Bacterial community profile and removal efficiency of a horizontal subsurface flow constructed wetland treating floriculture wastewater in Bishoftu town, Ethiopia

Abstract

Bacterial communities associated with wetland plants are major partners in the removal of pollutants from constructed wetland (CWs) treatment plants. However, there is still a knowledge gap on the link among bacterial community structure of wetland, plant composition and treatment efficiency under tropical conditions. The objective of this study was to characterize the bacterial community structure of wetland plants in a horizontal subsurface flow constructed wetland (HSSFCW) in relation to their removal efficiency of nutrients. Sediment samples were collected from the rhizosphere of single planted (Cp), mixed planted (Mp) and unplanted (Up) CW units. The bacterial diversity was analyzed using high throughput sequencing of the 16S rDNA gene. A total of 2793, 2554, and 2282 operational taxonomic units (OTUS) with 26, 28, and 22 phyla were identified from samples of Cp, Mp and Up operational units, respectively. The data showed Firmicutes were dominant in planted CW units, contributing to 40.9% of the bacterial flora in single planted (Cp) CW units; whereas Proteobacteria contributed to 56.4% of the flora in unplanted (Up) units. The genera Trichococcus, Chlorobium and Thermomonas were dominant representing 18.1%, 13.5%, and 14.2% of the microflora in the Cp, Mp, and Up units, respectively. The analysis, in general, showed that higher abundance of the different taxonomic groups (phylum, class, genus, etc) was recorded in planted CW units than the unplanted control units. Species richness (OTUs and Chao) and Shannon diversity index (H’) of the specific bacterial taxa were positively correlated with overall removal efficiencies of BOD₅, COD, NH₄⁺, NO^3- and TP which were 72.4 ± 2.7%, 70.3 ± 1.9%, 96.2 ± 2.2%, 71.3 ± 3.8% and 52.7 ± 7.3%, respectively. In general, the data showed that sediment bacterial communities were influenced by CW planting which, in turn, affected the pollutant removal efficiency of the CWs.

Key words/phrases: Diversity indices, Firmicutes, Illumina sequencing, Pollutant removal, Proteobacteria