Exopolysaccharides directed embellishment of diatoms triggered on plastics and other marine litter

Diatom-Bacteria Interactions in the Marine Environment: Complexity, Heterogeneity, and Potential for Biotechnological Applications

Diatom-bacteria interactions evolved during more than 200 million years of coexistence in the same environment. In this time frame, they established complex and heterogeneous cohorts and consortia, creating networks of multiple cell-to-cell mutualistic or antagonistic interactions for nutrient exchanges, communication, and defence. The most diffused type of interaction between diatoms and bacteria is based on a win-win relationship in which bacteria benefit from the organic matter and nutrients released by diatoms, while these last rely on bacteria for the supply of nutrients they are not able to produce, such as vitamins and nitrogen. Despite the importance of diatom-bacteria interactions in the evolutionary history of diatoms, especially in structuring the marine food web and controlling algal blooms, the molecular mechanisms underlying them remain poorly studied. This review aims to present a comprehensive report on diatom-bacteria interactions, illustrating the different interplays described until now and the chemical cues involved in the communication and exchange between the two groups of organisms. We also discuss the potential biotechnological applications of molecules and processes involved in those fascinating marine microbial networks and provide information on novel approaches to unveiling the molecular mechanisms underlying diatom-bacteria interactions.

Revealing the Bacterial Quorum-Sensing Effect on the Biofilm Formation of Diatom Cylindrotheca sp. Using Multimodal Imaging

Diatoms contribute to carbon fixation in the oceans by photosynthesis and always form biofouling organized by extracellular polymeric substances (EPS) in the marine environment. Bacteria-produced quorum-sensing signal molecules N-acyl homoserine lactones (AHLs) were found to play an important role in the development of Cylindrotheca sp. in previous studies, but the EPS composition change was unclear. This study used the technology of alcian blue staining and scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to directly observe the biofilm formation process. The results showed that AHLs promote the growth rates of diatoms and the EPS secretion of biofilm components. AHLs facilitated the diatom-biofilm formation by a forming process dependent on the length of carbon chains. AHLs increased the biofilm thickness and the fluorescence intensity and then altered the three-dimensional (3D) structures of the diatom-biofilm. In addition, the enhanced EPS content in the diatom-biofilm testified that AHLs aided biofilm formation. This study provides a collection of new experimental evidence of the interaction between bacteria and microalgae in fouling biofilms.

Environmental impacts of microplastic and role of plastisphere microbes in the biodegradation and upcycling of microplastic

Increasing usage of plastic has led to the deposition of plastic in the environment which later become microplastic, a pollutant of global concern. These polymeric particles affect the ecosystem bestowing toxicity and impede the biogeochemical cycles. Besides, microplastic particles have been known for their role in aggravating the effect of various other environmental pollutants including organic pollutants and heavy metals. These microplastic surfaces are often colonized by the microbial communities also known as "plastisphere microbes" forming biofilms. These microbes include cyanobacteria like Nostoc, Scytonema, etc., and diatoms like Navicula, Cyclotella, etc. Which become the primary colonizer. In addition to the autotrophic microbes, Gammaproteobacteria and Alphaproteobacteria dominate the plastisphere microbial community. These biofilm-forming microbes can efficiently degrade the microplastic in the environment by secreting various catabolic enzymes such as lipase, esterase, hydroxylase, etc. Besides, these microbes have shown great potential for the bioconversion of microplastic to polyhydroxyalkanoates (PHA), an energy efficient and sustainable alternative to the petroleum based plastics. Thus, these microbes can be used for the creation of a circular economy using waste to wealth strategy. This review provides a deeper insight into the distribution, transportation, transformation, and biodegradation of microplastic in the ecosystem. The formation of plastisphere by the biofilm-forming microbes has been described in the article. In addition, the microbial metabolic pathways and genetic regulations involved in the biodegradation have been discussed in detail. The article suggests the microbial bioremediation and upcycling of microplastic along with various other strategies for effectively mitigate the microplastic pollution.

Time-course biofilm formation and presence of antibiotic resistance genes on everyday plastic items deployed in river waters

The plastisphere has been widely studied in the oceans; however, there is little information on how living organisms interact with the plastisphere in freshwater ecosystems, and particularly on how this interaction changes over time. We have characterized, over one year, the evolution of the eukaryotic and bacterial communities colonizing four everyday plastic items deployed in two sites of the same river with different anthropogenic impact. α-diversity analyses showed that site had a significant role in bacterial and eukaryotic diversity, with the most impacted site having higher values of the Shannon diversity index. β-diversity analyses showed that site explained most of the sample variation followed by substrate type (i.e., plastic item) and time since first colonization. In this regard, core microbiomes/biomes in each plastic at 1, 3, 6 and 12 months could be identified at genus level, giving a global overview of the evolution of the plastisphere over time. The measured concentration of antibiotics in the river water positively correlated with the abundance of antibiotic resistance genes (ARGs) on the plastics. These results provide relevant information on the temporal dynamics of the plastisphere in freshwater ecosystems and emphasize the potential contribution of plastic items to the global spread of antibiotic resistance.

Ecological insights into the resilience of marine plastisphere throughout a storm disturbance

Among numerous research about marine plastisphere, the community living on the surface of plastic debris, little attention was given to the ecological mechanisms governing prokaryotes compared to eukaryotes, and even less focused on their resilience in a changing climate with more storm prevalence. Our current research recruited an integrated approach involving community succession across temporal dimension, ecological mechanisms that govern the assembly, and resilience to environmental perturbations to highlight the ecology of different kingdoms in the plastisphere. Towards this goal, we examined the succession of the prokaryotic and eukaryotic communities on artificial plastic nets in a sidestream of seawater from the Gulf of Aqaba over 35 days. A robust local storm enabled investigation of the alterations before, during, and after this disturbance, aiming at the community's potential to recover. Data from 16S and 18S rRNA sequencing and microscopic analyses decrypted the plastisphere diversity, community assembly, and stochasticity, followed by further analyses of functional and co-occurrence networks for the prokaryotic group. Prokaryotic and eukaryotic communities underwent exact opposite ecological mechanisms. While determinism driven by a robust environmental selection dictated the prokaryotic community assembly, stochasticity prevailed when this condition was relaxed. Interestingly, resilience against disturbance was observed in prokaryotes but not in eukaryotes. The decrease in compositional, functional diversity and network complexity in the prokaryotic community was reversed, presumably due to the niche specification process and high dispersal. Niche specification following perturbation was evident in some bacteria by selected functions associated with plastic degradation, stress response, and antibiotic resistance. On the contrary, eukaryotes decreased in diversity and were dominated by the commonly found Chlorophyta towards the later successional period. Novel findings on the ecology of marine plastisphere during perturbation encourage the integration of this aspect into prediction research.

Epiplastic microhabitats for epibenthic organisms: a new inland water frontier for diatoms

Plastic pollution is widespread in each type of ecosystems. However, the colonization events of microorganisms on plastics seem to be neglected in inland waters. Therefore, in this study we analyze the possible colonization on the surface (hereafter epiplastic microhabitats) of two typology of plastic supports by diatom community. Specifically, we located 20 supports in expanded polystyrene and 20 in polyethylene terephthalate both floating and dipped (~ 1 m) in a central Italian shallow water pond, in order to evaluate the diachronic colonization of diatoms from November 2019 to August 2020. Our result showed the tendency in colonizing both epiplastic microhabitats without significant differences in number of species; additionally, depth does not appear to affect the number of species. As regard the temporal colonization, the number of species tends to increase over time from autumn-winter to spring-summer in both types of epiplastic microhabitats and depth. Instead, increase in dominance of some species over time has been demonstrated: only a few species keep a high number of individuals compared to the others; therefore, the number of individuals within the species is not uniformly distributed. These results suggest the tendency of diatom community to colonize plastic supports in lentic waters, and this evidence can be very important because artificial supports can increase the surface available for the settlement of the algae community with an increase of productivity and the colonization of new communities of different taxa. Further studies are mandatory to investigate the possible effects on the epiplastic community and the ecological implications in freshwater environments.

Sustainable treatment of dye wastewater by recycling microalgal and diatom biogenic materials: Biorefinery perspectives

Discharge of untreated or partially treated toxic dyes containing wastewater from textile industries into water streams is hazardous for environment. The use of heavy metal(s) rich dyes, which are chemically active in azo and sulfur content(s) has been tremendously increasing in last two decades. Conventional physical and chemical treatment processes help to eliminate the dyes from textile wastewater but generates the secondary pollutants which create an additional environmental problem. Microalgae especially the diatoms are promising candidate for dye remediation from textile wastewater. Nanoporous diatoms frustules doped with nanocomposites increase the wastewater remediation efficiency due to their adsorption properties. On the other hand, microalgae with photosynthetic microbial fuel cell have shown significant results in being efficient, cost effective and suitable for large scale phycoremediation. This integrated system has also capability to enhance lipid and carotenoids biosynthesis in microalgae while simultaneously generating the bioelectricity. The present review highlights the textile industry wastewater treatment by live and dead diatoms as well as microalgae such as Chlorella, Scenedesmus, Desmodesmus sp. etc. This review engrosses applicability of diatoms and microalgae as an alternative way of conventional dye removal techniques with techno-economic aspects.

A techno-economic approach for eliminating dye pollutants from industrial effluent employing microalgae through microbial fuel cells: Barriers and perspectives

Microbial fuel cells are biochemical factories which besides recycling wastewater are electricity generators, if their low power density can be scaled up. This also adds up to work on many factors responsible to increase the cost of running a microbial fuel cell. As a result, the first step is to use environment friendly dead organic algae biomass or even living algae cells in a microbial fuel cell, also referred to as microalgal microbial fuel cells. This can be a techno-economic aspect not only for treating textile wastewater but also an economical way of obtaining value added products and bioelectricity from microalgae. Besides treating wastewater, microalgae in its either form plays an essential role in treating dyes present in wastewater which essentially include azo dyes rich in synthetic ions and heavy metals. Microalgae require these metals as part of their metabolism and hence consume them throughout the integration process in a microbial fuel cell. In this review a detail plan is laid to discuss the treatment of industrial effluents (rich in toxic dyes) employing microbial fuel cells. Efforts have been made by researchers to treat dyes using microbial fuel cell alone or in combination with catalysts, nanomaterials and microalgae have also been included. This review therefore discusses impact of microbial fuel cells in treating wastewater rich in textile dyes its limitations and future aspects.

Design and Development of Fluorinated and Biocide-Free Sol–Gel Based Hybrid Functional Coatings for Anti-Biofouling/Foul-Release Activity

Biofouling has destructive effects on shipping and leisure vessels, thus producing severe problems for marine and naval sectors due to corrosion with consequent elevated fuel consumption and higher maintenance costs. The development of anti-fouling or fouling release coatings creates deterrent surfaces that prevent the initial settlement of microorganisms. In this regard, new silica-based materials were prepared using two alkoxysilane cross-linkers containing epoxy and amine groups (i.e., 3-Glycidyloxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane, respectively), in combination with two functional fluoro-silane (i.e., 3,3,3-trifluoropropyl-trimethoxysilane and glycidyl-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononylether) featuring well-known hydro repellent and anti-corrosion properties. As a matter of fact, the co-condensation of alkoxysilane featuring epoxide and amine ends, also mixed with two opportune long chain and short chain perfluorosilane precursors, allows getting stable amphiphilic, non-toxic, fouling release coatings. The sol–gel mixtures on coated glass slides were fully characterized by FT-IR spectroscopy, while the morphology was studied by scanning electron microscopy (SEM), and atomic force microscopy (AFM). The fouling release properties were evaluated through tests on treated glass slides in different microbial suspensions in seawater-based mediums and in seawater natural microcosms. The developed fluorinated coatings show suitable antimicrobial activities and low adhesive properties; no biocidal effects were observed for the microorganisms (bacteria).

Plastisphere development in relation to the surrounding biotic communities

To study the early colonization processes, polyethylene terephthalate (PET) microfragments were immersed in Lake Sakadaš and the Drava River and sampled weekly together with the surrounding biotic communities - phytoplankton, zooplankton, epixylon in the lake and epilithon in the river. At the end of the study, a rise in water level occurred in the river, which altered the environmental conditions and plankton communities. In studied environments, all of the sampled biotic communities were diverse and abundant. Plastispheres formed in both waters by the seventh day of incubation and developed rapidly, reaching a peak in abundance on the last day of the study. Initial colonization was supported equally by planktonic and periphytic taxa in both environments, but after initial settlement, plastisphere assemblages were affected differently in the river and lake. This study suggests that PET microfragments are a suitable substrate for microphyte settlement and may provide an important pathway for their transport in dynamic freshwater floodplains and river systems.

Plastic Interactions with Pollutants and Consequences to Aquatic Ecosystems: What We Know and What We Do Not Know

Plastics are a group of synthetic materials made of organic polymers and some additives with special characteristics. Plastics have become part of our daily life due to their many applications and uses. However, inappropriately managed plastic waste has raised concern regarding their ecotoxicological and human health risks in the long term. Due to the non-biodegradable nature of plastics, their waste may take several thousands of years to partially degrade in natural environments. Plastic fragments/particles can be very minute in size and are mistaken easily for prey or food by aquatic organisms (e.g., invertebrates, fishes). The surface properties of plastic particles, including large surface area, functional groups, surface topography, point zero charge, influence the sorption of various contaminants, including heavy metals, oil spills, PAHs, PCBs and DDT. Despite the fact that the number of studies on the biological effects of plastic particles on biota and humans has been increasing in recent years, studies on mixtures of plastics and other chemical contaminants in the aquatic environment are still limited. This review aims to gather information about the main characteristics of plastic particles that allow different types of contaminants to adsorb on their surfaces, the consequences of this adsorption, and the interactions of plastic particles with aquatic biota. Additionally, some missing links and potential solutions are presented to boost more research on this topic and achieve a holistic view on the effects of micro- and nanoplastics to biological systems in aquatic environments. It is urgent to implement measures to deal with plastic pollution that include improving waste management, monitoring key plastic particles, their hotspots, and developing their assessment techniques, using alternative products, determining concentrations of micro- and nanoplastics and the contaminants in freshwater and marine food-species consumed by humans, applying clean-up and remediation strategies, and biodegradation strategies.

Employing newly developed plastic bubble wrap technique for biofuel production from diatoms cultivated in discarded plastic waste

Algal culturing in photobioreactors for biofuel and other value-added products is a challenge globally specifically due to expensive closed or open photobioreactors associated with the high cost, problems of water loss and contamination. Among the wide varieties of microalgae, diatoms have come out as potential source for crude oil in the form of Diafuel™ (biofuel from diatoms). However, culturing diatoms at large scale hypothesized as diatom solar panels for biofuel production is still facing a need for facile and economical production of value-added products. The aim of this work was to culture diatom (microalgae) in a closed system by sealing the reactor rim tightly with very cheap priced and used plastic bubble wrap material which is generally discarded in a lodging and transportation of goods. To optimize it, different plastic wraps discarded from a plastic industry were tested first for their permeability to gases and impermeability to water loss. It was found that among different varieties of plastic bubble wraps, low density polyethylene (LDPE) bubble wrap material which was used to seal glass containers as photobioreactors allowed harvest of maximum Diafuel™ (37%), lipid (35 μgmL^-1), highest cell count (1152 × 10² cells mL^-1), maximum CO₂ absorbance (0.084) with almost no water loss and nutrient uptake for 40 days of experiments. This was due to its permeability to gases and impermeability to water. To check usability of such LDPE bubble wrap on other microalgae it was therefore tested on the red-green microalgae Haematococcus pluvialis, which showed scope to be scaled up for astaxanthin production using discarded bubble wrap packing material. This study thus would open up a new way for decreasing plastic disposal and with reuse for sustainable development and application of diatom in biofuel production which could find applications in environmental and industrial sectors.

Microalgal drugs: A promising therapeutic reserve for the future

Over the decades, a variety of chemically synthesized drugs are being used to cure existing diseases but often these drugs could not be effectively employed for the treatment of serious and newly emerging diseases. Fortunately, in nature there occurs immense treasure of plants and microorganisms which are living jewels with respect to their richness of medically important metabolites of high value. Hence, amongst the existing microorganism(s), the marine world offers a plethora of biological entities that can contribute to alleviate numerous human ailments. Algae are one such photosynthetic microorganism found in both marine as well as fresh water which are rich source of metabolites known for their nutrient content and health benefits. Various algal species like Haematococcus, Diatoms, Griffithsia, Chlorella, Spirulina, Ulva, etc. have been identified and isolated to produce biologically active and pharmaceutically important high value compounds like astaxanthin, fucoxanthin, sulphur polysaccharides mainly galactose, rhamnose, xylose, fucose etc., which show antimicrobial, antifungal, anti-cancer, and antiviral activities. However, the production of either of these bio compounds is favored under conditions of stress. This review gives detailed information on various nutraceutical metabolites extracted from algae. Additionally focus has been made on the role of these bio compounds extracted from algae especially sulphur polysaccharides to treat several diseases with prospective treatment for SARS-CoV-2. Lastly it covers the knowledge gaps and future perspectives in this area of research.

Perovskite-based solar cells fabricated from TiO2 nanoparticles hybridized with biomaterials from mollusc and diatoms

Perovskite solar cells (PVSCs) convert solar energy into electrical energy. Current study employs fabrication of PVSCs using calcium titanate (CaTiO₃) prepared by co-precipitation of TiO₂ nanoparticle (NP) and CaCO₃ NP with later synthesized from mollusc shell. Furthermore, frustules of diatom, Nitzschia palea were used to prepare silica doped CaTiO₃ (Si-CaTiO₃) nanocomposite. CaTiO₃ NP and Si-CaTiO₃ nanocomposites film were made on fluorine doped tin oxide (FTO) glass plate using spin coater separately for two different kinds of PVSCs tested at different intensities of light. The perovskite materials were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray (EDX) spectroscopy. Thickness of the film was measured by profilometer. The maximum power density (PD_max) of CaTiO₃ made PVSCs was 0.235 mW/m² under white LED light and 0.041 mW/m² in broad spectrum light. Whereas, PD_max of PVSCs with Si-CaTiO₃ was higher about 0.0083 mW/m² in broad spectrum light and was 0.0039 mW/m² in white LED light. This is due to the fact that CaTiO₃ allowed blue and red light in broad spectrum to pass through it without being absorbed compared to white LED light which gets reflected. On the offset, in PVSC made of Si-CaTiO₃ since diatoms frustules are made up of nanoporous architecture it increases the overall porosity of PVSC making them potentially more efficient in broad spectrum of light compared to white LED light.

Impact of light on microalgal photosynthetic microbial fuel cells and removal of pollutants by nanoadsorbent biopolymers: Updates, challenges and innovations

Photosynthetic microbial fuel cells (PMFCs) with microalgae have huge potential for treating wastewater while simultaneously converting light energy into electrical energy. The efficiency of such cells directly depends on algal growth, which depends on light intensity. Higher light intensity results in increased potential as well as enhancement in generation of biomass rich in biopolymers. Such biopolymers are produced either by microbes at anode and algae at cathode or vice versa. The biopolymers recovered from these biological sources can be added in wastewater alone or in combination with nanomaterials to act as nanoadsorbents. These nanoadsorbents further increase the efficiency of PMFC by removing the pollutants like metals and dyes. In this review firstly the effect of different light intensities on the growth of microalgae, importance of diatoms in a PMFC and their impact on PMFCs efficiencies have been narrated. Secondly recovery of biopolymers from different biological sources and their role in removal of metals, dyes along with their impact on circular bioeconomy have been discussed. Thereafter bottlenecks and future perspectives in this field of research have been narrated.

Diatom microalgae as smart nanocontainers for biosensing wastewater pollutants: recent trends and innovations

Microalgae have been recognized as one of the most efficient microorganisms to remediate industrial effluents. Among microalgae diatoms are silica shelled unicellular eukaryotes, found in all types of water bodies and flourish very well even in wastewater. They have their silica cell wall made up of nano arrayed pores arranged in a uniform fashion. Therefore, they act as smart nanocontainers to adsorb various trace metals, dyes, polymers, and drugs which are hazardous to human as well to aquatic life. The beautiful nanoarchitecture in diatoms allows them to easily bind to ligands of choice to form a nanocomposite structure with the pollutants which can be a chemical or biological component. Such naturally available diatom nanomaterials are economical and highly sensitive compared to manmade artificial silica nanomaterials to help in facile removal of the toxic pollutants from wastewater. This review is thus focused on employing diatoms to remediate various pollutants such as heavy metals, dyes, hydrocarbons detected in the wastewater. It also includes different microalgae as biosensors for determination of pollutants in effluents and the perspectives for nanotechnological applications in the field of remediating pollutants through microalgae. The review also discusses in length the hurdles and perspectives of employing microalgae in wastewater remediation.

Live diatoms as potential biocatalyst in a microbial fuel cell for harvesting continuous diafuel, carotenoids and bioelectricity

Microbial fuel cell (MFC) with live diatoms (Nitzschia palea) displacing bacteria in the anodic chamber generated electrical potential. Unlike other microalgae, diatoms fix 25% of atmospheric CO₂, thus releasing O₂. They perform photolysis of water by photosynthesis in the plastid during light photoperiod and cellular respiration in the mitochondria during dark, producing electrons and protons, respectively. The electrogenic property of diatom was explored and evaluated by comparing the potential changes with reference fuel cell without diatoms and that operated with diatoms in the anodic chamber. Such photosynthetic diatom microbial fuel cell (PDMFC) employed f/2 media rich in nitrates, phosphates, metasilicates, trace metals and vitamins as the anolyte and potassium permanganate as catholyte enhanced the output voltage by 3^rd day. The maximum power density for PDMFC was 12.62 mWm^-2 and coulombic efficiency of 22.95%. Besides this, the fixed diatom cells at anode showed about 64.28% increase in lipid production on 15th day compared to that on 1st day along with the increment in formation of complex fatty acid methyl esters and carotenoids during its operation. Hence, diatoms can be envisaged to substitute bacteria in the anodic chamber of MFC to simultaneously produce bioelectricity and other valuable compounds. Further their silica nanoporous architecture serve as good absorbents for heavy metal removal found in many wastewaters.

Insights into diatom microalgal farming for treatment of wastewater and pretreatment of algal cells by ultrasonication for value creation

Wastewater management and its treatment have revolutionized the industry sector into many innovative techniques. However, the cost of recycling via chemical treatment has major issues especially in economically poor sectors. On the offset, one of the most viable and economical techniques to clean wastewater is by growing microalgae in it. Since wastewater is rich in nitrates, phosphates and other trace elements, the environment is suitable for the growth of microalgae. On the other side, the cost of harvesting microalgae for its secondary metabolites is burgeoning. While simultaneously growing of microalgae in photobioreactors requires regular feeding of the nutrients and maintenance which increases the cost of operation and hence cost of its end products. The growth of microalgae in waste waters makes the process not only economical but they also manufacture more amounts of value added products. However, harvesting of these values added products is still a cumbersome task. On the offset, it has been observed that pretreating the microalgal biomass with ultrasonication allows easy oozing of the secondary metabolites like oil, proteins, carbohydrates and methane at much lower cost than that required for their extraction. Among microalgae diatoms are more robust and have immense crude oil and are rich in various value added products. However, due to their thick silica walls they do not ooze the metabolites until the mechanical force on their walls reaches certain threshold energy. In this review recycling of wastewater using microalgae and its pretreatment via ultrasonication with special reference to diatoms is critically discussed. Perspectives on circular bioeconomy and knowledge gaps for employing microalgae to recycle wastewater have been comprehensively narrated.