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The occurrences of microplastics in the ocean
After the plastics end up into the ocean, based on plastic density, it could be floated in the surface, suspended in the ocean column or settled to the bottom of the ocean. Polyethylene and polypropylene are main composition in the surface layer and shorelines (Bond et al., 2018). Plastic in the ocean surface is very patchy, and mostly accumulates in the convergence zones of the each of the five subtropical gyres with comparable density (North Pacific, North Atlantic, South Pacific, South Atlantic, Indian Ocean), and enclosed sea (Mediterranean Sea), the microplastic with the highest concentration in the surface water are from the Mediterranean sea and North Pacific with more than 108 pieces per square kilometer (Figure 2) (Eriksen et al., 2014; Van Sebille et al., 2015), around half of the microplastic afloat in subtropical gyres (Van Sebille et al., 2015). Evidence also showed that the subtropical gyre is rapidly accumulating plastics (Lebreton et al., 2018), while any global estimation of total accumulated floating microplastic debris only accounted for 1% or less of the amount of plastic waste emitted into the ocean annually (Jambeck et al., 2015; Van Sebille et al., 2015). The speculation for the plastic distribution is that surface waters are not the final destination for buoyant plastic debris in the ocean. Nano-fragmentation, predation, biofouling, or shore deposition have been proposed as possible mechanisms of plastic removal from the surface (Cózar et al., 2014). Previous study also emphasized that most of the ocean surface is under-sampled for microplastics, uncertainties in the Southern hemisphere basins illustrate the lack of data even in the high concentration subtropical gyre (Van Sebille et al., 2015). As most of the plastic is missing from the sea surface, it is estimated that seafloor is the area accumulating majority of the plastics (Thompson et al., 2004). The seashore or littoral sediment is also the accumulating the microplastics, average of 60 and 128 microplastic particles per kg sand (around 0-1.8 cm depth) were found between low tide and high tide line from coast of the southeastern United states and Bohai sea of China, respectively (Yu et al., 2016, 2018). Average of 1445 microplastic particles per kg littoral sediment (0-5 cm depth) were found in the from northeastern Italian coast (Vianello et al., 2013), 141-461microlastics particles per kg littoral sediment (around 0-2 cm depth) were found from the littoral zone of the north Tunisia coast (Mediterranean Sea) (Abidli et al., 2018). In summary, the beach and the littoral sediment are highly contaminated by the microplastics.
Canonical bacterial colonization process
Research argues that most bacteria (if not all) are capable of forming a biofilm and a large fraction of their lifetime is probably spent in the biofilm. The biofilm were considered as themicrobial development stage, analogous to the microbial spore formation (Monds and O’Toole, 2009). It is estimated that 40-80% of prokaryotes residing in biofilm, and drives all biogeochemical process (Flemming and Wuertz, 2019).
The formation of biofilm had been viewed as the processed governed by the chemical and physical process (such as hydrophobicity and/or surface charge) imposed by the solid surface and composition of bacterial cell surface (Teughels et al., 2006). The advance of the genetic characterization revealed that it could be an active bacterial colonization process with the genetic pathway dedicated for the surface attachment regulation (Monds and O’Toole, 2009). Recent study strongly support that bacteria can indeed sense the complex surface topographies, and then reside on the favorable area (O’Toole and Wong, 2016).
Free-living lifestyle
For free-living bacteria, Grossart et al. considered as the bacterial groups which spend their while life cycle as the individual cell (Grossart, 2010). The free-living bacteria have developed several strategies to survival. It tends to be streamlined both in cell size and genome size to reduce the expenditures for the maintenance energy and duplication, and their vulnerability to grazing (Hurst, 2019). SAR11 is the representative for the free-living bacteria. They are the most abundant bacteria (approximately 25% of all bacterioplankton). The SAR11 have the smaller bacterial size, and are generally regarded as the defense generalist. In another point, bacteriochlorophyll and proteorhodopsin are the common feature of free-living bacteria, since these pigments could be involved in getting extra light energy source to cope with extreme oligotrophic condition, the proteorhodopsin was also found to be continuously expressed for bacteria of SAR11 (Kirchman, 2016; Giovannoni, 2017).
Surface-associated lifestyle
Surface-associated bacteria have some different traits compared to free-living bacteria (Dang and Lovellc, 2015). Surface-associated bacteria have bigger cell compared to free-living one, it has been proposed that the surface-associated microorganisms are mainly copiotrophic, whereas free-living bacteria are mainly oligotrophic. The mobility and chemotactic behaviors are considered essential for surface-associated bacteria to reach the nutritious microhabitats.
There are several advantage aspects for surface-associated lifestyle. Surface colonization and the formation of biofilm provides bacteria the shielding matrix, which provide the protection from predators, viruses, antibiotic and the environmental stresses such as UV radiation, pH shifts, osmotic shock (Davey and O’toole, 2000; Matz et al., 2008). Moreover, bacteria with surface-associated lifestyle possess a wild repertoire of genes coding for membrane attachment and extracellular enzyme for digestion of phytoplankton EPS (Hurst, 2019). The elaborate architecture of biofilm could also provide the opportunity of metabolic cooperation, gene exchange (via horizontal gene transfer) (Davey and O’toole, 2000; Madsen et al., 2012).
Cell density in bacterial biofilm tends to be several orders of magnitude higher than the freeliving bacteria. In another perspective, there are also competition within the biofilm. To acquire limited resource and space on the surface, more than 50% of bacteria were found to have the antagonistic activity, which was more common for particles-associated bacteria than the freeliving bacteria. Members of Alteromonadales are one of the most prolific producers for inhibitory materials (Grossart et al., 2004; Long and Azam, 2001). For instance, member of marine roseobacter clade were characterized to be an important surface-associated bacteria, and it can produce antimicrobial substance, such as tropodithietic acid (TDA) and indigoidine (Buchan et al., 2014). Some surface-associated bacteria have contact-dependent growth inhibition system (CDI), which is a member of type V secretion system, and could be used for the intra- and interspecies competition or to coordinate the bacteria growth within biofilm. Type VI secretion system (T6SS) is similar to a phage injection system, and it is used for deliver toxins to neighboring bacteria cell, it was presented in more than a quarter of bacteria (mainly in Proteobacteria) (Bingle et al., 2008; Hayes et al., 2010). Thus, it is reasonable to deduce that the cooperation and competition within the biofilm will shape the bacteria diversity, and to be a greater extent, influence the biogeochemical recycling processes.
Technique for the research on the microbial community
Concerning on the technique on determination of bacterial abundance and identification of microbial eukaryotes living on plastisphere, microscopy such as scanning electron microscopy (SEM) is a powerful technique widely used on the research on plastisphere, flow cytometry is less used, because it is difficult to detach all the microbes from plastisphere due to the sticky nature of EPS (Salta et al., 2013). For the identification of microbial composition, biomarkers of 16S and 18S rRNA gene were used more common nowadays (Jacquin et al., 2019). Only one study used the metagenomic sequencing to study the metabolite potential on plastisphere from North Pacific subtropical gyre (Bryant et al., 2016).
A new niche for the marine microorganisms
Whatever the polymer type, recent studies emphasized the difference between the bacteria living on plastics and living in free-living state (Debroas et al., 2017) or on organic particles in the surrounding seawater (Dussud et al., 2018; Oberbeckmann et al., 2018). Similar observations have been made for fungal communities (Kettner et al., 2017).
Microbial community abundance and composition
The SEM and next generation sequencing data revealed distinct microbial groups on plastisphere, including bacteria, diatoms, fungi, bryozoans, dinoflagellates, radiolarians, barnacles, isopods, marine worms and marine larvaes etc. (Table 1). During the surveys by SEM, bacteria and diatoms presented on plastisphere wherever the sample from the coastal area or pelagic ocean (North Atlantic or Pacific Ocean). The diatoms had different abundance, ranging from several counts to thousands per square millimeter (Table 1). The bacteria had the counts of thousands per square millimeter (Carson et al., 2013; Dussud et al., 2018). In some situation, the bacteria and diatom could have the comparable abundance (table 1(Carson et al., 2013).
According to the nutritional type, the microbe on plastisphere could be grouped as phototrophs heterotrophs, predators, symbionts and saprotrophs (Figure 4).
Table of contents :
Abstract:
Résumé en français :
Introduction
1. Microplastics and its distribution in the ocean
1.1 Plastic production and classification
1.2 Microplastics and Source of the oceanic microplastics
1.3 The occurrences of microplastics in the ocean
2. microbes and “plastisphere”
2.1 Microbes in the ocean
2.2 Canonical bacterial colonization process
2.3 Bacterial lifestyle and adaptation
2.4 Zooming in on “Plastisphere”
2.5 Factors driving the formation of bacterial plastisphere
2.6 Impact of plastic and plastic biofilm in ecosystem
2.7 Plastic degradation
3. Toxicity of plastics to marine creatures
3.1 Plastics ingestion and entanglement
3.2 Toxic aspects of microplastics
Summary of introduction:
Thesis Objectives:
Chapter 2: Relative influence of plastic debris size and shape, chemical composition and phytoplankton-bacteria interactions in driving seawater plastisphere abundance, diversity and activity
Chapter 3: Beneficial or detrimental effects of microplastics on the marine filter-feeder amphioxus (Branchiostoma lanceolatum)?
Chapter 4: Biodegradability under marine conditions of bio-based and petroleum-based polymers as substitutes of conventional microbeads
CHAPTER 5: General discussions and perspectives
1. Main results and general discussions
2. Perspectives
References
