F. Van Wambeke

Longitudinal and vertical trends of bacterial limitation by phosphorus and carbon in the Mediterranean Sea.

The effect of phosphate (P), nitrate (N), and organic carbon (C, glucose) enrichment on heterotrophic bacterial production was examined along two longitudinal transects covering the whole Mediterranean Sea during June and September 1999. During these cruises, integrated bacterial production ranged from 11 to 349 mgC m(-2) d(-1) for the 0-150 m layer. P was found to stimulate bacterial production (BP) in 13 out of 18 experiments, in the eastern and in the western Mediterranean Sea. Organic carbon stimulation of bacterial production was observed at two stations in the Alboran Sea, where the highest bacterial production was recorded (216 and 349 mg C m(-2) d(-1)) and in the Sicily Strait. Maximum rates of alkaline phosphatase (AP) increased from the Alboran to the Levantine Sea whereas AP turnover time decreased. Moreover, alkaline phosphatase activity was not systematically reduced following additions of P. In cases of P limitation, however, the alkaline phosphatase activity to bacterial production ratio was severely reduced in the P and NPC enrichments. Generally, the addition of the limiting factor--whether P or C--had a synchronous stimulating effect on bacterial production and ectoaminopeptidase activity and induced a decline in the amino acid respiration percentage. At two selected stations in the eastern and northwestern Mediterranean, response to enrichment was tested on vertical profiles. Bacteria shifted from P to C limitation at a depth where soluble reactive phosphorus was still undetectable, but corresponding to a strong increase in alkaline phosphatase turnover time. Our results showed that values of AP turnover time lower than 100 h corresponded to situations of P limitation of bacterial production.

Major outputs of the recent multidisciplinary biogeochemical researches undertaken in the Aegean Sea

The main outputs of a multidisciplinary and integrated studies are summarised. The results incorporate the latest biogeochemical researches, at basin scale, in the Aegean Sea (including thermohaline circulation studies, SPM dynamics, mass and energy fluxes, acknowledge biochemical processes in the euphotic and the benthic layer and benthic response to downward fluxes). The data were acquired within five (seasonal) research cruises, during 1997–1998. Data analysis and evaluation hence provided important new information on the functional processes of the Aegean ecosystem. In terms of water circulation, no new deep water formation in the Aegean Sea was observed, during 1997–1998, but rather intermediate water, due mainly to the mild winter conditions. All the biochemical parameters of the euphotic zone (nutrients, Particulate Organic Carbon (POC), chlorophyll-a, phytoplankton, primary and bacterial production), although high in the N. Aegean Sea reflect clearly the highly oligotrophic character of the Aegean Sea. In the N. Aegean, microbial food web was the main pathway of carbon, whereas in the S. Aegean, the food web could be classified as multivorous. An important Black Sea Water (BSW) signal was observed in the dissolved phase; this was especially pronounced in the Dissolved Organic Carbon (DOC), Mn and to a lesser degree to Cd, Cu and Ni concentrations. The downward material fluxes are higher in the N. Aegean, relative to the S. Aegean. Substantially higher values of near-bottom mass fluxes were measured in the deep basins of the N. Aegean, implying significant deep lateral fluxes of POM. The N. Aegean could be classified as a “continental margin” ecosystem, whilst the S. Aegean is a typical “oceanic margin” environment. There is a close relationship and, consequently, coupling between the near-bottom mass fluxes and the accumulation rates of organic matter (OM), with the near-bottom mineralisation, bioturbation, redox potential, oxygen consumption rates, the sediment microbiological and enzymatic activity and the meio-macro- and mega fauna abundances in the Aegean Sea. The N. Aegean is characterised by higher fluxes of labile POC and higher rates of benthic mineralisation and displays much higher benthic productivity and diversity relatively to the S. Aegean. The deep isolated N. Aegean basins should be regarded as “hot spots” of organic matter accumulation, benthic abundances and diversity. There is a general N–S gradient in the biogeochemical and benthic processes in the Aegean that is especially pronounced during winter–early spring, implied mainly by the of Black Sea Water (BSW) inflow (through the dissolved phase) and the lateral marginal inputs.

Carbon flow in the planktonic food web along a gradient of oligotrophy in the Aegean Sea (Mediterranean Sea)

Abstract This work is a study of plankton food web structure and carbon flow in March and September 1997 in the Aegean Sea, area of outflow of Black Sea waters in the Mediterranean Sea. Biomass and production of autotrophs were measured by size fraction as well as bacterial biomass and production; furthermore, we studied heterotrophic nanoflagellates (HNAN), ciliates and mesozooplankton biomass, copepod production and grazing impact on phytoplankton. The obtained low values of nutrients and plankton biomass and production confirmed the oligotrophic character of this region. Despite the fact that there was no significant horizontal variability in the spatial distribution of nutrients throughout the study area, the planktonic biomass and production revealed a gradual decrease from the Northeast Aegean (NEA) towards the South Aegean (SA). In the Northeast Aegean, a large part of the fixed carbon was channelled through the microbial food web towards copepods; in contrast there was a low transfer of energy in the South Aegean where the multivorous food web was developed. Throughout the study area, almost 60–70% of autotrophic biomass and primary production was performed by cells

Effect of salinity on petroleum biodegradation

SummaryThe biodegradation of Ashtart crude oil by a mixed bacterial community (EH1) isolated from a marine sediment was investigated in varying concentration of sodium chloride (0 to 2 mol/l). Each fraction of Ashtart crude oil was biodegraded after a 30 day incubation period for a NaCl concentration equivalent to the one of seawater (0.4 mol/l). Saturates were more readily degraded than aromatics. The amount of oil degraded increased initially with increasing salt concentrations to a maximum level for 0.4 mol/l NaCl concentration. Thereafter the amount of oil degraded decreased with increasing salt concentrations. The polar fraction increased for the 0.4 to 1.4 mol/l NaCl concentration range. Asphaltenes were only slightly degraded (10%) for a NaCl concentration equivalent to the one of seawater (0.4 mol/l). NaCl concentrations seemed to affect the relative biodegradation of pristane and phytane. GC, UV fluorescence and FTIR techniques were used to study the evolution of each fraction of Ashtart crude oil.

Total organic carbon dynamics in the Aegean Sea

Abstract During the Mater oceanographic cruise, seawater samples were collected during September 1997 in the northern and southern parts of Aegean Sea and analyzed for total organic carbon (TOC) by using a high temperature catalytic oxidation method. Our results indicate that TOC concentrations decreased from the surface to deep waters from 52–128 to 48–56 μM C in the North and from 55–87 to 47–56 μM C in the South. Standing stocks of TOC in the upper 100 m, ranged from 6300 to 9120 mmol C m −2 in the North and from 6563 to 7483 mmol C m −2 in the South. Basically, higher concentrations were observed in the North and particularly for waters where the signature of Black Sea water was more pronounced. From salinity–TOC relationships, we estimated that TOC input from the Marmara and Black Sea to the Mediterranean Sea through the Dardanelles Strait varied from 1.6 to 23.7×10 10 mol C year −1 . Our computations of TOC budget suggest that TOC inflow through Dardanelles Strait, is slightly lower than TOC input from Mediterranean rivers and from the atmosphere, whereas it is one order of magnitude lower than that reported for the Atlantic Ocean via the Gibraltar Strait. We also discuss the distribution of TOC in relation to bacterial production (BP) and associated bacterial carbon demand (BCD). Assuming a bacterial growth efficiency (BGE) of 0.30, BCD in the upper 100 m was on average 18.1 mmol C m −2 day −1 in the North and 15.5 mmol C m −2 day −1 in the South. Turnover times of excess TOC (calculated as the difference between surface water TOC and a mean value of TOC below 200 m) in the upper 100 m were higher in the northern basin (0.28–0.59 years; av. 0.47 years) than in the central Aegean basin (0.20 years) and the southern basin (0.20–0.36 years; av. 0.28 years). These results may vary and need to be cautiously taken into account because of the uncertainties associated to the BGE. However, this study indicates that bacterial cycling of TOC is more effective in the South Aegean than in the North, which leads to a higher accumulation of TOC in North Aegean surface waters.

Small-scale variability in the coupling/uncoupling of bacteria, phytoplankton and organic carbon fluxes along the continental margin of the Gulf of Lions, Northwestern Mediterranean Sea

A High Frequency Flux (HFF) experiment was conducted during spring 1997 on the continental slope of the Gulf of Lions (Northwestern Mediterranean Sea) with the aim of examining the dynamical and biological processes controlling particle transfer in this margin environment. Within this general framework, a special attention was paid to short temporal and small spatial variations of phytoplankton and bacterial production through six hydrological and biological surveys performed during a 7-week period at nine sampling stations located on a 10×20-mile grid. Downward fluxes of particulate organic carbon at each station were measured by traps deployed at 240 m depth. The f-ratio and the ratio of integrated bacterial to primary production (IBP/IPP ratio), computed as indexes of biological export for each survey and station, did not provide a clear, unambiguous understanding of the importance of biological processes in the cycling of carbon in the upper water column. However, the data collected allowed to draw up carbon budgets for the different phases of the experiment. The comparison of primary production with measured and estimated organic carbon removal terms (sinking, cycling through the microbial food web, grazing by ciliates and metazoans) showed that a balance was never reached between fluxes of production and removal of organic carbon during the course of the experiment. The system shifted from an initial situation of ‘missing’ carbon (removal>production) to one of ‘excess’ carbon (removal<production). Factors such as horizontal advection of carbon into and out of the experimental area and accumulation of dissolved organic carbon (dissolved biological pump) are invoked to explain the observed imbalances. A sensitivity test of the budget to the variations of the different parameters involved showed that bacterial growth efficiency was the most important factor affecting the budget.

Numération et taille des bactéries planctoniques au moyen de l'analyse d'images couplée à l'épifluorescence

Abstract The interest of the image analysis procedure is the time-saving in automated planktonic bacterial counting and sizing, with the possibility of manual visual field control at all times. Bacterial biomass (in number and volume) and bacterial projected area histograms were determined with a microcomputer. Performance limits of image-analysed epifluorescence microscopy were: camera sensitivity, considering the very low fluorescence levels on stained bacteria; pixel-μm conversion factor, and the impossibility of the apparatus distinguishing between bacteria and fluorescent small particles. This method is not of interest for counting sediment bacteria.

Influence of phytoplankton lysis or grazing on bacterial metabolism and trophic relationships

Experimental microcosms were used to study the dynamics of heterotrophic bacterial populations with respect to phytoplankton loss. In a two-stage linked culture system, we artificially separated production and loss processes of a diatom Phaeodactylum tricornutum. In the first (productive) stage, the algae developed axenically and continuously. The outflow was fluxed in two degradation stages, where phytoplankton-derived detritus resulted respectively from: (1) excretion and by-products of phagotrophic organisms (protozoans), and (2) bacterial degradation through bacterial attachment and lysis. According to the phytoplankton decay mode, i.e., lysis or grazing, bacterial adaptations were different. The study of bacterial productivity and aminopeptidase activity showed specific bacterial evolution during the succession of different prey-predator relationships. The occurrence of aggregates allowed nanoflagellates to develop an alternative diet; they fed not only on bacteria, but also on partially degraded phytoplankton detritus, inducing a strong short-cut in the food chain. Sources and controls of extracellular proteolytic activity are discussed. Such experimental approaches are interesting because they separate bacterial lysis and protozoan grazing of phytoplankton, as well as the fates of their corresponding phytoplankton detritus in the microbial food web.