Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

Abstract

The impact of viral lysis and grazing by flagellates on bacterioplankton production was assessed during a mesocosm experiment in the Eastern Mediterranean Sea, in response to Saharan dust (SD) vs. mixed aerosols (A) addition. The results highlight a positive effect on bacterial abundance, production and growth rate (∼1.2, ∼2.4, and ∼1.9 -fold higher than the controls) in both SD and A, which was also confirmed by the increased portion of high DNA content bacteria (up to 48% of the bacterial community). Lytic viral production and the portion of bacterial production lost due to viral lysis were lower in SD and A after dust addition than in the controls (0.33 ± 0.17 × 10 6 virus-like particles mL −1 h −1 and 6 ± 4%, respectively). Potential ingestion rate of bacteria by flagellates increased upon dust enrichment, but did not differ between mesocosms. Larger predators possibly down regulated flagellate abundance, and the calculated portion of bacterial production lost due to flagellate grazing was probably an artifact. Higher frequency of lysogenic cells in A compared to SD and the controls four days after dust addition may reflect faster phosphorus limitation in A, due to receiving less dissolved inorganic phosphorus and more dissolved inorganic nitrogen than SD.

Planktonic Lipidome Responses to Aeolian Dust Input in Low-Biomass Oligotrophic Marine Mesocosms

“…While chlorophyll concentrations returned to pre-addition values by day 6 (Tsagaraki et al, under revision, this SI; Tsiola et al, 2016), total IPL concentration in all treatment mesocosms exhibited a net increase of 4-24% (40-220 ng L −1 ). This is in contrast to control mesocosms, in which the chlorophyll response was lower than in treatment mesocosms and IPL concentrations fell by 12-35% by day 6 (140-530 ng L −1 ; p < 0.01; Figure 6).…”

“…During the MESOAQUA experiment in 2012 (Heraklion, Greece), concentrations of bacteria, Synechococcus, and chlorophyll increased significantly in response to the addition of SD and A to oligotrophic seawater mesocosms Tsiola et al, 2016;Tsagaraki et al, under revision, this SI). In parallel with the stimulation of picoplankton, Chl-a concentrations representative of larger phytoplankton (Chl>2 µ m) had increased by >50% by day 3.…”

“…Biogeochemical responses to the addition of SD or A, including plankton community composition and production, pigment concentration, and cycling of macro-nutrients and trace elements, were compared in triplicate to unamended control mesocosms over a period of 8 days (Guo et al, 2016;Herut et al, 2016;Rahav et al, 2016;Tsiola et al, 2016;Tsagaraki et al, under revision, this SI). In addition to these bulk biogeochemical measures of food web dynamics, the diversity of algal and bacterial cell membrane lipids provides a moderate level of taxonomic and physiological information (e.g., Sato, 1992;Guschina and Harwood, 2006;Sohlenkamp and Geiger, 2015) that can be applied to assess organic matter reactivity (Harvey and Macko, 1997;Wakeham et al, 1997), plankton communities in the ocean (e.g., Van Mooy and Fredricks, 2010), and carbon and nutrient fluxes in marine food webs (Dalsgaard et al, 2003;Sebastián et al, 2016).…”

“…Protocols for the determination of chlorophyll concentration, bacterio-and picoplankton cell abundances, primary and bacterial production, and phosphate concentration, uptake and turnover time in the mesocosms were reported by Herut et al (2016) as well as Guo et al (2016) and Tsiola et al (2016). Tsagaraki et al (under revision, this SI) additionally describe measurements of nutrient concentration, abundances of autotrophic picoplankton and zooplankton, and alkaline phosphatase activity.…”

“…Changes in the structure of the plankton community in response to SD or A addition were thoroughly assessed by companion studies (Guo et al, 2016;Herut et al, 2016;Tsiola et al, 2016;Tsagaraki et al, under revision, this SI) and can be summarized as follows. Chlorophyll a (Chl-a) concentrations had doubled in mesocosms on days 2 and 3 following the addition of SD or A, which were significantly higher compared to the control; this increase coincided with significant increases in Synechococcus and autotrophic picoeukaryotes (Guo et al, 2016).…”