ballast for sinking particles, [97] which may augment the export flux resulting from increased nutrient supply by atmospheric deposition.…”