Bacterial dynamics during the transition from spring bloom to oligotrophy in the northwestern Mediterranean Sea: relationships with particulate detritus and dissolved organic matter

Abstract

The variability in microbial communities (abundance and biomass), bacterial production and ectoaminopeptidase activity, particulate and dissolved organic carbon (POC, DOC), and particulate and dissolved lipids was examined in spring 1995 in the northwestern Mediterranean, where a transition from the end of a bloom to pre-oligotrophic conditions was observed. Four time series of 36 h each and 4 h sampling intervals were performed at 5 m and at the chlorophyll maximum (30 m) between 11 and 31 May. Simultaneous measurements of pigments, abundance of hetero-and autotrophic flagellates, bacteria and POC enabled the estimation of living POC (defined as autotrophic-C plus heterotrophic-C biomass), and thus the detrital organic carbon. During the first 2 time series (11 to 15 May), the bacterial-C biomass was higher than the autotrophic-C biomass at 5 m (ratio 1.4 and 1.7), whereas the opposite trend was observed in the chlorophyll peak (ratio 0.7 for the first cycle). However, at the end of May, autotrophic-C biomass was equivalent to bacterial-C biomass at both depths studied. The detrital pool remained a more or less constant fraction of the POC (52, 53 and 47% on 11-12 May, 14-15 May and 30-31 May) at the chlorophyll peak, whereas it decreased significantly with time (62 to 53%) at 5 m. Relationships between bacterial activities and evolution of available resources were not systematically evidenced from our 36 h diel cycle data. Nevertheless, at the monthly scale, comparison of bacterial carbon demand (BCD) to potential carbon resources (detrital POC and DOC) showed that bacteria fed differently on the various pools. From ectoaminopeptidase turnover rates and detrital POC, the potential hydrolysis rate of detritus was calculated. Depending on the choice of conversion factors for bacterial production and estimates of hydrolysis turnover rates, it was shown that bacterial hydrolysis of detritus could be one of the DOC accumulation sources. We observed that the percentage of BCD supplied by detrital POC hydrolysis increased in the surface and decreased in the chlorophyll peak. An index of lipid degradation in POC, the lipolysis index, increased during the month at 5 m, also indicating a higher hydrolysis of POC. The opposite trend was observed in the chlorophyll maximum layer. The selective decrease in dissolved lipids in DOC in the chlorophyll maximum layer, particularly free fatty acids, also suggests that bacteria utilized increased fractions of carbon sources from the DOC. We concluded that partitioning between DOC and detritus as resources for bacteria can change during the rapid transition period from mesotrophy to oligotrophy in the northwestern Mediterranean.

Short-scale temporal variability of physical, biological and biogeochemical processes in the NW Mediterranean Sea: an introduction

“…Abundance of bacteria was lower than that found in bloom and post-bloom conditions in this area Van Wambeke et al, 2001). Bacterial production, in carbon units averaged about 29 mg C m −2 d −1 over the top 150 m of the water column , equivalent to 0.2 mg C m −3 , suggesting that bacterial production was very low relative to both total bacterial biomass and phytoplankton production.…”

“…This permitted some interesting comparisons to be made between the variability encountered and that recorded during a previous multi-disciplinary study carried out during the spring to summer transition in 1995 detailed in Andersen and Prieur, 2000;Copin-Montégut, 2000;Goutx et al, 2000;Pérez et al, 1997Pérez et al, , 2000Stemmann et al, 2000;Vidussi et al, 2000;Andersen et al, 2001a, b;Van Wambeke et al, 2001. Below we will first very briefly review the cruise plan then highlight some of the findings described in detail in the individual papers of this special section.…”

Seasonal to hour variation scales in abundance and production of total and particle-attached bacteria in the open NW Mediterranean Sea (0–1000 m)

“…These results together with hypothesis mentioned above suggest that bacterial abundance and production was not directly linked to the diel changes in dissolved organic matter produced by phytoplankton (bottom-up control), thus reinforcing the role of top-down control by predation or viral lysis in our sampling conditions. At the DYFAMED station, diel changes in bacterial production was also observed during spring phytoplanktonic bloom (Ghiglione et al 2007), but not during spring to summer transition (Van Wambeke et al, 2001). Such discrepancy can result from seasonal factors or meteorological conditions (rain, cloudy days, wind) that could superimpose their effects on diel variability and consequently, probably prevent us from obtaining reproducible diel patterns (Coffin et al, 1993).…”

“…The seasonal variations of the biogeochemical production regimes have been well studied in this area: deep convection occurs during winter leading to a spring bloom; oligotrophic conditions prevail during summer while perturbations in the meteorological forcing generate a secondary bloom in autumn (Marty, 2002). The vertical and temporal dynamics of heterotrophic bacteria (Van Wambeke et al, 2001;Lemée et al, 2002;Tanaka and Rassoulzadegan, 2004;Misic and Fabiano, 2006) and the contribution of particle-attached bacteria (Turley and Stutt, 2000;Harris et al, 2001) have already been reported in the NW Mediterranean Sea. In a previous study, we have shown that the contribution of attached bacteria to total bacterial activity can reach up to 83% under mesotrophic condition, then reinforcing the biogeochemical role of this fraction in the cycling of particulate organic carbon in the NW Mediterranean Sea (Ghiglione et al, 2007).…”

Composition and degradation of marine particles with different settling velocities in the northwestern Mediterranean Sea

“…Evolution of composition during settling-Although previous studies report on the role of bacterial decomposition of particles by bacteria attached to settling or suspended particles at DYFAMED (Sempéré et al 2000;Van Wambeke et al 2001;Ghiglione et al in press), the MedFlux experiment is the first decomposition study using natural settling marine particles separated by settling velocity. As the particles with different settling velocity decomposed, their compositions changed with time, yet those compositional changes were related to initial com- Table 7.…”