Carbon demand, utilization, and metabolic diversity of bacterioplankton in the frontal regimes of the Indian sector of the Southern Ocean

Abstract

Bacterial production, respiration and metabolic diversity were measured up to 120 m depth in the Sub-Antarctic Front (SAF) and Polar Fronts I and II (PFI and PFII) of the Indian Ocean sector of the Southern Ocean during 2010 Austral Summer. Prokaryotic cell count was maximum at PFI and PFII (~10⁹ cells L⁻¹) and minimum at SAF (~10⁷ cells L⁻¹). Furthermore, integrated bacterial production was higher at PFI (1.07 mg C m⁻² h⁻¹) and PFII (0.72 mg C m⁻² h⁻¹) compared to SAF (0.61 mg C m⁻² h⁻¹). At PFII, integrated bacterial growth efficiency was higher (8.96) compared to PFI (7.42) and SAF (7.17), signifying that the net contribution of PFII to the microbial loop could be relatively pronounced. Enhanced cell numbers and production at polar fronts indicate that the dissolved organic matter could be converted to secondary biomass through the microbial loop. However, integrated bacterial respiration rate at PFII (0.83 mg C m⁻² h⁻¹) was lower than that at PFI (1.84 mg C m⁻² h⁻¹) resulting in higher growth efficiency at PFII. Metabolic flexibility at SAF was clearly brought about by utilization of carboxylic acids like D-malic acid and itaconic acid, and carbohydrates like N-acetyl D-glucosamine, D-cellobiose and D-lactose. Utilization of amino acids like glycyl L-glutamic acid and L-threonine, and an amine, phenylethylamine, was critical in determining the metabolic variability at PFI. PFII hosted microbes that utilized phenolic compounds (2-hydroxy benzoic acid and 4-hydroxy benzoic acid) and polymers (like Tween 80). Utilization of polyols over carbohydrates in polar waters indicates a niche with lesser influence of the Antarctic melt waters on the bacterioplankton metabolism.