Ocean acidification modifies the effect of antibiotics on natural marine bacteria communities

Chair: Alistair Hobday

Triranta Sircar (1), Maria Asplund (1), Sam Dupont (1), Maria E. Granberg (2)

1 University of Gothenburg, Department of Biology and Environmental Sciences, Fiskebäckskil, 450 34, Sweden
2 Norwegian Polar Institute, Tromsø, 9296, Norway

Background
Polar (amphoteric) organic contaminants, like antibiotics, change net charge with pH in ways depending on molecular structure. The neutral form is considered most bioavailable and therefore most toxic. In future more acidic oceans, polar organic contaminants will be differently charged than today. The contaminant “toxicity map” may thus have to be redrawn. We investigated the combined effect of antibiotics and current (average 8.1) and reduced (predict ed average in 90 years 7.7) sea water pH on natural pelagic bacterial communities collected during spring bloom and winter conditions in the Gullmar fjord, Sweden.

Methods
Seawater with intact pelagic bacteria communities were exposed to amoxicillin (Amox), ciprofloxacin (Cip) or no antibiotics at pH 8.1 and 7.7 (intermittent CO2 bubbling). Amox and Cip were chosen due to their frequent use, occurrence in the marine environment, and amphoteric properties acting opposite within the relevant pH span. We predicted Cip to have a stronger bactericidal effect at pH 8.1, since a larger fraction then is neutral compared to at pH 7.7. The opposite was predicted for Amox. Bacterial abundance was measured using epifluorescence microscopy (DAPI-staining) and bacterial production was quantified with 14C-leucine incorporation techniques. Bacterial community diversity was identified using DGGE on rpoβ gene amplicons.

Findings
Acidification alone reduced bacterial abundances and stimulated production. A combined stress effect of pH and antibiotics was observed acting on all measured variables, where the presence of Amox augmented the pH effect while Cip hampered or even reversed it. The less diverse winter bacteria community appeared more sensitive, i.e. less resilient, compared to the more diverse spring bloom community. As predicted, the bactericidal effect of Amox was greater at low pH while the opposite was true for Cip.

Conclusions
Impact assessments of ocean acidification on marine organisms should consider pH induced change in contaminant bioavailability and toxicity.