Diatom Biofilms Research Articles

Diatom Cooccurrence Shows Less Segregation than Predicted from Niche Modeling.

Species cooccurrence patterns give significant insights into the processes shaping communities. While biotic interactions have been widely studied using cooccurrence analyses in animals and larger plants, studies about cooccurrences among micro-organisms are still relatively rare. We examined stream diatom cooccurrences in France through a national database of samples. In order to test the relative influence of environmental, biotic and spatial constraints on species’ incidence distribution, cooccurrence and nestedness patterns of real communities were compared with the patterns generated from a set of standard and environmentally constrained null models. Real communities showed a higher level of segregation than the most conservative standard null models, but a general aggregation of cooccurrences when compared to environmentally constrained null models. We did not find any evidence of limiting similarity between cooccurring species. Aggregations of species cooccurrences were associated with the high levels of nestedness. Altogether, these results suggested that biotic interactions were not structuring cooccurrences of diatom species at our study scale. Instead, the patterns were more likely to be related with colonization patterns, mass effect, and local temporal dynamics of diatom biofilms. We further highlight that the association of standard and environmentally constrained null models may give realistic insight into the cooccurrence patterns of microbial communities.

Efficiency of copper and cupronickel substratum to resist development of diatom biofilms

This study on the population dynamics of fouling diatoms from a biofouling rich monsoon influenced tropical bay revealed that irrespective of the season and exposure period, the highest diatom abundance and species were encountered on non-toxic (fiberglass > glass) compared to toxic (copper–Cu and cupro-nickel–Cu-Ni) substrata. Though the diatom recruitment on toxic substrata is noticed within 24 h, the multiplication of the settled diatoms was not significant, even with an increase in the exposure period (days to weeks). The diatoms recorded on toxic substrata are the result of fresh recruitment. Results indicated that the potential of Cu and Cu–Ni panels to resist diatom fouling are the same. The dominance of raphid-pennate diatoms (Navicula, Amphora, Nitzschia and Thalassionema) throughout the year indicated them as ideal candidates for antifouling studies. The negligible contribution of araphid-pennate diatoms (Licmophora and Grammatophora) and the influence of tycho-pelagic/bloom forming diatoms (Fragillariopsis and Skeletonema) on fouling community are the other noteworthy observations. Concern with the adverse impact of Cu pollution on the ecosystem this study recommends the incorporation of Cu alloy (Cu–Ni) in the marine structures to combat biofouling as Cu–Ni has a slower leaching rate of the toxic product such as Cu2O.

Diatoms and Other Epibionts Associated with Olive Ridley (Lepidochelys olivacea) Sea Turtles from the Pacific Coast of Costa Rica.

Although the sea turtles have long been familiar and even iconic to marine biologists, many aspects of their ecology remain unaddressed. The present study is the first of the epizoic diatom community covering the olive ridley turtle’s (Lepidochelys olivacea) carapace and the first describing diatoms living on sea turtles in general, with the primary objective of providing detailed information on turtle epibiotic associations. Samples of turtle carapace including the associated diatom biofilm and epizoic macro-fauna were collected from Ostional beach (9° 59´ 23.7´´ N 85° 41´ 52.6´´ W), Costa Rica, during the arribada event in October 2013. A complex diatom community was present in every sample. In total, 11 macro-faunal and 21 diatom taxa were recorded. Amongst diatoms, the most numerous were erect (Achnanthes spp., Tripterion spp.) and motile (Haslea sp., Navicula spp., Nitzschia spp., Proschkinia sp.) forms, followed by adnate Amphora spp., while the most common macro-faunal species was Stomatolepas elegans (Cirripedia). Diatom densities ranged from 8179 ± 750 to 27685 ± 4885 cells mm-2. Epizoic microalgae were either partly immersed or entirely encapsulated within an exopolymeric coat. The relatively low diatom species number, stable species composition and low inter-sample dissimilarities (14.4% on average) may indicate a mutualistic relationship between the epibiont and the basibiont. Dispersal of sea turtle diatoms is probably highly restricted and similar studies will help to understand both diatom diversity, evolution and biogeography, and sea turtle ecology and foraging strategies.

Grain sizes retained by diatom biofilms during erosion on tidal flats linked to bed sediment texture

Size-specific sediment retention by diatom biofilms was measured by eroding intertidal muds at increasing shear stresses (0.01–0.60Pa) using a Gust microcosm. The grain sizes eroded from biofilm-covered sediment were compared to those from control cores from which the biofilms were destroyed using bleach. Biofilms were quantified using carbohydrate measurements. Cores from an intertidal mud flat in the Minas Basin of the Bay of Fundy (Canada) showed biofilms preferentially retained clays and very fine silts relative to fine and medium silts. In contrast, prior field observations on an intertidal sand flat indicated that fine and medium silts were preferentially retained by biofilms relative to clays and very fine silts. These contrasting results suggest a link between size-specific sediment retention and sediment texture, where sand biofilms retain coarser, non-cohesive sediment grains, while mud biofilms retain finer, cohesive sediment grains. This relationship implies that biofilms could contribute to a positive feedback that would preserve existing sediment texture.

The Alternating Current Scanning Electrochemical Microscopy Characterization of Diatom Cell Activity Affected by Paeonol

The alternating current scanning electrochemical microscopy (AC-SECM) is an important method to characterize the electrochemical activity of living cells. In this paper, we apply AC-SECM to measure the electrochemical activity of diatom biofilm. We find that the diatom biofilm presents insulating effect to decrease the current, when the electrode is close to the biofilm. The alternating current impedance of diatom biofilm is overall smaller than that of the bare glass. To investigate the possible effect of chemical stimulus on cellular activity, the diatoms were immersed into paeonol solution (100 μg/mL), and AC-SECM imaging experiments were conducted when immersion time reached 20 h, 25 h and 35 h. The AC measurements performed at high frequencies show that phase angle decreases over time, which preliminarily indicates that the cellular activity of diatoms is affected by paeonol.

Protection of cells from salinity stress by extracellular polymeric substances in diatom biofilms

Diatom biofilms are abundant in the marine environment. It is assumed (but untested) that extracellular polymeric substances (EPS), produced by diatoms, enable cells to cope with fluctuating salinity. To determine the protective role of EPS, Cylindrotheca closterium was grown in xanthan gum at salinities of 35, 50, 70 and 90 ppt. A xanthan matrix significantly increased cell viability (determined by SYTOX-Green), growth rate and population density by up to 300, 2,300 and 200%, respectively. Diatoms grown in 0.75% w/v xanthan, subjected to acute salinity shock treatments (at salinities 17.5, 50, 70 and 90 ppt) maintained photosynthetic capacity, Fq′/Fm′, within 4% of pre-shock values, whereas Fq′/Fm′ in cells grown without xanthan declined by up to 64% with hypersaline shock. Biofilms that developed in xanthan at standard salinity helped cells to maintain function during salinity shock. These results provide evidence of the benefits of living in an EPS matrix for biofilm diatoms.

Ecological and social modeling for migration and adhesion pattern of a benthic diatom

A biofilm is an aggregation of microorganisms that involves the adhesion of cells to each other on a surface. Because of their ecological advantages and disadvantages, it is important to understand the rules that govern the formation and evolution of diatom biofilms. In this paper, the cell migration of the benthic diatom Navicula sp. was quantitatively investigated, and the results show that the cumulative migration angles and step lengths can be fitted with a normal distribution and an exponential distribution, respectively. These statistical parameters were used to simulate the adhesion pattern of the diatoms in the NetLogo platform to better understand the cellular interactions and formation of the diatom biofilm. Variance-to-mean ratio (VMR) was used as an index of dispersion, and an analysis of the significance in the differences of VMR between the in vitro measurements and the simulated results was conducted. By comparing the simulation results and observed experiments, the parameters characterizing cellular interactions were adjusted. As a conclusion, the clustered pattern shown in the in vitro experiments is verified to be caused by both the cellular interactions and the cell reproduction.

Impacts of biogenic structures on benthic assemblages: microbes, meiofauna, macrofauna and related ecosystem functions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 465:85-97 (2012) - DOI: https://doi.org/10.3354/meps09915 Impacts of biogenic structures on benthic assemblages: microbes, meiofauna, macrofauna and related ecosystem functions Claire Passarelli1,*, Frédéric Olivier1,2, David M. Paterson3, Cédric Hubas1 1Muséum National d’Histoire Naturelle, UMR BOREA 7208 MNHN/CNRS/IRD/UPMC, 75231 Paris Cedex 5, France 2ISMER / UQAR, CP3300 Rimouski, Quebec G5L 3A1, Canada 3Sediment Ecology Research Group, Scottish Oceans Institute, University of St Andrews, St Andrews, KY16 8LB, UK *Email: claire.passarelli@mnhn.fr ABSTRACT: Species that modify their surroundings are known as ecosystem engineers. For example, patches of polychaete tubes enhance soft-bottom intertidal habitat complexity, modifying water flow, promoting sediment accretion and affecting nutrient fluxes at the water–sediment interface. Understanding how such structures affect the benthic ecosystem’s functioning requires the assessment of their influence on all benthic components and how the related ecosystem services may be modified. We performed an in situ experimental study, involving the use of artificial mimics of polychaete tubes, to investigate the purely physical impacts of the structures without the complexity of worm activity. Benthic chambers of different mimic densities were used, and their effect on the recolonisation of defaunated natural sandy sediments by microorganisms, meiofauna and macrofauna was monitored. We also measured air–sediment CO2 fluxes and sediment stability as they constitute crucial ecosystem services provided by benthic habitats. We showed that the biogenic structures stimulated the development of diatom biofilms (microphytobenthos) and their associated extracellular polymeric substances (EPS). Impacts of tubes on meiofaunal and macrofaunal assemblages were significant; in most cases, species and groups were more abundant in treatments with few or no tubes. In response to the tube density increase, the whole system tended towards heterotrophy and higher sediment stability, probably as a consequence of the development of the diatom biofilm. Biogenic structures are, therefore, of critical importance for soft-bottom intertidal communities in terms of both structure and function. KEY WORDS: Biogenic structures · Polychaete tubes · Biofilm · Microphytobenthos · Meiofauna · Macrofauna · Carbon flux · Sediment stability Full text in pdf format PreviousNextCite this article as: Passarelli C, Olivier F, Paterson DM, Hubas C (2012) Impacts of biogenic structures on benthic assemblages: microbes, meiofauna, macrofauna and related ecosystem functions. Mar Ecol Prog Ser 465:85-97. https://doi.org/10.3354/meps09915 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 465. Online publication date: September 28, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Microphytobenthos biomass mapping using the optical model of diatom biofilms: Application to hyperspectral images of Bourgneuf Bay

A new fast mapping approach based on an optical model was developed and applied to hyperspectral airborne HySpex images of Bourgneuf Bay (French Atlantic coast). The aims were (1) to discriminate diatom biofilms (from the Bacillariophyceae class) constituting microphytobenthos on any mudflat and (2) to estimate their biomass expressed in mgChla·m−2 by remote sensing of poorly accessible mudflats.Due to the distinct absorption peaks of certain pigments of different microalgae classes constituting microphytobenthos, the high spectral resolution of hyperspectral images was employed in order to detect the pigment assemblage characterizing diatom biofilms. Thus, the HySpex 1600 VNIR camera was used both in the laboratory for estimating biomass under controlled conditions and in an airborne setting. 1) The mapping approach used known spectral indices (NDVI and MPBI) and new ones (IDiatom and IEuglenid) to characterize quickly the key pigments of two main microphytobenthos classes, for a fast selection of diatom and euglenid biofilms by two different masks. 2) The inherent spectral properties of biofilms in the 700–900nm range were also used to calculate an original estimate of the substrate/environmental effects in each mask. 3) Biomass quantification of the diatom biofilms was finally performed by application of the microphytobenthos optical model (MPBOM).This approach was first tested on laboratory HySpex images of synthetic biofilms for which the in vitro biomass measurements by chromatography (HPLC) were available. The comparison between estimated and measured biomass values showed the reliability of the mapping approach for biomass estimation. Finally, a map of diatom biomass was obtained at the scale of the entire Bourgneuf Bay. The range of biomass variation (0–35mgChla·m−2) was in close agreement with the biomass maps presented in previous studies.

Differential responses of juvenile and adult South African abalone (Haliotis midae Linnaeus) to low and high oxygen levels

Marine invertebrates have evolved multiple responses to naturally variable environmental oxygen, all aimed at either maintaining cellular oxygen homeostasis or limiting cellular damage during or after hypoxic or hyperoxic events. We assessed organismal (rates of oxygen consumption and ammonia excretion) and cellular (heat shock protein expression, anti-oxidant enzymes) responses of juvenile and adult abalone exposed to low (~83% of saturation), intermediate (~95% of saturation) and high (~115% of saturation) oxygen levels for one month. Using the Comet assay, we measured DNA damage to determine whether the observed trends in the protective responses were sufficient to prevent oxidative damage to cells. Juveniles were unaffected by moderately hypoxic and hyperoxic conditions. Elevated basal rates of superoxide dismutase, glutathione peroxidase and catalase were sufficient to prevent DNA fragmentation and protein damage. Adults, with their lower basal rate of anti-oxidant enzymes, had increased DNA damage under hypoxic and hyperoxic conditions, indicating that the antioxidant enzymes were unable to prevent oxidative damage under hypoxic and hyperoxic conditions. The apparent insensitivity of juvenile abalone to decreased and increased oxygen might be related to their life history and development in algal and diatom biofilms where they are exposed to extreme diurnal fluctuations in dissolved oxygen levels.

Recovery potential of periphytic biofilms translocated in artificial streams after industrial contamination (Cd and Zn)

Metal wastes can significantly disturb aquatic communities, particularly photosynthetic organisms, the main primary producers in freshwater running ecosystems. In this study, biofilms and diatoms were used as bioindicators to characterize the kinetics of biofilm recovery. An experimental decontamination study was conducted under laboratory conditions, after biofilm colonisation at a site subject to discharge of industrial metals (Zn and Cd) and in parallel at an upstream site, metal-free, considered as a control. After 24 days of colonisation, biofilms were translocated and maintained in the laboratory for 56 days under clean conditions (control and decontamination) or metal contamination. Various tests were conducted from the community level--measures of metal bioaccumulation, cell densities and taxonomic investigations, to the individual level--measures of teratological forms. After 56 days of decontamination, Zn and Cd concentrations in decontaminated biofilms showed a sharp decline, respectively ranging from 6.7 ± 2 to 4 ± 2.5 mg Zn g⁻¹ DW and from 207.6 ± 24.5 to 45.4 ± 9.9 μg Cd g⁻¹ DW. However, at the end of the experiment bioaccumulations remained significantly higher than concentrations in control biofilms. Despite a diatom evolution in biofilm assemblages, taxonomic inventories did not demonstrate a complete restoration of diatom communities in biofilms under decontamination conditions compared with controls, since metal-resistant species initially present after colonisation at the contaminated site, such as Eolimna minima, persisted in high abundance in decontaminated biofilms. Biofilms kept under metal pressure showed very high bioaccumulation capacities and a sharp decline of species diversity which allowed identification of some resistant species. Regarding these first results on the behaviour of diatom biofilms under experimental decontamination conditions, improvement of the natural hydrosystem's chemical state appears quickly, but an eventual return to good ecological status appears delayed, with the persistence of metal-tolerant species even after 56 days.

Changes in diatom patch‐size distribution and degradation in a spatially self‐organized intertidal mudflat ecosystem

Self-organized spatial patterns have been proposed as possible indicators for regime shifts in ecosystems. Until now, this hypothesis has only been tested in drylands. Here, we focus on intertidal mudflats where regular spatial patterns develop in early spring from the interaction between diatom growth and sedimentation but disappear when benthic herbivore abundance increases in early summer, accompanied by a dramatic shift to a bare mudflat. We followed the patch-size distributions of diatom biofilms during this degradation process. As time progressed, we found a temporal change in the spatial configuration occurring simultaneously with the loss of the diatom-sediment feedback. This indicates a gradual failure in time of the self-organization process that underlies regular patterning in this ecosystem. The path to degradation co-occurred with the loss of the larger patches in the ecosystem, which resulted in a decrease of the truncation in the patch-size distribution. Hence, our study in mudflat ecosystems confirms the general hypothesis that spatial patterns can provide important clues about the level of degradation. Nevertheless, our study highlights the need for thorough study about the type of spatial patterns and the nature of the underlying feedbacks before a reliable assessment of ecosystem status can be made, as changes in patch-size distribution differed markedly with those observed in other ecosystems.

Photoautotrophic–heterotrophic biofilm communities: a laboratory incubator designed for growing axenic diatoms and bacteria in defined mixed‐species biofilms

Biofilm communities in the euphotic zone of aquatic habitats comprise photoautotrophic microorganisms, such as diatoms, green algae and cyanobacteria, which produce the organic carbon that fuels the life of a heterotrophic contingent of microorganisms, mostly bacteria. Such photoautotrophic-heterotrophic mixed-species biofilms have received little attention in biofilm research due to a lack of suitable pure-culture laboratory model systems. However, they offer important insight into microbial population dynamics and community interactions during a biofilm-developmental process that shapes highly structured, extremely well-adapted microbial landscapes. Here, we report on the development of a sterile incubation chamber for growing and monitoring axenic phototrophic biofilms, i.e. a sterilizable, illuminated, continuous-flow system for a routine work with pure cultures. The system has been designed to simulate the growth conditions in the shallow, littoral zone of aquatic habitats (horizontal surface, submerged in water, illuminated, aerated). Additional features of the concept include automated photometrical monitoring of biofilm density (as biofilm turbidity), analysis via confocal microscopy, direct harvesting of cells, and options to control illumination, flow velocity, and composition of culture fluid. The application of the system was demonstrated in growth experiments using axenic diatom biofilms, or axenic diatom biofilms co-cultivated with different bacterial strains isolated from epilithic biofilms of an oligotrophic freshwater lake.

Effect of chlorination on the development of marine biofilms dominated by diatoms

This study addressed the antifouling efficiency of commercially available chlorine at different concentrations (0.5%, 1%, and 2%) and exposure times (0.5 min, 1 min, 5 min, and 15 min). The rapid and non-destructive FIRe (fluorescence induction and relaxation) technique was used to evaluate the effects of the biocide on diatom dominated biofilms. The efficiency of chlorine in removing diatoms from the developed biofilms increased with an increase in concentration and exposure time. The fluorescence measurements revealed low F v /F m and high σPSII values for chlorine-treated Navicula and Amphora biofilms indicating that chlorination was efficient in damaging the photosystem-II reaction centers. Chlorination also caused mortality of diatom cells by damaging the cell body. In natural biofilms, the biocidal effect of chlorine was species specific; species of Amphiphrora, Navicula, Cylindrotheca, and Coscinodiscus showed an increase in the density of the population, but species of Pleurosigma, Amphora, and Thalassionema did not increase in density after chlorine treatment. It was also demonstrated that diatoms can colonize, grow and photosynthesize on chlorine-treated surfaces. Under pulse chlorination (treatment every 6 h), irrespective of chlorine concentration, the development of biofouling decreased with an increase in exposure time. Differences between exposure times of 1 to 15 min were not significant. Additionally, transmission levels of the control (non-chlorine-treated) fouled coupons reduced significantly (∼20%) compared to the chlorine-treated fouled coupons (<2%). These results suggest that chlorine can be used as a biocide to control the development of diatom biofilms.

Development of formulated diet for post-larval abalone, Haliotis diversicolor supertexta

This study examined the effects of formulated diets with/without emulsified fish oil on the survival and growth of post-larval Haliotis diversicolor supertexta. Agar was used as a binding agent and served as a carbohydrate source to make formulated diets. Post-larval abalone were settled on plastic plates and cultured in fiberglass reinforced plastics (FRP) tanks treated with 0.2 ppm ozone. Two experimental conditions, one with shading (90% light shading) and the other without shading were provided. The post-larval abalone were fed with (a) natural diatom biofilm (without diet supplementation, (b) formulated diet containing only agar, (c) formulated diet containing agar and Chlorella powder and (d) formulated diets containing 0.5, 1.0 or 2.0% emulsified fish oil and agar. Experimental duration was 35 days and survival, growth rate, daily growth rate, the amount of bacteria and attached diatoms were identified every 7 days. In the shading condition, post-larval abalone fed a formulated diet supplemented with 0.5% emulsified fish oil and a diet supplemented with Chlorella powder had a significantly higher daily growth rate than those fed other diets. Their daily growth rate in shell length was 78.5 μm/day and 77.5 μm/day, respectively. Post-larval abalone fed the diet with 0.5% emulsified fish oil had a significantly higher survival (35.1%) than those fed diets supplemented with only agar, Chlorella powder or natural diatom biofilm. For the treatment groups without shading, post-larval abalone fed a diet with 2% emulsified fish oil had significantly higher final size (3708 μm) and daily growth rate (98.8 μm /day shell length) than those fed diets supplemented with only agar or natural diatom biofilm.

Complementarity effects drive positive diversity effects on biomass production in experimental benthic diatom biofilms

Summary1. Positive effects of species diversity on ecosystem functioning have often been demonstrated in ‘macrobial’ communities. This relation and the responsible mechanisms are far less clear for microbial communities. Most experimental studies on microorganisms have used randomly assembled communities that do not resemble natural communities. It is therefore difficult to predict the consequences of realistic, non‐random diversity loss.2. In this study, we used naturally co‐occurring diatom species from intertidal mudflats to assemble communities with realistically decreasing diversity and analysed the effect of non‐random species loss on biomass production.3. Our results demonstrate a highly positive biodiversity effect on production, with mixtures outperforming the most productive component species in more than half of the combinations. These strong positive diversity effects could largely be attributed to positive complementarity effects (including both niche complementarity and facilitation), despite the occurrence of negative selection effects which partly counteracted the positive complementarity effects at higher diversities.4. Facilitative interactions were, at least in part, responsible for the higher biomass production. For one of the species,Cylindrotheca closterium, we show its ability to significantly increase its biomass production in response to substances leaked into the culture medium by other diatom species. In these conditions, the species drastically reduced its pigment concentration, which is typical for mixotrophic growth.5. Synthesis. We show that both species richness and identity have strong effects on the biomass production of benthic diatom biofilms and that transgressive overyielding is common in these communities. In addition, we show mechanistic evidence for facilitation which is partly responsible for enhanced production. Understanding the mechanisms by which diversity enhances the performance of ecosystems is crucial for predicting the consequences of species loss for ecosystem functioning.

Surface adhesion measurements in aquatic biofilms using magnetic particle induction: MagPI

Natural sediment stability is a product of interacting physical and biological factors, and whereas stability can be measured, few techniques allow sensitive assessment of the sediment surface as conditions change. For example, stability gradually increases as a biofilm develops or as salinity rises, or it might be influenced by toxic compounds. This article introduces a new technique (magnetic particle induction: MagPI) based on the magnetic attraction of specially produced fluorescent ferrous particles. The test particles are added to a surface and subjected to an incrementally increasing magnetic field produced by permanent magnets or electromagnets. There is a strong correlation between magnetic flux density (mTesla) and distance from the surface (r2 = 0.99) for permanent magnets and between magnetic flux density and the current supplied to an electromagnet (r2 &gt; 0.95). The magnetic force at which the particles are recaptured is determined as a measure of surface adhesion. MagPI therefore determines the “stickiness” of the surface, whether a biofilm, sediment, or other material. The average magnetic flux density required to remove test particles from diatom biofilms (15.5 mTesla) was significantly greater than from cyanobacterial biofilms (10 mTesla). Controls of fine glass beads showed little adhesion (2.2 mTesla). Surface adhesion is an important bed property reflecting the sediment system's potential to capture and retain new particles and accumulate material. MagPI offers a straightforward and economic way to determine the surface adhesion of a variety of surfaces rapidly and with precision. The technique may have applications in physical, environmental, and biomedical research.

Adaptations of microphytobenthos assemblages to sediment type and tidal position

Tagus intertidal microphytobenthos (MPB) assemblages were characterized over a wide range of sediment type and tidal height and the possible effects of these variables on MPB spatial distribution and photo-adaptation mechanisms were investigated. Two transects with six different sediment type and different tidal height sites were sampled once every two months from 2002 to 2004. Upper shore and sandy sites showed higher chlorophyll a (chl a) content, with sandy sediments showing a biomass peak in late winter–early spring, and muddy sites showing no obvious seasonal pattern. Stepwise multiple linear regressions showed that only SiO 2, tidal height and sediment particle size <63 μm were significant variables ( p<0.05), explaining 50% of MPB biomass spatial–temporal variability. However, when data were separated by transect, only tidal height remained significant at both transects. Sandier sediments exhibited higher zeaxanthin/chl a and lower fucoxanthin/chl a ratios characteristic of a mixed cyanobacteria/diatom assemblages, showing an alternate seasonality with cyanobacteria increasing in summer and diatoms dominating in spring. Diatom biofilms showed contrasting features depending on the sediment type. Epipsammic diatoms were small with an average length of around 10 μm, while epipelic diatoms showed a wider size range with size distribution peaks at 10–15 μm, 25–35 μm and >60 μm. Epipelic biofilms showed evidence of being low light-acclimated (high fucoxanthin/chl a) and of photo-regulating by vertical migration movements (presence of endogenous vertical rhythms and lower diatoxanthin/diadinoxanthin). Epipsammic biofilms showed higher diatoxanthin/diadinoxanthin ratios and no vertical migration rhythms. Thus, the two diatom biofilm types had distinct strategies to photo-regulate: epipelic diatoms using vertical migration to position themselves at the sediment depth of optimum light conditions, and epipsammic diatoms using the xanthophyll cycle to photo-regulate. Further studies comparing epipsammic and epipelic assemblages are necessary to better understand MPB photo-regulation mechanisms.

Bacteria Associated with Benthic Diatoms from Lake Constance: Phylogeny and Influences on Diatom Growth and Secretion of Extracellular Polymeric Substances

The composition of diatom-associated bacterial communities was studied with 14 different unialgal xenic diatom cultures isolated from freshwater epilithic biofilms of Lake Constance, Germany. A clear dominance of Alphaproteobacteria was observed, followed by Betaproteobacteria, Gammaproteobacteria, Bacteroidetes, and Verrucomicrobia. Pure cultures of the diatom Cymbella microcephala, which was found to be dominant in epilithic biofilms in Lake Constance, were cocultivated with six associated bacterial strains. All these bacterial strains were able to grow in C. microcephala cultures in the absence of organic cosubstrates. Diatom growth was generally enhanced in the presence of bacteria, and polysaccharide secretion was generally increased in the presence of Proteobacteria. The monomer composition of extracellular polysaccharides of C. microcephala changed in relation to the presence of different bacteria, but the dominant monomers were less affected. Our results indicate that these changes were caused by the diatom itself rather than by specific bacterial degradation. One Bacteroidetes strain strongly influenced carbohydrate secretion by the alga via extracellular soluble compounds. Biofilms were formed only in the presence of bacteria. Phylogenetic analysis and coculture studies indicate an adaptation of Proteobacteria and Bacteroidetes to the microenvironment created by the diatom biofilm.

THE ROLE OF NITRIC OXIDE IN DIATOM ADHESION IN RELATION TO SUBSTRATUM PROPERTIES(1).

Adhesion of raphid diatoms to surfaces, mediated by the secretion of extracellular polymeric substances (EPS), is an important strategy for growth and survival. Diatom biofilms are also important in the context of biofouling. Diatoms exhibit selectivity in adhering to surfaces, but little is understood about how they perceive the properties of a substratum and translate that perception into altered adhesion properties. In this study, we demonstrate that Seminavis robusta Danielidis et D. G. Mann, like many other pennate diatoms, adheres more strongly to hydrophobic surfaces (such as silicone elastomer foul-release coatings) than to hydrophilic surfaces. To explore the cellular mechanisms that may underlie this selectivity, we tested the hypothesis that diatoms may perceive a hydrophilic surface as unconducive to adhesion through a form of stress response involving nitric oxide (NO) production. Single-cell imaging with the fluorescent indicator DAF-FM DA (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate), revealed NO levels that were 4-fold higher in cells adhered to a hydrophilic surface (acid-washed glass) compared with a hydrophobic surface (polydimethylsiloxane elastomer, PDMSE). Elevated levels of NO caused by the addition of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) did not affect growth, but cells showed reduced adhesion strength to both glass and PDMSE. Addition of the nitric oxide synthase inhibitor NG-monomethyl-l-arginine (NMMA) caused a small but significant increase in adhesion strength. Overall, the results suggest that NO acts as a signal of the wettability properties of substrata for Seminavis.