the ability of specific plants species and plant-microbes interactions in improving the
removal efficiency of pharmaceuticals in CWs is considered unclear [27, 60].
In previous studies, indirect approaches like mass balance estimations without
detailed process identification have been applied to assess microbial degradation
[50]. Thus, little information is currently available on integrating microbial community structure with pharmaceutical pathways of metabolism in CWs [60]. One of the
key questions posed by the scientific community was whether pharmaceuticals,
namely, antibiotics, could induce toxic effects to the microbial community. Weber
et al. [99] findings indicated that the presence of ciprofloxacin had an adverse effect
on the bacterial communities in CW systems, initially reducing their ability to
assimilate anthropogenic carbon-based compounds, but the bacterial communities
returned to normal functioning after a 2–5 week acclimation period. Fernandes et al.
[100] saw that microbial communities in CWs treating livestock wastewaters with
veterinary antibiotics (enrofloxacin and tetracycline) were able to adapt without
significant changes. Overall, it seems that CWs microbial community may be
affected by short pulses of higher levels of antibiotics but should be able to recover
easily. For other pharmaceuticals, e.g. ibuprofen and iohexol, no effects to the
microbial community have been noted [101].
Regarding the microbial degradation itself, there is overall (not only for CWs) a
lack of studies on the specific organisms and genes involved in the biodegradation of
pharmaceuticals. For instance, it is reported that polar-acidic pharmaceutically active
compounds such as ibuprofen can be degraded by indigenous microbial communities in the wetland ecosystem, as well as Serratia quinivorans, Corynebacterium
segmentosum, and Escherichia coli (species commonly found in municipal wastewater) [60]. Despite the wide consortium of microorganisms present in inflowing
wastewaters and in biofilm communities, only a few microorganisms are expected to
transform organic micropollutants. Studies in other types of biofilm reactors have
suggested that only around 0.1% of the biomass was actually contributing to the
degradation of this type of compounds [102]. First, the relatively low concentration
of pharmaceuticals compared to other pollutants in wastewater may be insufficient to
induce enzymes that are capable of degrading the pharmaceuticals. Thus, it is
unlikely that pharmaceuticals will be favourable energy or carbon sources for
microorganisms [50]. So, co-metabolism has been suggested as a possible mechanism for the biotransformation of pharmaceuticals and other organic
micropollutants. It is known that autotrophic ammonia oxidizing bacteria and/or
heterotrophic microbes plays a role in pharmaceuticals degradation activity
[57, 60]. Balcom et al. [103] found that the relative abundance of sequences
associated with ammonia monooxygenase, which was able to hydroxylate ibuprofen, was highest in biofilm samples from planted tanks. A study focused on removal
of ibuprofen by CWs found that both the interstitial water and biofilm microbial
community metabolic function were influenced by CW design, plant presence and
species, but design had a greater influence than plants. Moreover, canonical correlation analysis indicated that biofilm microbial communities in three designs (mimicking HSSF, VF and aerated HSSF) played a key role in ibuprofen degradation. The
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P. N. Carvalho
removal efficiency of pharmaceuticals in CWs is considered unclear [27, 60].
In previous studies, indirect approaches like mass balance estimations without
detailed process identification have been applied to assess microbial degradation
[50]. Thus, little information is currently available on integrating microbial community structure with pharmaceutical pathways of metabolism in CWs [60]. One of the
key questions posed by the scientific community was whether pharmaceuticals,
namely, antibiotics, could induce toxic effects to the microbial community. Weber
et al. [99] findings indicated that the presence of ciprofloxacin had an adverse effect
on the bacterial communities in CW systems, initially reducing their ability to
assimilate anthropogenic carbon-based compounds, but the bacterial communities
returned to normal functioning after a 2–5 week acclimation period. Fernandes et al.
[100] saw that microbial communities in CWs treating livestock wastewaters with
veterinary antibiotics (enrofloxacin and tetracycline) were able to adapt without
significant changes. Overall, it seems that CWs microbial community may be
affected by short pulses of higher levels of antibiotics but should be able to recover
easily. For other pharmaceuticals, e.g. ibuprofen and iohexol, no effects to the
microbial community have been noted [101].
Regarding the microbial degradation itself, there is overall (not only for CWs) a
lack of studies on the specific organisms and genes involved in the biodegradation of
pharmaceuticals. For instance, it is reported that polar-acidic pharmaceutically active
compounds such as ibuprofen can be degraded by indigenous microbial communities in the wetland ecosystem, as well as Serratia quinivorans, Corynebacterium
segmentosum, and Escherichia coli (species commonly found in municipal wastewater) [60]. Despite the wide consortium of microorganisms present in inflowing
wastewaters and in biofilm communities, only a few microorganisms are expected to
transform organic micropollutants. Studies in other types of biofilm reactors have
suggested that only around 0.1% of the biomass was actually contributing to the
degradation of this type of compounds [102]. First, the relatively low concentration
of pharmaceuticals compared to other pollutants in wastewater may be insufficient to
induce enzymes that are capable of degrading the pharmaceuticals. Thus, it is
unlikely that pharmaceuticals will be favourable energy or carbon sources for
microorganisms [50]. So, co-metabolism has been suggested as a possible mechanism for the biotransformation of pharmaceuticals and other organic
micropollutants. It is known that autotrophic ammonia oxidizing bacteria and/or
heterotrophic microbes plays a role in pharmaceuticals degradation activity
[57, 60]. Balcom et al. [103] found that the relative abundance of sequences
associated with ammonia monooxygenase, which was able to hydroxylate ibuprofen, was highest in biofilm samples from planted tanks. A study focused on removal
of ibuprofen by CWs found that both the interstitial water and biofilm microbial
community metabolic function were influenced by CW design, plant presence and
species, but design had a greater influence than plants. Moreover, canonical correlation analysis indicated that biofilm microbial communities in three designs (mimicking HSSF, VF and aerated HSSF) played a key role in ibuprofen degradation. The
404
P. N. Carvalho