Anthropogenic versus mineral aerosols in the stimulation of microbial planktonic communities in coastal waters of the northwestern Mediterranean Sea

Aerosol Nutrients and Their Biological Influence on the Northwest Pacific Ocean (NWPO) and Its Marginal Seas

Simple Summary

With intensifying human activities in the past decades, East Asia has recorded increasingly severe air pollution and become the second largest aerosol source on earth. The large quantity of aerosol emissions is not only a major health threat to humans, but can also be transported for a long distance and deposited in downwind seas and oceans. The aerosol contains major ions, heavy metals, and organic matters that are important external nutrients in upper oceans and potentially influence marine microbes and biogeochemical cycles. Therefore, the role of atmospheric deposition to oceans has received growing attention in recent years. In this paper, the current state of knowledge on the atmospheric nutrients and the biological effect of East Asian aerosol deposition on the northwest Pacific Ocean are reviewed, which could help us better understand the comprehensive influence of East Asian aerosols on marine ecosystems, and give insights into future research directions, especially under the future scenarios of changing human activities and climate.

Abstract

Atmospheric deposition is recognized as a significant source of nutrients in the surface ocean. The East Asia region is among the largest sources of aerosol emissions in the world, due to its large industrial, agricultural, and energy production. Thus, East Asian aerosols contain a large proportion of anthropogenic particles that are characterized by small size, complex composition, and high nutrient dissolution, resulting in important influences on marine microbes and biogeochemical cycles in the downwind areas of the northwest Pacific Ocean (NWPO). By using remote sensing, modeling, and incubation experimental methods, enhanced primary production due to the East Asian aerosol input has been observed in the NWPO, with subsequent promotion and inhibition impacts on different phytoplankton taxa. Changes of bacterial activity and diversity also occur in response to aerosol input. The impact of East Asian aerosol loadings is closely related to the amount and composition of the aerosol deposition as well as the hydrological condition of the receiving seawater. Here, we review the current state of knowledge on the atmospheric nutrients and the effects of the East Asian aerosols on microbes in the NWPO region. Future research perspectives are also proposed.

Desert and anthropogenic mixing dust deposition influences microbial communities in surface waters of the western Pacific Ocean

The western Pacific Ocean is particularly affected by dust aerosols due to the transport of desert-natural sand and industrially derived particulate matter with aerodynamic diameter < 2.5 μm (PM2.5) from continental Asia. Both oligotrophic and nutrient-sufficient surface water occurs in this region and these are speculated to support different microbial community dynamics. Here, we report evidence from four shipboard experiments in the western Pacific Ocean supplying oligotrophic and nutrient-sufficient surface waters with aerosol particles obtained from the nearby coastal mountains, to simulate dust and anthropogenic aerosol inputs in the ocean region. A sharp increase in nitrate for surface waters after addition of dust aerosols resulted in large increases in diatom abundance in oligotrophic waters, whilst in nutrient-sufficient waters the response of diatom population was reduced. The increase in organic matter provided by aerosol inputs and/or increase in phytoplankton biomass induced the growth of heterotrophic prokaryotes, such as Rhodobacteraceae and Alteromonadaceae populations, in both oligotrophic and nutrient-sufficient seawater. Anthropogenic and desert-natural dust is an important source of nitrate and organics to oceanic waters and such inputs can directly affect primary production and heterotrophic prokaryotic abundance in the ocean, implying consequences for the carbon cycle in these aerosol-affected waters.

Effect of East Asian atmospheric particulate matter deposition on bacterial activity and community structure in the oligotrophic Northwest Pacific

Large amounts of anthropogenic East Asian (EA) particulate matters (PM), containing inorganic nutrients and organic matter, are deposited in the oligotrophic Northwest Pacific Ocean. However, the effects of such deposition on marine microbes remain unclear. In this study, the effect of EA PM deposition on marine bacteria was assessed by five on-board microcosm experiments, conducted in oligotrophic basins of the South China Sea. The addition of EA PM to the sampling water induced a clear shift in bacterial community composition from prevailing oligotrophs (i.e., SAR 11 clade, Prochlorococcus, AEGEAN-169 marine group) to less common copiotrophs (i.e., Alteromonas, Ruegeria, Flavobacteriaceae) and thus a slight increase in bacterial diversity. The shift to more active community composition, as well as stimulation of PM nutrients, resulted in a large increase in cell-specific and bulk bacterial production. In contrast, there were only minor changes in bacterial abundance, possibly due to increased top-down mortality. The EA PM also exhibited a stronge toxic effect on pico-cyanobacteria, leading to a significant decrease in their proportion. Moreover, the responses of bacterial metabolism and community composition exhibited significant relationships with the hydrographic condition of the locations. Stronger promotion effects of the EA PM on bacterial production and community shift from oligotrophs to copiotrophs was demonstrated at the more oligotrophic sites with lower chlorophyll a concentrations. These results suggest that PM deposition from polluted areas has the potential to alter the typical oligotrophic microbiomes and change the net metabolic balance of the bacterial community. These will then influence the dynamics of carbon flow in microbial food webs and biogeochemical cycles, especially with the trend of global warming and expansion of low-chlorophyll regions.

Atmospheric Deposition Impact on Bacterial Community Composition in the NW Mediterranean

Atmospheric deposition is a source of inorganic nutrients and organic matter to the ocean, and can favor the growth of some planktonic species over others according to their nutrient requirements. Atmospheric inputs from natural and anthropogenic sources are nowadays increasing due to desertification and industrialization, respectively. While the impact of mineral dust (mainly from the Saharan desert) on phytoplankton and bacterial community composition has been previously assessed, the effect of anthropogenic aerosols on marine bacterial assemblages remains poorly studied. Since marine bacteria play a range of roles in the biogeochemical cycles of inorganic nutrients and organic carbon, it is important to determine which taxa of marine bacteria may benefit from aerosol fertilization and which not. Here, we experimentally assessed the effect of Saharan dust and anthropogenic aerosols on marine bacterioplankton community composition across a spatial and temporal range of trophic conditions in the northwestern Mediterranean Sea. Results from 16S rDNA sequencing showed that bacterial diversity varied significantly with seasonality and geographical location. While atmospheric deposition did not yield significant changes in community composition when all the experiments where considered together, it did produce changes at certain places and during certain times of the year. These effects accounted for shifts in the bacterial community's relative abundance of up to 28%. The effect of aerosols was overall greatest in summer, both types of atmospheric particles stimulating the groups Alphaproteobacteria, Betaproteobacteria, and Cyanobacteria in the location with the highest anthropogenic footprint. Other bacterial groups benefited from one or the other aerosol depending on the season and location. Anthropogenic aerosols increased the relative abundance of groups belonging to the phylum Bacteriodetes (Cytophagia, Flavobacteriia, and Sphingobacteriia), while Saharan dust stimulated most the phytoplanktonic group of Cyanobacteria and, more specifically, Synechococcus.

Growth impacts of Saharan dust, mineral nutrients, and CO2 on a planktonic herbivore in southern Mediterranean lakes

Rising levels of CO₂ can boost plant biomass but reduce its quality as a food source for herbivores. However, significant uncertainties remain as to the degree to which the effect is modulated by other environmental factors and the underlying processes causing these responses in nature. To address these questions, we carried out CO₂-manipulation experiments using natural seston from three lakes under nutrient-enriched conditions (mimicking eutrophication and atmospheric dust-input processes) as a food source for the planktonic Daphnia pulicaria. Contrary to expectations, there were no single effects of rising CO₂ on herbivorous growth. Instead, synergistic CO₂ × nutrient interactions indicated that CO₂ did not support higher zooplankton growth rates unless supplemented with dust or inorganic nutrients (nitrogen, N; phosphorus, P) in two of three studied lakes. The overall positive correlation between zooplankton growth and seston carbon (C), but not seston C:P, suggested that this was a food quantity-mediated response. In addition, we found that this correlation improved when the data were grouped according to the nutrient treatments, and that the response was largest for dust. The synergistic CO₂ × nutrient effects reported here imply that the effects of rising CO₂ levels on herbivorous growth may be strongly influenced by eutrophication processes and the increase in dust deposition predicted for the Mediterranean region.

Coastal Bacterioplankton Metabolism Is Stimulated Stronger by Anthropogenic Aerosols than Saharan Dust

In oligotrophic regions, such as the Mediterranean Sea, atmospheric deposition has the potential to stimulate heterotrophic prokaryote growth and production in surface waters, especially during the summer stratification period. Previous studies focused on the role of leaching nutrients from mineral particles of Saharan (S) origin, and were restricted to single locations at given times of the year. In this study, we evaluate the effect of atmospheric particles from diverse sources and with a markedly different chemical composition [S dust and anthropogenic (A) aerosols] on marine planktonic communities from three locations of the northwestern Mediterranean with contrasted anthropogenic footprint. Experiments were also carried out at different times of the year, considering diverse initial conditions. We followed the dynamics of the heterotrophic community and a range of biogeochemical and physiological parameters in six experiments. While the effect of aerosols on bacterial abundance was overall low, bacterial heterotrophic production was up to 3.3 and 2.1 times higher in the samples amended with A and S aerosols, respectively, than in the controls. Extracellular enzymatic activities [leu-aminopeptidase (AMA) and β-glucosidase (β-Gl)] were also enhanced with aerosols, especially from A origin. AMA and β-Gl increased up to 7.1 in the samples amended with A aerosols, and up to 1.7 and 2.1 times, respectively, with S dust. The larger stimulation observed with A aerosols might be attributed to their higher content in nitrate. However, the response was variable depending the initial status of the seawater. In addition, we found that both A and S aerosols stimulated bacterial abundance and metabolism significantly more in the absence of competitors and predators.