Description: 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.

Description: The response of the phytoplankton and bacterial spring succession to the predicted warming of sea surface temperature in temperate climate zones during winter was studied using an indoor-mesocosm approach. The mesocosms were filled with winter water from the Kiel Fjord, Baltic Sea. Two of them were started at ~2°C and the temperature was subsequently increased according to the decadal temperature profile of the fjord (ΔT 0°C, baseline treatment). The other mesocosms were run at 3 elevated temperatures with differences of ΔT +2, +4 and +6°C. All mesocosms were exposed to the same light conditions. Timing of peak phytoplankton primary production (PP) during the experimental spring bloom was not significantly influenced by increasing temperatures, whereas the peak of bacterial secondary production (BSP) was accelerated by about 2 d per °C. This suggests that, in case of warming, the spring peak of bacterial degradation of organic matter (in terms of BSP) would occur earlier in the year. Furthermore, the lag time between the peaks of PP and BSP (about 16 d for ΔT 0°C) would diminish progressively at elevated temperatures. The average ratio between BSP and PP increased significantly from 0.37 in the coldest mesocosms to 0.63 in the warmest ones. Community respiration and the contribution of picoplankton (〈3 µm fraction) to this also increased at elevated temperatures. Our results lead to the prediction that climate warming during the winter/ early spring in temperate climate zones will favor bacterial degradation of organic matter by tightening the coupling between phytoplankton and bacteria. However, if PP is reduced by warming, as in our experiments, this will not necessarily lead to increased recycling of organic matter (and CO2).