Impacts of chemical contamination on bacterio-phytoplankton coupling

Abstract

Phytoplankton and bacterioplankton are the key components of the organic matter cycle in aquatic ecosystems, and their interactions can impact the transfer of carbon and ecosystem functioning. The aim of this work was to assess the consequences of chemical contamination on the coupling between phytoplankton and bacterioplankton in two contrasting marine coastal ecosystems: lagoon waters and offshore waters. Bacterial carbon demand was sustained by primary carbon production in the offshore situation, suggesting a tight coupling between both compartments. In contrast, in lagoon waters, due to a higher nutrient and organic matter availability, bacteria could rely on allochthonous carbon sources to sustain their carbon requirements, decreasing so the coupling between both compartments.Exposure to chemical contaminants, pesticides and metal trace elements, resulted in a significant inhibition of the metabolic activities (primary production and bacterial carbon demand) involved in the carbon cycle, especially in offshore waters during spring and fall, inducing a significant decrease of the coupling between primary producers and heterotrophs. This coupling loss was even more evident upon sediment resuspension for both ecosystems due to the important release of nutrients and organic matter. Resulting enrichment alleviated the toxic effects of contaminants as indicated by the stimulation of phytoplankton biomass and carbon production, and modified the composition of the phytoplankton community, impacting so the interactions between phytoplankton and bacterioplankton.

Chemical contamination alters the interactions between bacteria and phytoplankton

“…Like most of the Mediterranean coastal lagoons, the Bizerte lagoon (lagoon station, L), to the North of Tunisia, is a polluted coastal ecosystem subject to intense industrialization, urbanization, and agriculture pressures, as well as pressures from commercial and naval shipping harbors (Pringault et al, 2020). Consequently, sediments are strongly contaminated by a wide range of pollutants, including metals and organic compounds (polycyclic aromatic hydrocarbons (PAHs) and pesticides) (Barhoumi et al, 2014a(Barhoumi et al, , 2014bTriki et al, 2017).…”

“…Consequently, sediments are strongly contaminated by a wide range of pollutants, including metals and organic compounds (polycyclic aromatic hydrocarbons (PAHs) and pesticides) (Barhoumi et al, 2014a(Barhoumi et al, , 2014bTriki et al, 2017). The Bizerte bay (offshore station, O) is less contaminated than the lagoon (Pringault et al, 2020), although polar pesticides including herbicides have been detected in the lagoon watershed (Mhadhbi et al, 2019) and local PAH contamination has been recorded in the effluent from an oil refinery located on the shore of the bay (Zrafi-Nouira et al, 2009. Water sampling was -5-carried out from 2 m depth in a lagoon station (station L, 37° 12' 43.96'" N, 9° 50' 79.78" E) and an offshore station (station O, 37° 16' 46.46" N, 9° 53' 50.98" E) (Fig.…”

“…Briefly, polluted sediment was sampled using a Van Veen grab at 4 m depth in front of a cement factory in the lagoon channel (station CH,37° 15' 40.22" N,9° 51' 30.49" E Fig. S1), one of the most polluted sites in the Bizerte lagoon (Ben Said et al, 2010;Pringault et al, 2020;Triki et al, 2017). Overlying channel water was also sampled at the same time following the sampling procedure used for the offshore and lagoon waters.…”

“…For that purpose, microcosm incubations were performed during different seasons with offshore and lagoon waters exposed to a sediment elutriate or an artificial mixture of -4-contaminants mimicking the main contaminants present in sediment elutriate (pesticides and trace metal elements (TME)). Previous works have shown that chemical contamination and sediment resuspension impacted the coastal marine chemical environment differently (Bancon-Montigny et al, 2019) depending on the water masses (open or semi enclosed) with important consequences for ecosystem functioning (Pringault et al, 2020(Pringault et al, , 2016. The present study is based on the same sampling and experimental approach extensively described in Bancon-Montigny et al…”

“…(2019) and Pringault et al (2020), dealing respectively with contaminant dynamics in water and plankton, and with biogeochemical coupling between bacteria and phytoplankton. Here we focus on the consequences of contamination for phytoplankton and bacterioplankton diversity, and aim to decipher the phytoplankton-bacterioplankton interactions and the resulting microbial networks.…”