Introduction

Metals are introduced into the coastal environs by anthropogenic activities and are absorbed /incorporated into the water and sediments. A major part of the metals accumulate in the estuarine and continental shelf regions which are the important sinks for the land derived activities. These metals tend to accumulate in water and sediment and move up the aquatic food chain, ultimately reaching human being, and cause chronic and acute ailments (Forstner and Wittmann, 1979). The marine organisms that contribute directly to pollutant degradation are principally bacteria, and to a lesser degree (or indirectly) the fungi, protozoa and benthic invertebrates (Jaysankar et al., 2006). The basis of indication is the changes that occur either in their metabolism or in phenotype of the organisms due to the uptake of these elements. Solubility or bioavailability of metal may be a more accurate predictor of ecosystem toxicity and more specifically a predictor of microbial metal resistance and bioremediation potential (Roane, 1999).The innershelf region of Bay of Bengal off Chennai was examined for microbial diversity and its relation to heavy metal pollution over a period of three years (2006-09).

Metal ion content in Chennai Coast

The metal ion content of water samples was always lower than that found in sediments. The availability of metal ions in sea water did not follow any specific trend in the study area. The bottom water recorded lower copper content than the surface waters and the surface sediments recorded upto 98.49 mg/kg. Cadmium, cobalt and nickel were present at very low concentration in the waters but the sediment had upto 26 and 47 mg/kg respectively. Lead concentration was present both in water and sediments. The highly toxic element mercury was recorded to be in very low concentrations both in water and sediments.

Bacterial populations in the seawater off Chennai differed greatly with the stations. The total microbial population in three seasons of three consecutive years scored maximum during post monsoon (POM) when compared to that of pre monsoon (PRM) and monsoon (MON) seasons.

Total bacterial population in the water samples of the study area was in the range of 104 to 105 CFU/ml. Total bacterial population showed increasing trend during the period of study. Sample collected at shoreline of all traverses showed higher total bacterial populations. Highest total bacterial population was recorded in surface water samples of the Fishing Harbour Traverses (FHT) during the period of study. The sediment samples generally harbour more bacterial populations than water samples and population was in the range of 10⁵ to 10⁶ CFU/ml. The highest total bacterial population of 1.56 x 10⁶ CFU/g soil was observed in the sediment collected from the shoreline of Chennai Harbour Traverses (CHT) during POM season of the year (2008-09).

The total fungal population recorded in water samples was always low and was in the range of 101 to 102 CFU/ml. A slight increase in total fungal population was recorded during the three years of study. The highest total fungal population of 4.2 x 10² CFU/ml was scored in water samples of CHT shoreline during POM season (2008 - 2009), but the sediment collected at CHT shoreline during the same period exhibited 5.6 x 10² CFU/g soil. Total bacterial and fungal populations and metal resistant bacterial and fungal populations were high in sediment samples than in water samples of the study area in all the seasons of the three consecutive years.

Nickel resistance

Among the different metals resistance studied, Ni resistance was dominant among microbial populations (Figs .1 & 2). A total of 1.24 x 10⁵ CFU/ml bacterial colonies showed Ni resistance in the water samples of CHT collected during POM of 2008-09. But in sediment of CHT for the year of 2008-09 it was recorded as 1.88 x 10⁵ CFU/g soil. POM samples generally scored high for both total and metal resistant population in comparison with PRM and MON seasons. Among the Ni resistant bacterial isolates, Vibrio spp. was the most dominant organism recorded from all traverse of the study area which was followed by Bacillus sp., Halomonas sp., Micrococcus sp., Marinobacter sp., Pseudomonas sp., Alteromonas sp., Acinetobacter sp. and  Flavobacterium sp. The highest number of Ni resistant fungal population was scored in water samples of Cooum River Traverses (CRT) and Adyar River Traverses (ART) as 9 x 10¹ CFU/ml for the year 2008-09, and a further higher number of 2.2 x 10² CFU/g was recorded again in sediments of CRT collected during the same year. The Ni resistant fungal population was always higher in POM samples than PRM and MON samples. Among the fungal isolates Penicillium spp. formed the most dominant Ni resistant organism followed by A. niger, A. terreus, A. japonicus, A. fumigatus, Mucor spp., Trichoderma spp. and non-sporulating fungal isolates.

Fig. 1 Total number of nickel resistant bacterial isolates in pre monsoon and post monsoon season. A – Vibrio spp., B – Pseudomonas sp., C – Halomonas sp., D – Flavobacterium sp., E – Acinetobacter sp., F – Alteromonas sp., G – Marinobacter sp., H – Bacillus sp., I – Micrococcus sp.

Fig. 2 Total number of nickel resistant fungal isolates in pre monsoon and post monsoon season.
A - Penicillium spp., B - Aspergillus niger, C - A. terreus, D - A. japonicus, E - A. fumigatus, F - Trichoderma spp., G - Mucor spp., H - Non-sporulating fungal isolates.

Nickel resistance

Lead resistance was the second dominant metal resistance recorded among the microbial population in the study area during the study period. The water and sediment samples of CHT for the year 2008-09 scored 1.71 x 105 CFU/ml and 2.88 x 106 CFU/g as Pb resistant bacterial populations respectively. Vibrio spp. formed the most dominant group resistant to Pb followed by Bacillus spp., Halomonas sp., Micrococcus sp., Pseudomonas sp., Alteromonas sp., Marinobacter sp., Acinetobacter sp. and Flavobacterium sp. The highest Pb resistant fungal population of 8 x 10¹ CFU/ml was recorded in the water samples of CRT during 2008-09 and at the same period the sediment samples scored as high as 1.4 x 10² CFU/g. Among the fungal isolates resistant to Pb, Penicillium spp. formed the most dominant group followed by Aspergillus niger, A. terreus, A. japonicus, A. fumigatus, Trichoderma spp., Mucor spp. and non-sporulating fungal Isolates.

Mercury resistance

Mercury resistant bacterial population of 1.12 x 10⁵ CFU/ml and 1.48 x 10⁵ CFU/g were scored in water and sediment samples of CHT respectively for the year 2008-09. The Hg resistant bacterial population was higher during POM than that of PRM and MON seasons. Vibrio spp. formed the most dominant group of organisms resistant to Hg followed by Bacillus sp., Pseudomonas sp., Micrococcus sp., Halomonas sp., Alteromonas sp., Marinobacter sp., Marinomonas sp., Flavobacterium sp., Alcaligenes sp. and Acinetobacter sp. The Hg resistant fungal population of 5 x 10¹ CFU/ml was scored in water samples of CRT for the year 2008-09 and the shoreline sediment samples of Ennore Traverses (ET) scored a population of 7 x 10¹ CFU/g during the same period. Penicillium spp. formed the most dominant group followed by A. niger, A. terreus, A. japonicus, A. fumigatus, Mucor spp., Trichoderma spp. and non-sporulating fungal isolates.

Cadmium resistance

The cadmium resistant bacterial population were 8.6 x 10⁴ CFU/ml and 10.8 x 10⁴ CFU/g in water and sediment samples collected at CHT for the year 2008-09 respectively. Vibrio spp. was the most dominant group of organisms resistant to Cd isolated from all transects of the study area followed by Bacillus sp., Pseudomonas sp., Micrococcus sp., Halomonas sp., Alteromonas sp., Marinobacter sp., Flavobacterium sp. and Acinetobacter sp. The Cd resistant fungal populations of 6 x 10¹ CFU/ml and 1.3 x 10²CFU/g were scored in water samples of CRT and the sediment samples of ET respectively for the year 2008-09. Penicillium spp. formed the most dominant Cd resistant group followed by A. niger, A. terreus, A. japonicus, A. fumigatus, Mucor spp., Trichoderma spp. and nonsporulating fungal isolates.

Cobalt resistance

The cobalt resistant bacterial population of 8.8 x 10⁴ CFU/ml and 1.28 x 10⁵ CFU/g were scored in water samples at ET and sediment samples of CRT respectively. Vibrio spp. was the most dominant group of organisms resistant to Co which were observed to be present in all traverses of the study area followed by Bacillus sp., Pseudomonas sp., Micrococcus sp., Halomonas sp., Marinobacter sp., Alteromonas sp., Acinetobacter sp. and Flavobacterium sp. The high Co resistant fungal populations of 6 x 10¹ CFU/ml and 1.3 x 10² CFU/g were scored in water samples and the sediment samples of CRT respectively (2008-09). While Penicillium spp. formed the most dominant group of Co resistant fungal isolates, A. niger, A. terreus, A. japonicus, A. fumigatus, Mucor spp., Trichoderma spp. and non-sporulating fungal isolates were in the order of their frequency of occurrence.

Copper resistance

The high copper resistant bacterial population of 7.3 x 10⁴ CFU/ml and 10.3 x 10⁴ CFU/g was scored in water and sediment samples of CHT respectively (2008-09). Vibrio spp. was the most dominant group of organisms resistant to Cu isolated from all transects of the study area followed by Bacillus sp., Halomonas sp., Pseudomonas sp., Marinobacter sp., Micrococcus sp., Alteromonas sp., Acinetobacter sp. and Flavobacterium sp. in the order of frequency of occurrence. The high Cu resistant fungal population of 8 x 10¹ CFU/ml and 1.4 x 10² CFU/g were scored in water and sediment samples collected at CRT respectively for the year 2008-09 during POM season. While Penicillium spp. formed the most dominant group of Cu resistant organisms, A. niger, A. terreus, A. japonicus, A. fumigatus, Mucor spp., Trichoderma spp. and non-sporulating fungal isolates followed in the order of their frequency of occurrence. Almost all metal resistant bacterial and fungal isolates were found to have increased in POM samples collected in the near shore region when compared to samples collected during PRM and MON season. The bacterial and fungal heavy metal resistance isolates were found to have increased during these three consecutive years.

Conclusion

The concentration of metal ions in the Chennai coast was found to be increasing during the three year study period. The microbial population also was found to be higher and the incidence of metal resistance among the microbial population was very significant and the resistance level seems to be increasing over the period of study indicating the continuous stress of metal pollution on microbial diversity. Vibrio spp. and Bacillus sp. formed the major groups of metal resistant populations. Among the fungal population Penicillium sp. and Aspergillus sp. formed the major groups of metal resistant population. The results strongly indict anthropogenic sources for strongly to extremely polluted with respect to Cu and Cd and unpolluted to moderately polluted with respect to Ni and Cr. The level of both enrichment and contamination factor are in the order of Cd > Cu > Cr > Ni > Pb > Co > Zn > Mn > Fe > Hg. The results of the study implies that continuous monitoring has to be carried out to ascertain the long term impact of anthropogenic inputs to take remedial measures so as to ensure the health of our invaluable marine ecosystem.

References

Forstner, U. and Wittmann, G. T. W. (1979) Metal pollution in the aquatic environment. Springer- Verlag, NewYork.

Jaysankar, De., Sarkar, A. and Ramaiah, N. (2006) Bioremediation of toxic substances by mercury resistant marine bacteria. Ecotoxicology 15:385- 389.

Roane, T. M. (1999) Lead resistance in two bacterial isolates from heavy metal - contaminated soils. Microbial Ecology 37:218-224.