Penicillin-mediated changes in viable benthic diatom assemblages - insights about the relevance of bacteria across spatial and seasonal scales

Penicillin, a -lactam antibiotic, cannot affect diatoms directly but does so through bacteria. Its effects on viable benthic diatom assemblages in a tropical environment were evaluated across spatial (intertidal sandflat, mangrove site, port environment) and seasonal (post-monsoon, pre-monsoon, monsoon) scales. Penicillin treatment resulted in bacterial suppression and a reduction in diatoms at the intertidal and mangrove sites having stable pennate-dominated assemblages. Diatom response at the port site, dominated by transient centric diatoms, ranged from total inhibition to enhancement. The monsoon diatom assemblages, which differed in composition from those in other seasons, showed reduced susceptibility to penicillin. These observations highlight the bacterial influence on diatom assemblages and the significance of species composition of diatom assemblages in these interactions. Epipsammic diatoms (Cocconeis, Grammatophora and Thalassionema) were more sensitive to penicillin than epipelic forms, probably due to: (1) their inability to escape unfavourable bacterial metabolites because of limited diffusion in benthic environments; and (2) the interference of penicillin with the bacterial conditioning' film required for their attachment. Overall, the observations from the present, preliminary, ecological study provide insights into the relevance of bacteria in influencing viable benthic diatom assemblages across spatial and seasonal scales and emphasise the need for future mesocosm experiments on these aspects.