Abstract
The metabolic balance of the most extensive bioma on the Earth is a controversial topic of the global-change research. High ultraviolet radiation (UVR) levels by the shoaling of upper mixed layers and increasing atmospheric dust deposition from arid regions may unpredictably alter the metabolic state of marine oligotrophic ecosystems. We performed an observational study across the south-western (SW) Mediterranean Sea to assess the planktonic metabolic balance and a microcosm experiment in two contrasting areas, heterotrophic nearshore and autotrophic open sea, to test whether a combined UVR × dust impact could alter their metabolic balance at mid-term scales. We show that the metabolic state of oligotrophic areas geographically varies and that the joint impact of UVR and dust inputs prompted a strong change towards autotrophic metabolism. We propose that this metabolic response could be accentuated with the global change as remote-sensing evidence shows increasing intensities, frequencies and number of dust events together with variations in the surface UVR fluxes on SW Mediterranean Sea. Overall, these findings suggest that the enhancement of the net carbon budget under a combined UVR and dust inputs impact could contribute to boost the biological pump, reinforcing the role of the oligotrophic marine ecosystems as CO2 sinks.
Highlights
Several global studies during the last two decades have shown the high prevalence of heterotrophic metabolism into the ocean, in oligotrophic ocean areas[1,2]
In aquatic ecosystems, studies have reported that nutrients associated with dust inputs tend to stimulate primary production (PP)[15,16,17] and bacterial production[18], altering the structure of microbial planktonic communities[19,20] or biogeochemical cycles[21]
Autotrophic organisms use photosynthetically active radiation (PAR) to drive photosynthesis, but in the upper part of this layer, cells are exposed to high levels of ultraviolet radiation (UVR)
Summary
Several global studies during the last two decades have shown the high prevalence of heterotrophic metabolism into the ocean, in oligotrophic ocean areas[1,2]. The metabolic balance of oceans poses a heated debate, as reflected in the studies of Williams et al.[3] and Duarte et al.[4] where their autotrophy or heterotrophy, respectively, are defended. Most of the results published to date on this issue derive from oceanographic transects and short-term (hours) experiments Most of these studies have involved almost exclusively open-ocean areas, whereas less (or scarce) attention has been directed towards coastal areas, where previous studies have shown a contrasting response of planktonic metabolism (autotrophy or heterotrophy)[6]. Positive effects of UVR on both compartments have been shown, such as increased photorepair DNA damage[25,26] or enhanced photosynthesis[27]
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