Christiane Barranguet

Microphytobenthos biomass and community composition studied by pigment biomarkers : importance and fate in the carbon cycle of a tidal flat

Abstract Microphytobenthos biomass and community composition were studied by the use of pigment biomarkers during one year at a tidal flat located in the Westerschelde (SW Netherlands). Benthic microphytes appeared to be an important carbon source in the Westerschelde, especially in the central part of the flat with a mean biomass as high as 65 mg Chl a m−2 in the superficial 1 mm of sediment. Diatoms (fucoxanthin, Chl c) dominated the population during both spring and autumn blooms. In summer microphytobenthic biomass decreased, and diatoms coexisted with Cyanobacteria (zeaxanthin) and Euglenophyceae (Chl b, zeaxanthin and lutein). The shift in the community composition may possibly be linked with the decrease in silicon concentration in the overlying water. The degradation and recycling of microphytobenthic biomass was assessed by the study of degraded chlorophyll pigments, and pigments in deeper sediment layers. The fate of microphytobenthos appears to include rapid bacterial degradation in spring, episodic grazing by benthic animals in surface layers of sediments in the centre of the flat, and export by resuspension in sediments more exposed to currents.

Divergent composition of algal-bacterial biofilms developing under various external factors

The influence of external factors other than nutrients on biofilm development and composition was studied with a combination of optical (Confocal Laser Scanning Microscopy, PAM fluorometry) and chemical methods (EPS extraction, HPLC, TOC determination). The development of algal-bacterial biofilms was followed from bare surfaces to mature biofilms in two water reservoirs on Dutch filtration dunes. Biofilms developed under the influence of grazing, light limitation or undisturbed by either of these two factors. Biofilms appeared similar at the beginning of the colonization period at the three sites and laser microscopy observations revealed the predominance of bacteria and capsular EPS (extracellular polymeric substances) in young biofilms. After 3 weeks development, the biofilms had a higher number of taxa; undisturbed biofilms presented the highest biomass, the longest developmental period and showed a significant correlation between organic carbon, chlorophyll and EPS production, indicating a close coupling between autotrophic carbon production and EPS. In light-limited biofilms, this coupling was weaker (although the organic carbon content was similar to that of the undisturbed biofilms) and a lower algal biomass was reached. Light-limited biofilms were mostly composed of diatoms, which are more efficient in low irradiances than green microalgae. Biofilms grazed by the snail Potamopyrgus antipodarum presented the lowest biomass level, but the highest proportion of EPS. Grazing seemed to favour the predominance of EPS-rich algae, as well as firmly attached diatoms. Although filamentous cyanobacteria were found in mature biofilms at the three locations, they were more abundant in the grazed biofilms. The differences in carbon uptake with respect to its allocation indicated that external factors influencing biofilm development affect the cycling and transport of carbon in biofilms and hence influence the effect of biofilm metabolism on the overlying water quality.

Analysis of structural and physiological profiles to assess the effects of Cu on biofilm microbial communities

ABSTRACT We investigated the effects of copper on the structure and physiology of freshwater biofilm microbial communities. For this purpose, biofilms that were grown during 4 weeks in a shallow, slightly polluted ditch were exposed, in aquaria in our laboratory, to a range of copper concentrations (0, 1, 3, and 10 μM). Denaturing gradient gel electrophoresis (DGGE) revealed changes in the bacterial community in all aquaria. The extent of change was related to the concentration of copper applied, indicating that copper directly or indirectly caused the effects. Concomitantly with these changes in structure, changes in the metabolic potential of the heterotrophic bacterial community were apparent from changes in substrate use profiles as assessed on Biolog plates. The structure of the phototrophic community also changed during the experiment, as observed by microscopic analysis in combination with DGGE analysis of eukaryotic microorganisms and cyanobacteria. However, the extent of community change, as observed by DGGE, was not significantly greater in the copper treatments than in the control. Yet microscopic analysis showed a development toward a greater proportion of cyanobacteria in the treatments with the highest copper concentrations. Furthermore, copper did affect the physiology of the phototrophic community, as evidenced by the fact that a decrease in photosynthetic capacity was detected in the treatment with the highest copper concentration. Therefore, we conclude that copper affected the physiology of the biofilm and had an effect on the structure of the communities composing this biofilm.

Organisation of microbenthic communities in intertidal estuarine flats, a case study from the Molenplaat (Westerschelde Estuary, The Netherlands)

Summary The microbenthic communities of a tidal flat in the Westerschelde estuary were studied at 4 stations in late spring and early autumn 1996. Additional information on the diatom component of these communities was obtained from a one-year survey of these organisms. Total biomass of pigmented (PIG) protists greatly exceeded that of non-pigmented (NPIG) protists in late spring, especially at the more silty stations. However, in autumn, the ratio of PIG/NPIG protists was much lower and is In general, silty sediments appear to be characterized by considerable temporal changes in microbenthic biomass and composition, related to predictable seasonal changes in environmental conditions as well as episodic and stochastic events resulting in severe disturbance and resuspension. Sandy sediments may have more complex and resilient microbenthic communities, adapted to a continuous regime of disturbance in the top layers of sediments and with a less pronounced seasonality.

Microphytobenthos production in the Gulf of Fos, French Mediterranean coast

Microphytobenthic oxygen production was studied in the Gulf of Fos (French Mediterranean coast) during 1991/1992 using transparent and dark benthic chambers. Nine stations were chosen in depths ranging from 0.5 to 13 m, which represents more than 60% of bottoms in the Gulf. Positive net microphytobenthic oxygen production was seasonally detected down to 13 m; the maximum value attained was 60 mg O2 m−2 h−1 (0.7–0.8 g O2 m−2 d−1) in sediments at 0.5 m depth during spring and winter. Respiration rates were maximum in the sediments located at the mussel farm (5 m), in the center of the Gulf, with 135 mg O2 m−2 h−1 in spring (3.2 g O2 m−2 d−1); in the other locations, it ranged from 3.3 to 58.2 mg O2 m−2 h−1 (0.08–1.4 g O2 m−2 d−1).Compared to phytoplankton, microphytobenthos production was higher only in the bottoms < 1 m depth. In deeper bottom waters, phytoplankton production could be absent due to light limitation, while microphytobenthos was still productive. Phytoplankton production m−2 was generally higher than microphytobenthic production. Microphytobenthic biomass, higher than phytoplanktonic, varied from 27 to 379 mg Chl a m−2, the maximum in the mussel farm sediments, with the minimum in sandy shallow bottoms. Pigment analysis showed that microphytobenthos consisted mainly of diatoms (Chl c and fucoxanthin) but other algal groups containing Chl b could become seasonally important. A Principal Component Analysis suggested that the main statistical factors explaining the distribution of our observations may be interpreted in terms of enrichment in phaeopigments and light; the role of Chl a appearing paradoxically as secondary in benthic production rates. Phaeopigments are mainly constituted by phaeophorbides, which indicate grazing processes. The influence of the mussel farm on the oxygen balance is noticeable in the whole Gulf.

Linking ecological and ecotoxicological techniques to support river rehabilitation.

Human activities in river catchments interfere with natural fluxes of water and materials. Diffuse inputs and point-sources of toxicants have modified the ecological state of riverine communities considerably, and sanitation schemes are now under development for various rivers. To improve analysis, monitoring and prospecting the role of toxicants in river ecosystems a review of the available methods is undertaken. Ecotoxicological techniques are discussed in relation to basic ecological principles that are thought to regulate the functioning of communities. The response to toxicants among species is highly diverse and therefore the choice of test species (e.g. of typical riverine insects as caddisflies or mayflies) is critical, as it is the use of test-batteries. Long-term exposure may lead to developmental disturbances that may be assessed through morphometric techniques like analysis of asymmetry. Multi-generation exposure, although rarely studied, provides a useful insight into the genetic consequences of pollution. Selection for tolerant species or varieties has been experimentally assessed for smaller organisms such as insects, micro-algae, and bacteria. There is also perspective for multivariate analysis of species distribution in relation to pollutant exposure. Furthermore, a system approach to benthic ecology and sediment testing is needed. Such an approach reflects the strong linkage of ecological and ecotoxicological processes. Toxicants are transformed by biological activity; in some cases this alleviates toxicant stress, but in other cases degradation products are toxic as well. The risk of transformation to mutagenic products in the environment is indicated. The re-assessment of some of the classical ecotoxicological techniques is needed to adequately fulfil the needs of ecological recovery programs. To this purpose integration of ecotoxicological and ecological tools is needed.

Dominance of diatoms over cyanobacterial species in nitrogen-limited biofilms

Published observations, mainly on phytoplankton, suggest that (N 2 -fixing) cyanobacteria can outgrow diatoms under nitrate limitation. This study focussed on the competition for limiting nitrogen between 3 diatom species and 2 cyanobacteria species in biofilms. The potential of the diatom species Nitzschia perminuta (GRUNOw) M PERGALLO, Navicula trivialis LANGE-BERTALOT and Achnanthes lanceolata (BREBISSON) GRUNOw and the cyanobacterial species Leptolyngbya foveolarum (RABENHORST ex GOMONT) ANAGNOSTIDIS et KOMAREK and Cylindrospermum stagnate (KUTZING) RIPPKA et al. to grow in monocultures and in mixed cultures of a) the three diatoms and b) all five species was tested at nominal nitrate concentrations of 50 and 500 μM. Algal growth in biofilms was measured using a PHYTO-PAM, which measured fluorescence of biofilm suspensions, allowing simultaneous observations of diatoms and cyanobacteria in mixed cultures. Growth of all diatom species and the cyanobacterium L. foveolarum was similar in monoculture, reaching a maximum of 180 μg chl-a l -1 at 50 μM nitrogen and 950 μg chl-a l -1 at 500 μM nitrogen. Only the heterocyst forming cyanobacterium C. stagnate reached significantly higher densities, with about 800 μg chl-a l -1 at 50 μM nitrogen and 1500 μg chl-a l -1 at 500 μM nitrogen. However, in the mixed cultures, N. perminuta outgrew all other algae under the low nitrogen treatment while L. foveolarum dominated at the higher nitrogen concentration. It is concluded that interaction between species may impair the capacity of individual species to exploit nitrogen.