SOURCE:

Faculty: Biosences
Department: Applied Microbiology And Brewing

CONTRIBUTORS:

Okechi, R. N;
Chukwura, E.I;

ABSTRACT:

Otamiri is one of the major rivers that passes through Owerri urban and its environs. It serves as a source of aquatic food and water for domestic activities, irrigation among others. All the drainages discharge their untreated waste waters into this river. The study assessed the toxicity of heavy metals and sodium dodecyl sulphate (SDS) mixtures to environmental bacterial isolates from Otamiri river water and sediment. Physicochemical parameters of the river water and sediment were analysed using soxhlet extraction, atomic absorption spectrometry, and gas chromatography. Standard microbial techniques such as serial dilution, spread plate culturing techniques, plate counts, morphological and biochemical characterization were used in determining the preponderant bacterial isolates from the river and its sediment. The identities of the preponderant isolates were further confirmed using 16S rRNA gene partial sequencing and were subsequently adopted for the toxicity assay. The toxicities of the heavy metal ions, Pb(II), Cd(II), Ni(II), Zn(II) and Co(II), as individuals, and in binary, ternary, quaternary, quinary and senary mixtures with SDS against the preponderant bacteria from the river water and sediment were assessed, using inhibition of dehydrogenase activity as the response. Similarly, fixed ratio design [Arbitrary concentration ratio (ABCR) and EC50 equieffect concentration ratio (EECR50)]was employed in evaluating the toxicities of the mixtures to the preponderant bacteria. The effects of the mixtures on the dehydrogenase activity were assessed using toxic index, model deviation ratio and isobolographic analyses. In addition, the toxicities of the mixtures were predicted with concentration addition (CA) and independent action (IA) models. The experimentally-derivedEC50S for each toxicants as well as for the four-mixture ratios in each mixture type were compared. Similarly, within each mixture ratio, the experimentally-derived EC50, CA- and IA-models predicted EC50S were equally compared using Duncan post-hoc tests, implemented with SPSS Statistics 21 at P<0.05.In Otamiri river water,iron(Fe) recorded the highest value among the heavy metals (1.972 mg/l), followed by zinc (Zn) (1.556 mg/l), while cobalt (Co) was not detected. Similarly, lead (Pb), cadmium (Cd), nickel (Ni), mercury (Hg), conductivity and turbidity recorded values higher than WHO recommended quality standards for drinking water. In the sediment, Fe and Cd recorded the highest and least values 19.82 and 0.025 mg/kg respectively. The pH of the river and sediment were 6.42 and 5.40. Similarly, SDS was the predominant anionic surfactant in both the river water (0.100 µg/l) and sediment (0.453 µg/kg), while perfluorobutane sulfate was not detected in the river water. The bacteriological analysis showed the presence of Serratia marcescens (SerEW01) (33.33%), Staphylococcus (22.20%), Streptococcus (22.20%), Enterobacter (11.11%), Escherichia coli (11.11%) as well as Acinetobacterseifertii (42.10%), Bacillus (15.80%), Escherichia coli (15.80%), Klebsiella (10.53%) and Streptococcus species (5.30%), in the river water and sediment respectively, with their percentage occurrences. The responses of both bacteria to the inhibitory effects of the individual toxicants and their various mixtures were concentration-dependent, increasing progressively as the concentrations increased.All the dose-response relationships of the ABCR and EECR50 mixtures and the individual toxicantswere described by logistic function.The experimental EC50S ranged from 0.046 ± 0.003 mM (Zn(II)) to 2.329 ± 0.092 mM (SDS) againstS. marcescens (SerEW01) as well as from 0.011 ± 0.000 mM (Cd(II)) to 2.810 ± 0.140 mM (SDS) againstA.seifertii.Duncan tests for both bacteria indicated that the EC50S of the individual toxicants differed significantly from oneanother and the order of decreasing toxicities were Zn(II) > Cd(II) > Co(II) > Ni(II) > Pb(II) > SDS for S. marcescens (SerEW01) and Cd(II) > Co(II) > Zn(II) > Pb(II) > Ni(II) > SDS for A.seifertii.In binary mixtures of SDS+metal ion against S. marcescens (SerEW01), SDS 98.08% + Co(II) 1.92% mixture ratio was hormetic, while CA and IA models predicted similar toxicities in SDS+Ni(II) binary mixtures. In the binary mixtures of SDS+metal ion against A. seifertii, SDS+Co(II) and ABCR3 mixture ratio of SDS+Cd(II) mixture type showed no statistical differences between CA and IA-model predicted EC50S. SDS+Zn(II) binary mixtures were also hormetic at low concentrations. The CA and IA models underestimated the binary mixture toxicities against both organisms. All the ternary mixtures of SDS and two metals were very toxic against S. marcescens (SerEW01), even at low concentration, with EC50S raging from 0.102 ± 0.006 mM to 0.203 ± 0.009 mM. There was no significant difference between CA and IA-models predicted EC50S in ABCR1 and ABCR3 mixture ratios of SDS+Ni(II)+Cd(II) and SDS+Co(II)+Cd(II) ternary mixtures respectively, against A.seifertii. In ABCR1 mixture ratio of SDS+Ni(II)+Cd(II) ternary mixture, both models almost correctly predicted the experimentally-derived data, while the models overestimated the mixture toxicities in the other ternary mixtures. In all quaternary mixtures, both models predicted lower toxicities compared to the experimentally-derived data against S. marcescens (SerEW01). Similarly, the CA model correctly predicted the experimentally-derived data at low concentrations in SDS+Cd(II)+Zn(II)+Pb(II) quaternary mixtures against A.seifertii. In quinary mixtures, ABCR2 mixture ratio of SDS+Cd(II)+Zn(II)+Pb(II)+Co(II) mixture was stimulatory against A.seifertii at low concentrations and both models underestimated the interactive effects of the mixtures on both bacteria. Similarly, senary mixtures of SDS+five metal ions were also toxic against both organisms even at low concentrations. In most mixtures, the interactive effect was strongly synergistic against both bacteria. Otamiri river water and sediment were contaminated by heavy metals and sodium dodecyl sulfate. Some of these heavy metals and SDS inhibited the dehydrogenase activities in the preponderant bacteria from the river water and sediment, both as individual toxicants and their mixtures. The mostly synergistic effect reported in mixtures in this study demonstrates the potential danger of co-contamination of the aquatic ecosystems by SDS and heavy metals.