# Microalgae Water Bioremediation: Trends and Hot Topics
## 3 Bibliometric Performance Analysis
### Content Analysis
For instance, the growth of the microalgae _Chlorella vulgaris_ (Chlorophyta) was inhibited in the presence of 400 mg/L of phenol [54]. Nevertheless, at tolerable conditions, microalgae have also been shown to remediate pollutant-rich effluents [55; 56; 57; 58]. Green microalgae _Chlorella_ spp., _T. obliquus_ (Chlorophyta) and _Limospira maxima_ (formerly _Spiurulina maxima_) (Cyanobacteria) showed a capability to degrade organic compounds such as phenol [55; 56]. Moreover, _Auxencholorella pyrenoidosis_ (formerly _Chlorella pyrenoidosis_), _Chlamydomonas reinhardtii_, and _Stichococcus bacillaris_ (Chlorophyta) were shown to have more affinity to metal compounds such as iron (Fe) and copper (Cu) due to their importance as micronutrients to microalgae metabolism. Uptake rates of 70% and 98% of Fe and Cu were obtained after exposure of the mentioned species at 4 mg/L metal concentration [57]. Other microalgae, such as _Tetraselmis marina_, _Stigeocolonium tenue_ and _Spirogyra_ spp. (Chlorophyta), have shown potential to metal pollution bioremediation [59]. Apart from chemical toxicity, the removal of pollutants from effluents can be affected by several biotic factors, namely, the production of toxic compounds, enzyme induction, the composition of the microbial community and the number of microorganisms [11; 26; 27; 28]. Therefore, it was important to address such issues in order to developed efficient remediation systems. Apart from microalgae interaction with pollutants, the biological interactions among the bacterial-microalgal community have also been the target of study [26]. Bacteria produce extracellular polymeric substances that act as a bio-flocculation agent of the microalgae [59] which can be an advantage to overcome the harvesting of microalgae. In a swine effluent, co-culture of the bacteria _Sphingobacteria_, _Flavobacteria_, _Terrimonas_ and _Hyphomonas_ with the green microalgae _C. vulgaris_ was shown to induce the formation of microalgal-bacteria aggregates. In this assay, a 94% flocculation activity, at a pH 11, was registered within the co-culture of the mentioned microorganisms [60]. Furthermore, there is evidence that some microalgae species, like _Phaeodactylum tricornutum_ (Bacilariophyta), also present the ability to flocculate as a response to several abiotic factors, such as light, pH, temperature or carbon dioxide concentration [61; 62]. Between the periods 2001-2010 and 2011-2018, the themes \"cultivation\", \"wastewater\", \"nutrient removal\" and \"biofilms\" were interconnected. One of the challenges of water bioremediation is the harvesting of microalgae, since traditional methods, such as centrifugation, are expensive and energy-demanding procedures. For this reason, in the last few years, there has been an increasing interest of the scientific community in developing different approaches and designs of bioreactors and methods of microalgae cultivation which tackle this need [63; 64]. A new bioreactor design combined with a culture method-designated microalgae attachment was developed, namely, comprising microalgal cells attached to a substratum through the secretion of exopolysaccharide, producing a biofilm. In a nutrient-rich environment, such as wastewater, microalgae were shown to reproduce, augmenting the thickness of the biofilm [65; 66]. Furthermore, a previous study had also revealed that a microalgae-bacteria consortium, attached on kenaf fibers as a substratum, was successful in removing nitrogen, phosphorous and atrazine (a toxic compound) from an effluent, registering removal rates of 91%, 85% and 84%, respectively [67]. Herein, several features influencing water bioremediation with microalgae attachment must still be addressed, such as substratum properties and orientation, microalgal-bacteria community and interaction, environmental and operational conditions [65]. A liaison between the themes \"pharmaceuticals\" and \"toxicity\" was also found.