An indoor study of the combined effect of industrial pollution and turbulence events on the gut environment in a marine invertebrate
Introduction
Most of the immune system is located in the gut, where it coexists and interacts with a rich community of commensal and symbiotic microbes. Altogether, the gut environment is becoming recognized as an important control center of a range of processes in organism biology, including immune functions, metabolism and neurobiology (Holmes et al., 2011; Kinross et al., 2011; Semova et al., 2012; Tremaroli and Backhed, 2012; Portune et al., 2017). Environmental stressors may disrupt the composition and metabolic activities of both gut immunity and microbiota, eventually compromising the health of host organisms (Snedeker and Hay, 2012; Kan et al., 2015; Jin et al., 2017). The potential for host-microbiota interactions to influence physiology, behaviour, and ecology of marine animals may be altered by exposure to environmental stressors, including climate change and anthropogenic contaminants. In particular, soft-bottom species living near operating or dismissed industrial plants are exposed to a wide range of chemicals that remain persistent in the shallow water sediments and lipophilic substrates (Arienzo et al., 2017; Trifuoggi et al., 2017). These substances tend to bio-accumulate in aquatic organisms, leading to oxidative stress, immunotoxic effects (Livingstone and Gallacher, 2003; Salo et al., 2007; Baussant et al., 2009; Hogan et al., 2010; Madison et al., 2015; Geraudie et al., 2016; Breitwieser et al., 2018) and ultimately cancer (Aas et al., 2000). Polluted sediments may affect not only soft-bottom species and assemblages living within contaminated sediments, but also organisms that populate surrounding hard substrates. This phenomenon can occur when rough sea conditions determine the reworking of sediments (Sunamura and Kraus, 1984; Sherman et al., 1994), causing release and transportation of sediment-associated pollutants. Such a propagation of anthropogenic disturbance assumes a central importance under climate-related changes, as the intensity and frequency of storms are expected to increase (Easterling et al., 2000; Trapp et al., 2007; Wolff et al., 2016; Aumann et al., 2018) with a range of potential impacts (Burge et al., 2014). A detailed understanding of the nature and extent of the interaction of marine pollution and changes of meteorological patterns is a key and urgent requirement of basic and applied research.
From early 1900s to mid-1990s, one of the largest Italian steel, cement and asbestos plant released contaminants in the Bagnoli area within the Gulf of Naples (Tyrrhenian Sea) (Bertocci et al., 2019, and references therein). Several studies have reported high levels of organic and inorganic pollutants in nearby shallow water sediments (Damiani et al., 1987; Sharp and Nardi, 1987; Albanese et al., 2010; Romano et al., 2004, 2009; Arienzo et al., 2017; Trifuoggi et al., 2017; Qu et al., 2018). In particular, baseline levels of polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and heavy metals were described (Damiani et al., 1987; Albanese et al., 2010).
The exposure to such contaminants may induce histopathological and molecular effects in the bivalve Mytilus galloprovincialis, a main model organism for ecotoxicological studies (Rossi et al., 2016; Arienzo et al., 2019). It may also alter the abundance, diversity and morphology of several prokaryotic, meiofaunal and macrozoobenthic groups (Romano et al., 2008, 2009; Bertocci et al., 2019; Gambi et al., 2020; Tangherlini et al., 2020). Yet, the toxicological relevance of Bagnoli area contaminants on the gut environment of marine organisms remains unexplored, especially in light of the ability of strong waves and currents to mobilize large volumes of polluted sediments that can be ingested by suspension feeders.
The ecology and life cycle of filter-feeding animals have major influence on the dynamics of aquatic ecosystems (Sánchez et al., 2016). Ascidians (phylum Chordata, subphylum Tunicata) feed by pumping seawater from the oral siphon to the branchial basket and collecting suspended matter and food particles in the digestive apparatus that consists of oesophagus, stomach, and intestine. The mucosal surface of the ascidian gut compartments is a complex and integrated system with the highest exposure to the external environment. Its integrity is critically important for nutrition, ion transport, as well as for maintaining a homeostatic relationship between immune cells and the gut microbiota (Galloway and Depledge, 2001). Ciona robusta is a cosmopolitan ascidian, deeply studied in developmental biology, immunology and toxicology (Caputi et al., 2007). In recent years, this species has been investigated for host-microbial interactions within the gut (Dishaw et al., 2012, 2016; Leigh et al., 2016, 2018; Liberti et al., 2018, 2019). Aspects of its gut environment were described, including a monolayered epithelium with different cell types (Millar, 1953; Thomas, 1970; Burighel and Cloney, 1997) covered by mucus rich in mucin and chitin fibers (Dishaw et al., 2016; Nakashima et al., 2018). Of note, ascidian genomes lack the expansion of innate immune genes that occurred in some invertebrate deuterostome lineages, such as those leading to amphioxus and sea urchin (Dehal et al., 2002; Azumi et al., 2003; Voskoboynik et al., 2013). The simpler immune system of ascidians may improve the comprehension of host immunity response to environmental challenges.
Based on this knowledge, the purpose of this manipulative study was to investigate, in aquaria, how varying the time interval between turbulence events causing suspension of contaminants exerted different effects on the gut environment, i.e., i) structure and organization of the epithelial layer, ii) alteration in mucus production or localization, iii) expression of molecules characteristic of the gut environment, and iv) antioxidant and immune response of the stomach. The knowledge of the nature and magnitude of potential adverse risks from exposure to resuspended pollutants is essential for evaluating the sub-lethal biological effects and for the management of marine populations exposed to current and predicted disturbance events superimposed to chronic environmental contamination.
Section snippets
Ethics statement and animal sampling
The ascidian Ciona robusta is commonly found on hard substrates in several locations near the study area of Bagnoli-Coroglio, especially where nutrients reach high concentrations such as in marinas (Mergellina, Villaggio Coppola), coastal lagoons (Lago Fusaro), and harbors (Naples, Pozzuoli; Castellamare di Stabia) (Sordino et al., 2008; Affinito et al., 2015). The species is highly sensitive to environmental fluctuations, resulting in small interconnected populations with high local extinction
Results
In order to assess the effects of shifts in ecosystem functioning due to climate changes, an indoor system was developed. Adult specimens of C. robusta were exposed to ‘aggregate’ and ‘spaced’ turbulence events and the repercussion on gut environment was evaluated by histological and molecular analyses as described below.
Discussion
The intact epithelial barrier and the balanced gut microbiota are both involved in the regulation of immune response in the GI, defined as gut homeostasis (Rescigno, 2011). The gut is one of the mucosal sites of the animal body, where the immune system plays a vital role in mucosal homeostasis and surveillance against potentially harmful agents. Different environmental factors, including chemical exposures and food contaminants, are well-recognized inducers of alterations in gut mucus layer,
Conclusions
We have shown that water turbulence and consequent release of contaminants from sediment to the overlying water may alter the diverse players that establish and maintain gut homeostasis and result in potentially adverse effects toward surrounding organisms. To understand the impact of climate extremes on aquatic ecosystems, it is important to evaluate changes in immune and microbiota functions of the gut environment in marine species and assemblages. In this context, the ascidian Ciona robusta
Funding
This study was supported by the project “Sperimentazioni pilota finalizzate al restauro ambientale e balneabilità del SIN Bagnoli-Coroglio (ABBaCo)” funded by the Italian Ministry for Education, University and Research (grant no. 508 C62F16000170001). The funding source had no role in study design, data collection, analysis and interpretation, writing of the report, and decision to submit the article for publication.
CRediT authorship contribution statement
Assunta Liberti: Conceptualization, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Iacopo Bertocci: Conceptualization, Writing - original draft, Writing - review & editing. Amandine Pollet: Investigation. Luigi Musco: Resources. Annamaria Locascio: Conceptualization, Writing - original draft. Filomena Ristoratore: Conceptualization, Writing - original draft. Antonietta Spagnuolo: Conceptualization, Writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Technical support was provided by Il Cozzaro Nero s.r.l. (Taranto, Italy), Alessandra Gallo (Stazione Zoologica Anton Dohrn, SZN), Fabio Conversano (SZN), Monia Renzi (Bioscience Research Center s.r.l.), Lorenzo Morroni (ISPRA, Italy) and the facilities for the maintenance of marine organisms, research infrastructures for marine biological resources, microscopy, and molecular biology of the SZN.
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