MEPS 323:21-33 (2006)  -  doi:10.3354/meps323021

ABSTRACT: This study deals with large spatial scale differences in the ratios between bacterial leucine incorporation (TLi: protein synthesis) and thymidine incorporation (TTi: DNA synthesis) in oligotrophic offshore and comparatively more mesotrophic inshore (sub)tropical regions of the Atlantic Ocean. Observations were derived from 2 RV ‘Polarstern’ cruises, one of which traversed a meridional mid-ocean transect while the other followed the African coast line. Average values (from 42°N to 35°S) of TLi, TTi and chlorophyll a (chl a) concentration were 40.3 pmol leucine l^–1 h^–1, 1.32 pmol thymidine l^–1 h^–1 and 0.18 µg chl a l^–1 along the offshore transect, compared to 51.8 pmol leucine l^–1 h^–1, 2.72 pmol thymidine l^–1 h^–1 and 0.29 µg chl a l^–1 along the inshore transect. Mean values of the TLi:TTi ratio (which defines bacterial growth characteristics) were 32.4 in offshore waters and 20.5 in inshore waters. Offshore ratios of TLi:chl a or TTi:chl a (proxy for bacterial substrate) were 274.1 and 8.5, compared to inshore ratios of 198.7 and 10.0, respectively. This means that, per unit of chl a, considerably higher bacterial protein synthesis was supported in water farther from the coast than near the coast, whereas bacterial DNA synthesis per unit chl a was slightly higher in the latter. Because temperature variability along the cruise tracts was rather similar (except in the Benguela upwelling region), we assume that substrate supply was mainly responsible for the observed significant differences in bacterial growth characteristics. In addition, the potential different contributions of picocyanobacteria to leucine uptake (TLi) must be considered. We conclude that the different TLi:TTi ratios in (sub)tropical offshore and inshore waters reflect reactions of the relevant bacterial communities to prevailing environmental conditions. Therefore, we did not interpret our results in the context of the currently used terms ‘balanced’ or ‘unbalanced’ growth. Bacterial community growth may be balanced in both regions of study, but at different levels of the TLi:TTi ratio.

KEY WORDS: Leucine uptake · Thymidine uptake · TLi:TTi ratio · Balanced growth · Marine bacteria · Cyanobacteria · Primary production · Atlantic