Seasonal variability of microbial respiration and bacterial and archaeal community composition in the upper twilight zone

The mesopelagic ('twilight') zone of the ocean is the sandwich layer between euphotic surface waters and the deep ocean. The physical, chemical, and biological processes in this layer strongly influ- ence the marine carbon cycle, although some surpris- ing knowledge gaps remain, e.g. with respect to micro- bial communities. We therefore determined biological parameters such as microbial abundances, community respiration, and bacterial and archaeal community composition (as assessed by 16S rRNA gene PCR dena- turing gradient gel electrophoresis, DGGE) over a sea- sonal time scale (at ca. 46 d intervals for 433 d) at a coastal mesopelagic site in the NW Mediterranean Sea (Point C, 300 m depth) and related them to physico- chemical parameters. Some chemical and most biolog- ical parameters showed strong variability, thus con- firming the emerging view that this water layer is more dynamic than previously thought. Community respira- tion was likely influenced by seasonal changes as indi- cated by its indirect relationship with water density. The high detected community respiration rates (0.23� 1.65 µmol O2 l �1 d �1 ) confirm that the twilight zone plays a critical role as a hub between surface and deep water. Statistical analyses indicate that water stratifica- tion, dissolved organic carbon, and transparent exo - polymeric particles were controlling factors of bacterial community composition, whereas the archaeal com- munity was likely controlled by other mechanisms. A sequence analysis of DGGE bands and a literature comparison suggests some specificity of the mesopela- gic zone regarding bacterial and archaeal community composition, thus further confirming the peculiarity of this water layer.