Changes in the microbial community based on seawater pH variations

Ocean acidification is an effect of the rise in atmospheric CO2, which causes a reduction in the pH of the ocean and generates a number of changes in seawater chemistry and consequently potentially impacts seawater life. The effect of ocean acidification on metabolic processes (such as net community production and community respiration and on particulate organic carbon (POC) concentrations was investigated in summer 2012 at Cap de la Revellata in Corsica (Calvi, France). Coastal surface water was enclosed in 9 mesocosms and subjected to 6 pCO2 levels (3 replicated controls and 6 perturbations) for approximately one month. No trend was found in response to increasing pCO2 in any of the biological and particulate analyses. Community respiration was relatively stable throughout the experiment in all mesocosms, and net community production was most of the time close to zero. Similarly, POC concentrations were not affected by acidification during the whole experimental period. Such as the global ocean, the Mediterranean Sea has an oligotrophic nature. Based on present results, it seems likely that seawater acidification will not have significant effects on photosynthetic rates, microbial metabolism and carbon transport.

Modeling, characterization and microalgal cultivation in an airlift panel photobioreactors.

Microalgae cultures are attracting great attentions in many industrial applications. However, one of the technical challenges is to cut down the capital and operational costs of microalgae production systems, with special difficulty in reactor design and scale-up. The thesis work open with an overview on the microalgae cultures as a possible answer to solve some of the upcoming planet issues and their applications in several fields. After the work offers a general outline on the state of the art of microalgae culture systems, taking a special look to the enclosed photobioreactors (PBRs). The overall objective of this study is to advance the knowledge of PBRs design and lead to innovative large scale processes of microalgae cultivation. An airlift flat panel photobioreactor was designed, modeled and experimentally characterized. The gas holdup, liquid flow velocity and oxygen mass transfer of the reactor were experimentally determined and mathematically modeled, and the performance of the reactor was tested by cultivation of microalgae. The model predicted data correlated well with experimental data, and the high concentration of suspension cell culture could be achieved with controlled conditions. The reactor was inoculated with the algal strain Scenedesmus obliquus sp. first and with Chlorella sp. later and sparged with air. The reactor was operated in batch mode and daily monitored for pH, temperature, and biomass concentration and activity. The productivity of the novel device was determined, suggesting the proposed design can be effectively and economically used in carbon dioxide mitigation technologies and in the production of algal biomass for biofuel and other bioproducts. Those research results favored the possibility of scaling the reactor up into industrial scales based on the models employed, and the potential advantages and disadvantages were discussed for this novel industrial design.

On the Mediterranean conveyor belt system

The Mediterranean Sea is a semi-enclosed sea, connected to the Atlantic Ocean through the Gibraltar Strait and subdivided in two different regions by the Sicily Strait. The geographical extension of the basin, the surface heat flux, and the water inflow from the Gibraltar Strait are some of the basic factors determining its horizontal and vertical circulation. In the Mediterranean strong salinity and temperature zonal gradients contribute to maintain the zonal-vertical circulation, while meridional-vertical cells are equally forced by winds and deep water mass formation in three regions, the Gulf of Lyon, the southern Adriatic and the Cretan Sea areas. The objective of this thesis is to study how these cells combine together to form the Mediterranean conveyor belt system. This has never been attempted before so the conclusions are necessarily preliminary. In the first part we discuss the vertical zonal and meridional circulation by reconstructing the Wust Mediterranean vertical salinity and temperature structures in an attempt to evaluate the water mass structure consistent with modern data. Our results confirm that Wust depicted vertical circulation from scarce data is reproduced by the past 27 years observations. The structure of both meridional and zonal circulations was discussed using velocity vertical streamfunctions with two different methods. The first one, eulerian, allowed us to observe vertical structures that were already reported in the literature. Recent studies in the Atlantic Ocean have shown that gyres and eddies have an important influence in the isopycnal vertical circulation. This is called the residual circulation which was computed in this study for the first time. A possible interpretation of horizontal connection between the meridional and zonal cells was discussed using horizontal streamfunction. In the last part of the thesis we have been developing an idealized numerical model to study the vertical circulation in the Mediterranean.