I come from Portugal and I did a Ph.D. in aquatic sciences in the University of Namur (Belgium) where I developed some research in the ecology of phytoplankton of the East African Great Lakes (especially in lakes Kivu and Tanganyika). The unusually high abundances of heterotrophic bacteria and photoautotrophic picoplankton in these lakes brought me in contact with the team of Josep M. Gasol and resulted in a collaborative work on these tropical freshwater systems. At present, I study the interactions between phytoplankton and heterotrophic bacteria in marine systems, mainly the bacterial assemblage responses to the dissolved organic carbon released by different phytoplankton species. I am developing the utilisation of "natural" substrates (as radiolabelled exudates from algae) to the application of the MAR-FISH (Micro autoradiography coupled with Fluorescent in situ hybridation) technique
Abstract: With the aim of designing photobioreactors (PBR) based on a smart exploitation of microalgae for the production of biofuels and metabolites of interest, this paper describes a novel approach where cyanobacteria are entrapped within highly porous silica matrices. With this concept, it would be possible to work with a constant population of organisms for a continuous (and increased) photoproduction of metabolites, in contrast to "one-shot" uses of liquid cultures. Different hybrid materials based on porous silica gels are described with a special emphasis on finding the most appropriate immobilisation conditions for prolonged cell survival. It is found that an aqueous route based on acid-exchanged sodium silicate combined with the use of silica nanoparticles as a gel-strengthening species shows the best results with a high primary production rate post immobilisation and a preservation of the photosynthetic pigments of up to 35 weeks. Oxygen production, though very low, could be evidenced up to 17 weeks after entrapment, demonstrating the suitability of using porous silica matrices in PBR design.
Abstract: Previsions of a warmer ocean as a consequence of climatic change point to a 2â6°C temperature rise during this century in surface oceanic waters. Heterotrophic bacteria occupy the central position of the marine microbial food web, and their metabolic activity and interactions with other compartments within the web are regulated by temperature. In particular, key ecosystem processes like bacterial production (BP), respiration (BR), growth efficiency and bacterialâgrazer trophic interactions are likely to change in a warmer ocean. Different approaches can be used to predict these changes. Here we combine evidence of the effects of temperature on these processes and interactions coming from laboratory experiments, space-for-time substitutions, long-term data from microbial observatories and theoretical predictions. Some of the evidence we gathered shows opposite trends to warming depending on the spatio-temporal scale of observation, and the complexity of the system under study. In particular, we show that warming (i) increases BR, (ii) increases bacterial losses to their grazers, and thus bacterialâgrazer biomass flux within the microbial food web, (iii) increases BP if enough resources are available (as labile organic matter derived from phytoplankton excretion or lysis), and (iv) increases bacterial losses to grazing at lower rates than BP, and hence decreasing the proportion of production removed by grazers. As a consequence, bacterial abundance would also increase and reinforce the already dominant role of microbes in the carbon cycle of a warmer ocean.
Abstract: Cyanidium caldarium (Tilden) Geitler SAG 16.91 has been encapsulated within a porous silica host structure to target novel photosynthetic hybrid materials suitable for use in solar cells or CO(2) fixation. C. caldarium cells are both thermophilic and acidophilic; on account of these tolerances the hybrid materials could be employed in more extreme heat conditions. TEM highlights that the external cell membrane can remain intact after encapsulation. The images reveal an alignment of silica gel around the external membrane of the cell, providing evidence that the cell wall acts as both a nucleation and polymerisation site for silica species and that the silica scaffold formed by the aggregation of colloidal particles, generates a porosity that can facilitate the transport of nutrients towards the cell. Epifluorescence microscopy and UV-visible spectroscopy have revealed the preservation of photosynthetic apparatus post-immobilisation. Productivity studies showed how the presence of silica nanoparticles within the matrix can adversely interact with the exterior cellular structures preventing the production of oxygen through photosynthesis.
Abstract: 1. Abundance and bacterial production (BP) of heterotrophic bacteria (HBact) were measured in the north and south basins of Lake Tanganyika, East Africa, during seasonal sampling series between 2002 and 2007. The major objective of the study was to assess whether BP can supplement phytoplankton particulate primary production (particulate PP) in the pelagic waters, and whether BP and particulate PP are related in this large lake. HBact were enumerated in the 0-100 m surface layer by epifluorescence microscopy and flow cytometry; BP was quantified using H-3-thymidine incorporation, usually in three mixolimnion layers (0-40, 40-60 and 60-100 m). 2. Flow cytometry allowed three subpopulations to be distinguished: low nucleic acid content bacteria (LNA), high nucleic acid content bacteria (HNA) and Synechococcus-like picocyanobacteria (PCya). The proportion of HNA was on average 67% of total bacterial abundance, and tended to increase with depth. HBact abundance was between 1.2 x 10(5) and 4.8 x 10(6) cells mL(-1), and was maximal in the 0-40 m layer (i.e. roughly, the euphotic layer). Using a single conversion factor of 15 fg C cell(-1), estimated from biovolume measurements, average HBact biomass (integrated over a 100-m water column depth) was 1.89 +/- 1.05 g C m(-2). 3. Significant differences in BP appeared between seasons, especially in the south basin. The range of BP integrated over the 0-100 m layer was 93-735 mg C m(-2) day(-1), and overlapped with the range of particulate PP (150-1687 mg C m(-2) day(-1)) measured in the same period of time at the same sites. 4. Depth-integrated BP was significantly correlated to particulate PP and chlorophyll-a, and BP in the euphotic layer was on average 25% of PP. 5. These results suggest that HBact contribute substantially to the particulate organic carbon available to consumers in Lake Tanganyika, and that BP may be sustained by phytoplankton-derived organic carbon in the pelagic waters.
Notes: Stenuite, Stephane Pirlot, Samuel Tarbe, Anne-Laure Sarmento, Hugo Lecomte, Melanie Thill, Sophie Leporcq, Bruno Sinyinza, Danny Descy, Jean-Pierre Servais, Pierre
Abstract: The majority of phytoplankton pigment studies are from marine, estuarine and oceanic waters, and commonly use estimates of the ratio between marker pigments and chlorophyll a (chl a) for calculating the contribution of phytoplankton groups to total chl a. In this study, we examined pigment ratios obtained with CHEMTAX processing of field data from a range of tropical and temperate freshwater bodies with contrasting water transparency, depth of the mixed layer, and trophic state. The pigment ratios obtained from processing data from fresh waters corresponded quite well with existing values from pure cultures, and were compared with the marine marker pigment: chl a ratio calculated with CHEMTAX using identical procedures. In deep, stratifying lakes a large variation of some pigment ratios with depth was observed, as well as seasonal variation relating to changes in water column structure. There was considerable variation of average phytoplankton pigment ratios among the freshwater bodies studied. Pigment ratios were significantly correlated to indicators of nutrient availability, to depth of the euphotic zone, or to a proxy of light availability. The substantial variation in marker pigment: chl a ratio confirms that algorithms which account for natural variation of pigments in phytoplankton groups are required for accurate assessment of phytoplankton groups based on marker pigment concentration alone.
Notes: Descy, Jean-Pierre Sarmento, Hugo Higgins, Harry W.
Abstract: The majority of phytoplankton pigment studies are from marine, estuarine and oceanic waters, and commonly use estimates of the ratio between marker pigments and chlorophyll <i>a</i> (chl <i>a</i>) for calculating the contribution of phytoplankton groups to total chl <i>a</i>. In this study, we examined pigment ratios obtained with CHEMTAX processing of field data from a range of tropical and temperate freshwater bodies with contrasting water transparency, depth of the mixed layer, and trophic state. The pigment ratios obtained from processing data from fresh waters corresponded quite well with existing values from pure cultures, and were compared with the marine marker pigment:chl <i>a</i> ratio calculated with CHEMTAX using identical procedures. In deep, stratifying lakes a large variation of some pigment ratios with depth was observed, as well as seasonal variation relating to changes in water column structure. There was considerable variation of average phytoplankton pigment ratios among the freshwater bodies studied. Pigment ratios were significantly correlated to indicators of nutrient availability, to depth of the euphotic zone, or to a proxy of light availability. The substantial variation in marker pigment:chl <i>a</i> ratio confirms that algorithms which account for natural variation of pigments in phytoplankton groups are required for accurate assessment of phytoplankton groups based on marker pigment concentration alone.
Abstract: Photosynthetic picoplankton (PPP), particularly picocyanobacteria (PCya), are known to be a major component of phytoplankton in oligotrophic waters. We investigated the dynamics of PPP and heterotrophic bacteria (HBact) in Lake Tanganyika during the dry and rainy seasons of 2004 to 2007, in the two basins of this large lake. Flow cytometry analyses showed that PPP was mainly composed by PCya of the phycoerythrin-rich Synechococcus type, with maximal abundances (2.3 x 104-8.2 x 105 cells mL-1) found in the mixolimnion (10-20 m depth). PPP biomass integrated over the water column depth ranged between 0.41 and 3.09 g C m-2, with maximal values in the south basin during the dry season (2.28 {+/-} 0.62 g C m-2 on average). The contribution of PPP to total phytoplankton biomass ranged from 41 to 99%, with highest values in the south basin in the dry season. Cellular measurements by image analysis of epifluorescence microscopy images showed a significant increase of the cell volume of the PCya during this period. Flow cytometry also allowed enumeration of photosynthetic picoeukaryotes. Assuming a conversion factor of 530 fg C cell-1, they contributed on average to 6% to PPP biomass, except during the dry season in the south basin, where their contribution increased to up to 20% of PPP biomass. Integrated over a 100-m water column depth, PCya biomass was on average 1.4 times higher than HBact biomass. This study establishes reference values for the biomass contribution of this part of the microbial food web, covering for the first time, spatial (different sites), vertical (profiles at different depths) and seasonal variations on a multi-year basis. The results strengthen the view of a major role of PPP in the pelagic food web of large tropical Lake Tanganyika.
Abstract: In the framework of designing novel bioreactors, the encapsulation of photosynthetic cyanobacterial strains of the genus Synechococcus, PCC 6301, PCC 7002 and Cyanothece PCC 7418, within mesoporous silica networks has been achieved via the acidification of aqueous colloidal silica precursors at ambient temperature. The effect of the silica matrix on the external membrane of the cells has been studied. The viability of the cells over a three month duration has been assessed using transmission electron microscopy, epifluorescence microscopy, UV-visible spectroscopy and high-performance liquid chromatography. The bioactivity of the encapsulated cyanobacteria was detected via the assimilation of NaH14CO3. Although most cells entrapped within the silica gel remain undivided, some cells continued to divide even when there was limited space. TEM studies have revealed an interaction between the silica gel and the cell membrane. HPLC studies highlight that the photoactive pigments in PCC 6301 and PCC 7002 can be preserved for up to 12 weeks whilst PCC 7418 lost its photosynthetic pigments after two weeks post-immobilisation. These results suggest that certain strains of cyanobacteria are able to photosynthesise within a hybrid gel yielding the possibility of novel photobioreactors.
Abstract: Phytoplankton is a key biological quality element for the establishment of the Water Framework Directive (WFD) ecological status in reservoirs and lakes. In freshwaters, inverted microscope examination is the traditional standard method for estimating phytoplankton and assessing taxonomic composition. Based on the enumeration of algal units and measurements for biovolume calculation, this technique is cumbersome and time-consuming. In large monitoring programmes, such as the application of the WFD in lakes and reservoirs, chemotaxonomy (HPLC pigment analysis and CHEMTAX treatment) is ideally suited as an alternative method because it allows the rapid processing of large numbers of samples from numerous locations and depths, thereby providing ideal temporal and spatial resolution. The low taxonomical detail obtained by HPLC and CHEMTAX (phytoplankton classes or phyla) can easily be overcome by a rapid inverted microscope screening with identification of the dominant species. Combining HPLC and microscopy provides a useful method for monitoring phytoplankton assemblages, which can be used to implement the WFD with respect to phytoplankton. Here, we present the application of a method combining marker pigments and microscopy to phytoplankton samples from 12 Belgian reservoirs. This method substantially reduced the workload and enabled us to assess the status of the phytoplankton assemblage in these lakes. The method complies with the WFD, as it takes into account taxonomic composition, assesses abundance and biomass of the phytoplankton taxa, and easily detects blooms. Additionally, a set of templates of probability of occurrence of phytoplankton functional groups at the maximal ecological potential for reservoirs from the Central/Baltic region is presented, based on reference conditions defined for natural lakes from other regions.
Abstract: 1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies. xD;2. Chlorophyll-a (chl-a) in the mixed layer ranged from 5 to 120 mg chl-a m(-2) and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C m(-3) in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m(-3), respectively). xD;3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare. xD;4. Measured particulate daily primary production ranged from 110 to 1410 mg C m(-2) day(-1); seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl-a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003. xD;5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 day(-1); these are lower than previously published values for Lake Tanganyika.
Abstract: The introduction into Lake Kivu of the planktivorous fish Limnothrissa miodon at the end of the 1950s raised major concern about the fate of mesozooplankton, but few data were available to assess the impact of predation. In this study, we followed zooplankton variations in Lake Kivu for 3.5 years. Present Lake Kivu mesozooplankton is dominated by cyclopoid copepods (Thermocyclops consimilis, Mesocyclops aequatorialis and Tropocyclops confinis), but cladocerans and rotifers are also present. Each year, total crustacean abundance in the plankton increased to a distinct seasonal maximum following a rise of phytoplankton production associated with a deep epilimnetic mixing in the dry season (August-September). This dependence on phytoplankton resource suggests that mesozooplankton dynamics in Lake Kivu is mainly bottom-up controlled, contrary to expectations from the food web structure. However, measurements of body size indicate that sardine predation affects the cladoceran Diaphanosoma excisum, whereas the larger copepods may efficiently escape predation by migration in the deep mixolimnion at daytime. Total biomass of mesozooplankton in Lake Kivu is lower than in lakes Tanganyika and Malawi. This may be related to the disappearance of a large grazer that existed before the sardine introduction, whereas the large lakes of the same region have a more complex pelagic food web, with piscivorous fish, and with calanoid copepods, which can more efficiently exploit phytoplankton production.
Abstract: This article reports a 3-year (2002-04) survey on limnologv and phyloplankton of Lake Kivu, a meromictic take of the East African Rift, with peculiar geophysical and geochemical features. The phytoplankton survey combined high-performance liquid chromatography (HPLC) analysis of marker pigments, flow cytometry and epifluorescence and electron microscopy. Availability of similar data from a parallel study on Lake Tanganyika allowed a detailed comparison of phytoplankton composition and ecology in these two lakes. Lake Kivu combines a relatively shallow euphotic layer, usually smaller than As mixed layer, with relatively low nutrient content of the mixolimnion and with unstable thermal stratification of the surface waters. With an annual average chlorophyll a (Chl a) in the mixed layer of 2.2 mg m(-3) and low nutrient levels in the euphotic zone, the lake is clearly oligotrophic. As in other large African Rift lakes, seasonal variations of algal biomass and composition occurred, with substantial interannual variations, mainly related to variabiliy of wind pattern and water column stability. Contrary to earlier reports that described Lake Kivu phytoplankton as dominated by cyanobacteria and green algae, we found that diatoms were the dominant group in the lake, particularly during the dry season (DS) episodes of deep mixing. During the rainy season (RS), the stratified water column, with high light and lower nutrient availability favoured dominance of filamentous, of diazotrophic cyanobacteria and of picocyanobacteria, which represented a substantial fraction of autotrophic biomass. Different phytoplankton functional groups were identified in Lake Kivu, which place it in an intermediate position between the oligotrophic lakes Tanganyika and Malawi and the more eutrophic Lake Victoria. However, the dominant diatoms of Lake Kivu (Urosolenia sp. and the needle-like Nitzschia bacata Hust. and Fragilaria danica Lange-Bert) are known from oligotrophic, P-deficient African lakes, and do not seem to be adequately included in the current functional classifications of freshwater phytoplankton.
