**2.2 Factors that influence bioremediators or bioturbators for pollutant remediation**

The activities of bioturbators are affected by some factors which modulate the rate of bioturbation for effective remediation of polluted environments. These factors include the variation in salinity, temperature, density, sediment grain size pH, and concentration.

## *2.2.1 Variation in salinity level*

Variation in salinities in the aquatic environment can influence the metabolism of nutrient and metal releases [59, 60], whether naturally and/or through human-related activities. Remaili et al. [61] noted that hypersalinity has a negative effect on the larger bioturbators which affects the activities of benthic organisms. Gonzalez et al. [62] study found that the salinity levels and tolerance of various bioturbators are distinct. The findings however suggest that ammonia release in the aquatic environment is significantly modified due to the effect of modulating conditions and distinguished by a higher salinity than other nutrients such as phosphorus [62, 63].

### *2.2.2 Temperature variation*

Regional variability in temperature is also a crucial factor that regulates the impact of bioturbation in pollutant remediation. In microbial response, metabolism, and degradation of organic matter and metals, temperatures played a fair modulatory function [64]. In the presence of bioturbation activities, the rise in temperatures increases the production of ammonium from the sediment, possibly due to the high level of hydrogenase in microbial species and the increased aerobic conditions in the sediment [64, 65]. Gonzalez et al. [62] reported that an increase in temperature is indirectly proportional to the nutrient dispersion as high temperature decreases nutrient flux (phosphorus) in the sediment but extreme temperatures may be devastating to the microbes. However, an increase in temperature corresponds to the increased rate of metal resuspension and metal solubility as a result of higher bioturbation rates [66, 67].

#### *2.2.3 Bioturbator density*

The bioturbator density influences bioturbation, control bioturbation efficiency for contaminant remediation, which correlates with the increased aerobic microbial activity and emission of pollutants. The increased bioturbation density increased phosphorus release and induced aerobic microbial activity but did not increase the release of ammonia. Animal density is a highly imperative factor, as study reveals that higher densities contribute toward greater degradation and mineralization of organic matter but may also increase nutrients in the overlying water and can, depending on the ecosystem studied, have counterproductive effects on recovery [66]. In response to pollution, the population of certain benthic species such as polychaetes [68] may increase as several systems are deprived of the use of other larger bioturbators.

#### *2.2.4 Sediment grain size*

Another element that influences the high level of organic matter and metals accumulation and the structure and metabolism of microbial communities and their metabolism is the sediment grain size [69, 70]. A recent study also shows a positive association between ammonia, phosphorus release, and aerobic microbial activity for the sediment grain size as Martinez-Garcia et al. [70] noted that the grain size showed less effect at low organic enrichments, but instead, at higher enrichments, coarse sediments contain less organic matter and nutrients while metabolism rate is enhanced. The contaminant bioavailability assessment can be affected by the susceptibility, grain size and behavior of microbes used in bioassays or observed on the ground, and the interaction between various species and microbial populations in highly polluted sediments depauperated by larger invertebrates [1, 71].

#### *2.2.5 Contaminant pH and concentration in the sediment*

The concentration of organic or inorganic contaminants is another factor that regulates the activities of the benthic organisms [72] which tend to either reduce or hinder the activities of the benthic organism at a high concentration, beyond the tolerable limit, which can result in the death of these organisms at extreme condition due to toxicity [5]. Benthic organisms have varying tolerance limits for sediment contaminations and tend to possess special features or activities (such as bioaccumulation or biosorption) to enable them to adapt and function effectively in high pollutant concentrations. For metal remediation, abiotic factor-like pH which works closely with concentration may be a crucial modulating variable that determines the impact of bioturbation in the marine environment which can alter metal speciation and reactivity [66].

Therefore, sediment properties like particle size and concentration as well as contaminant shape (sulphides or organic carbon) can affect the bioavailability of the contaminant. Also, in most environments, temperature and type of organism activity or population density can increase or decrease contaminant exposure or bioavailability for bioremediation [61, 73–75].
