**2.1 The role of bioturbation in bioremediation of organic and inorganic contaminants**

The role and effectiveness of bioturbators in bioremediation is dependent on several conditions, such as the chemical type and quantities of contaminants, the physicochemical properties of the environment, and their accessibility to microbes [38]. Bioturbators are responsible for vital changes in the biological and physicochemical aspects of soils and water [38, 39]. Additionally, aerobic bioturbation can increase benthic digestion and supplement components by stimulating oxygenconsuming bacterial networks that are concerned with pollutant mitigation [8, 11]. In other words, bioturbators are well-suited for a dual-purpose mechanism, namely the production of degradative enzymes for specific contaminants and resistance or protection from significant relative dangerous substances such as heavy metals [15, 38, 39]. Controlling and simplifying bioremediation procedures is a difficult process due to a large number of components including the presence of a microbial community with the ability to detoxify pollutants, the contaminants' accessibility to the microbial community, and abiotic conditions (soil type, temperature, pH, oxygen or other electron acceptors, and substrates) [6, 16, 39].

Bioturbation influences the sediment-water interface's biological, physical, and chemical properties which accounts for the high rate of mineralization of organic matter in the aquatic environment [40]. This operation changes the sediment column distribution of the contaminants [41]. Bioturbation and biotransport can affect the physicochemical characteristics of sediments and sediment pollutants [42–44]. Bioturbation controls the organic matter and nutrient digestion enhances pollutant mobility and transformation [45–48]. The biosorption of organic contaminants into the organic matter during bioremediation reduces its bioavailability for plants (phytoremediation) or degrading organisms (bioaugmentation) [49]. Atrazine removal from sediments is promoted and positively influenced by the adjustment of organic matter and earthworm bioturbation activities, which increases contaminant bioavailability and atrazine sorption rate on their microsites [46, 50]. Previous studies reported positive contributions of earthworm bioturbation to organic pollutant transformation and biodegradation [51, 52] by modifying pore size and metabolism of degrading bacteria groups or accelerating mineralization in bioaugmented soils [50].

Moreover, several studies showed that bioturbation alters the physicochemical characteristics of the water-sediment boundary which promotes the bioavailability of inorganic pollutants to degrading organisms. This is achieved through the modification of sediment particle sizes, pore spaces, moisture content, nutrient content, turbidity, and total organic carbon of the vadose water-sediment [41, 43, 53]. Also, the bioturbation of benthic invertebrates through the mixing of sediments in the underground zone enhanced the electron acceptors (oxygen, nitrate and sulphate) entrance into the vadose zone which triggers geochemical changes that influence metal behavior [54]. The presence of these electron acceptors in the unsaturated zone can activate the RedOx reaction to change the chelating of metals affinities between liquid and solid phases to enhance the quantitative distribution and bioavailability of metal in the sediment [55]. The changes created by the bioturbationattributed redox potentials, pH, organic content, pore spaces can affect metal sorption capacity and improve metal conversion from one phase to another e.g. Cd, Zn [56–58].
