Low-dose addition of silver nanoparticles stresses marine plankton communities

The release of silver nanoparticles (AgNPs) is expected to rise in the near future, with possible negative effects on aquatic life and enhancement of microbial resistance against AgNPs. However, a realistic evaluation of the toxicity of AgNPs to the marine ecosystem is currently missing. Therefore, we designed a mesocosm experiment to assess the impact of AgNP exposure on natural microbial plankton community dynamics in a coastal marine site at environmentally relevant concentrations. We monitored changes in the composition of the planktonic community, from viruses to protists. Further, we analyzed the concentration and properties of AgNPs for the total time of exposure. We found that the addition of AgNPs even at a low dose affected the plankton communities. Specifically, the growth of Synechococcus was inhibited and bacterial community composition significantly changed. Additionally, the amount of a lysogeny-related gene increased and viral auxiliary metabolic genes that are involved in cyanobacterial photosynthesis decreased, revealing a damaged photosynthetic potential after AgNP exposure. Microbial plankton was significantly affected due to both increased dissolved silver ions and decreased AgNP size. Our results highlight that the release of AgNPs alters the functioning of the marine food web by hampering important viral and bacterial processes.</p

Saharan Dust Deposition Effects on the Microbial Food Web in the Eastern Mediterranean: A Study Based on a Mesocosm Experiment

The effect of episodicity of Saharan dust deposition on the pelagic microbial food web was studied in the oligotrophic Eastern Mediterranean by means of a mesocosm experiment in May 2014. Two different treatments in triplicates (addition of natural Saharan dust in a single-strong pulse or in three smaller consecutive doses of the same total quantity), and three unamended controls were employed; chemical and biological parameters were measured during a 10-day experiment. Temporal changes in primary (PP) and bacterial (BP) production, chlorophyll a (Chl a) concentration and heterotrophic bacteria, Synechococcus and mesozooplankton abundance were studied. The results suggested that the auto- and hetero-trophic components of the food web (at least the prokaryotes) were enhanced by the dust addition (and by the nitrogen and phosphorus added through dust). Furthermore, a 1-day delay was observed for PP, BP, and Chl a increases when dust was added in three daily doses; however, the maximal values attained were similar in the two treatments. Although, the effect was evident in the first osmotrophic level (phytoplankton and bacteria), it was lost further up the food web, masked under the impact of grazing exerted by predators such as heterotrophic flagellates, ciliates and dinoflagellates. This was partly proved by two dilution experiments. This study demonstrates the important role of atmospheric deposition and protist grazing when evaluating the effect on oligotrophic systems characterized by increased numbers of trophic levels