Abstract
Algal-bacterial interactions include mutualism, commensalism, and predation. However, how multiple environmental conditions that regulate the strength and prevalence of a given interaction remains unclear. Here, we test the hypothesis that the prevailing algal-bacterial interaction shifted in two years (2005 versus 2015), due to increased temperature (T) and Saharan dust depositions in high-mountain lakes of Sierra Nevada (S Spain). Our results support the starting hypothesis that the nature of the prevailing algal-bacterial interaction shifted from a bacterivory control exerted by algae to commensalism, coinciding with a higher air and water T as well as the lower ratio sestonic nitrogen (N): phosphorous (P), related to greater aerosol inputs. Projected global change conditions in Mediterranean region could decline the functional diversity and alter the role of mixotrophy as a carbon (C) by-pass in the microbial food web, reducing the biomass-transfer efficiency up the web by increasing the number of trophic links.
Highlights
Algae and bacteria numerically dominate the ocean and freshwater communities[1], comprising the majority fraction of particulate organic carbon[2]
While ultraviolet radiation (UVR) striking the Sierra Nevada Mountains remained high during the summer period (300.25–306.47 W m−2; Supplementary Fig. S1), Saharan dust input and air T followed a positive trend during the 10 years covered by our metabolic measurements (Fig. 1)
The main finding of our study was a shift in the nature of the interaction between algae and bacteria in high-mountain lakes of Sierra Nevada in 2005 and 2015, with a consistent and greater predominance of autotrophic metabolism in 2015 supported by an increase in the PPP, and with the reinforcement of commensalistic algal-bacterial interaction against the weakening of bacterivory control (Fig. 6)
Summary
Algae and bacteria numerically dominate the ocean and freshwater communities[1], comprising the majority fraction of particulate organic carbon[2]. These characteristics, together with their oligotrophic state and simple trophic web structure, make these lakes sensitive indicators of the past and current global conditions and serve as models to predict future changes because they register environmental change more directly[38], being considered sentinels of global change[39] For these reasons, our objective was to assess whether the regulation of the algal-bacterial interaction has changed in 2015 respect to 2005, after a period in which Saharan dust transport to the Mediterranean basin and air T have increased, while chronic UVR levels remain high. Our hypothesis is that greater dust deposition to Sierra Nevada lakes and higher T have shifted the algal metabolism towards stricter autotrophy and higher PP, and we expect a reinforcement of the commensalistic interaction between algae and bacteria to the detriment of the predatory interaction
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