Resumen de Effects of atmospheric deposition on microbial dynamics and composition in two anthropogenically-influenced contrasted coastal sites
The Mediterranean Sea is an oligotrophic basin, while the atmosphere above is affected by continuous emissions of anthropogenic aerosols and episodic Saharan dust events. These atmospheric inputs finally deposit (as wet or dry deposition) into surface waters, delivering high amounts of macronutrients and trace metals to surface waters. This process can constitute a main source of nutrient supply at certain times of the year, especially during the stratification period of the column water (May - October). In this thesis, we have assessed the effect of atmospheric particles on coastal planktonic communities following two approaches.
On one side, we have characterized the atmospheric deposition fluxes of the main macronutrients that limit or co-limit plankton growth and production in Mediterranean surface waters (i.e. inorganic nitrogen, phosphate, silicate, and organic carbon). To do so, we have measured the total atmospheric deposition (wet and dry) at two coastal locations of the northwestern Mediterranean with a contrasted anthropogenic footprint - Barcelona, urban location, and Blanes, with a lower degree of human impact, using passive collectors. We carried out a time-series of 4.5 years in Barcelona, and 3 years in Blanes. We observed that the deposition of the studied nutrients from the atmosphere occurs preferentially during the spring-summer season, coinciding with the stratification of the water column. In addition, we found some significant correlations between the nutrients released from the atmosphere and the concentration of chlorophyll and bacteria in seawater. In all, these results suggest that atmospheric deposition is an important source of new nutrients in coastal waters of the Mediterranean, with the potential to increase primary (autotrophic microorganisms) and secondary production (heterotrophic microorganisms) at certain times of the year.
On the other hand, with the aim to assess directly the effect of aerosols on microbial communities, we carried out microcosm experiments at the two coastal locations and in open waters of the western Mediterranean. We evaluated the effect of atmospheric particles from mineral (i.e. from the Saharan desert) and anthropogenic origin on marine phyto- and bacterioplankton at different times of the year. We found that aerosols did not produce significant effects on the microbial community during winter conditions, whereas atmospheric particles, especially from anthropogenic sources, significantly stimulated plankton growth and production during spring and summer. Anthropogenic aerosols enhanced bacterial metabolic processes significantly more than Saharan dust, what we mainly attribute to their higher content in soluble inorganic (mainly nitrogen, but also phosphate) and organic compounds. Furthermore, anthropogenic particles favored the growth of certain taxa of heterotrophic bacteria - mainly from the groups Alphaproteobacteria and Bacteroidetes - more than Saharan dust. Saharan dust instead enhanced preferentially the growth of Cyanobacteria during summer. The overall effect of atmospheric particles on marine bacteria is dependent on the chemical composition of the aerosols, their solubility in the seawater, and the biogeochemical status of the seawater before the aerosol additions (deposition). Our results agree with others obtained so far in the Mediterranean, while we go one step further when assessing the role of anthropogenic aerosols on marine bacteria, a process that remains poorly studied.
