Dynamics of bacterial carbon metabolism at the entrance of a tropical eutrophic bay influenced by tidal oscillation

Abstract

Bacterial metabolic rates were assessed at the entrance to Guanabara Bay, SE Brazil, during the summer (i.e. wet) season, in order to evaluate short-term effects of tidal oscillation on the magnitude of carbon flow through the bacterioplankton. Bacterial production (BP), respiration (BR) and abundance, phytoplankton production and biomass, particulate and dissolved organic matter (POM and DOM) concentrations and elemental composition, nutrient concentrations and hydrological profiles were estimated at a fixed station during 3 consecutive days with intervals of 3 h, in surface and bottom (20 m) layers. The study period covered the end of the spring tide and the beginning of the neap tide. Bacterial metabolic rates were highly variable, and a small fraction of the dissolved organic carbon (DOC) pool was used by bacterioplankton (DOC turnover: 23 to 71 d). The input of allochthonous DOC, inferred from DOM elemental composition (C:N:P surface~1 50:9:1; C:N:P bottom 245:6:1), and temperature were the main controlling factors of bacterial carbon metabolism. Although the magnitude of carbon flow through the bacterioplankton was highly variable, there were nevertheless consistent differences between layers. At the surface, the particulate organic carbon (POC) production (~14.3 µM C h -1 ), mainly due to phytoplankton, was higher than BR, and bacterioplankton acted equally as POC producers and DOM remineralizers (bacterial growth efficiency ~52%). Near the bottom, BR was equivalent to total POC production (~0.3 µM C h -1 ) and higher than BP (~0.05 µM C h -1 ), and thus bacteria acted mostly as a DOM sink. This study highlights the importance of short-term variations in carbon flow through bacteria for understanding the carbon cycle of estuarine systems. KEY WORDS: Bacterial production · Bacterial respiration · Bacterial growth efficiency · Carbon flux · Guanabara Bay · EstuaryResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 50: 123-133, 2008 ration (BR), which represent, respectively, the amount of carbon incorporated into bacterial biomass and that used for ATP generation and thus lost as CO 2 (Jahnke & Craven 1995, del Giorgio et al. 1997.The sum of BP and BR provides an estimate of the total amount of organic carbon assimilated by bacteria, the bacterial carbon demand (BCD), as well as the proportion of carbon actually converted into bacterial biomass, the bacterial growth efficiency (BGE). The latter is a measure of the physiological state of bacterial cells, and can also be used as an indicator of the bacterioplankton role in a system: a high BGE indicates that bacteria are more important as a source of particulate organic matter, while a low BGE indicates that they act mainly as a sink of organic matter and a source of CO 2 and other inorganic nutrients (del Giorgio & Cole 1998).Estuaries and coastal bays are complex hydrodynamic systems, under both oceanic and terrestrial influence, driven by riverine input on the one hand and tidal oscillation on the ...