Primary production in a subtropical stratified coastal lagoon - contribution of anoxygenic phototrophic bacteria.

摘要

Anaerobic anoxygenic phototrophic bacteria can be found in the suboxic waters of shallow stratified coastal systems, and may play important roles in the total primary production of subtropical stratified coastal lagoons. We investigated the spatiotemporal variability of light CO₂ fixation and net oxygen production in the stratified Conceicao Lagoon (Brazil) in summer and fall of 2007, as well as the contribution of bacteriochlorophyll a (BChl a)-containing bacteria to photosynthetically driven electron transfer. Both chlorophyll a (Chl a) and BChl a varied in space, while only BChl a varied in time (three-fold increase from summer to fall). In summer, net oxygen production and light CO₂ fixation were correlated, with both having higher rates with higher Chl a concentrations in the enclosed region of the lagoon. In fall, CO₂ fixation was decoupled from oxygen production. Denaturing gradient gel electrophoresis revealed that bacterial communities of oxic site 12 and suboxic site 33 formed one cluster, different from other oxic samples within the lagoon. In addition, BChl a/Chl a ratios at these sites were high, 40% and 45%, respectively. Light acted as the main factor controlling the BChl a concentration and CO₂ fixation rates. High turbidity within the enclosed area of the lagoon explained high BChl a and decoupling between CO₂ fixation and oxygen production in oxygenated waters. Contribution of purple sulfur bacteria to total bacterial density in suboxic waters was 1.2%, and their biomass contributed to a much higher percentage (12.2%) due to their large biovolume. Our results indicate a significant contribution of anaerobic anoxygenic bacteria to the primary production of the "dead zone" of Conceicao Lagoon.