Linking phytoplankton pigment composition and optical properties: A framework for developing remote-sensing metrics for monitoring cyanobacteria

International audienceThis study has been performed in the framework of a research program aiming to develop a low-cost aerial sensor for the monitoring of cyanobacteria in freshwater ecosystems that could be used for early detection. Several empirical and mechanistic remote-sensing tools have been already developed and tested at large scales and have proven useful in monitoring cyanobacterial blooms. However, the effectiveness of these tools for early detection is hard to assess because such work requires the detection of low concentrations of characteristic pigments amid complex ecosystems exhibiting several confounding factors (turbidity, blooms of other species, etc.). We developed a framework for performing high-throughput measurements of the absorbance and reflectance of small volumes (~= 20 mL) of controlled mixtures of phytoplankton species and studied the potential of this framework to validate remote-sensing proxies of cyanobacteria concentration. The absorption and reflectance spectra of single and multiple cultures carried a specific signal that allowed for the quantitative analysis of culture mixes. This specific signal was shown to be related to known pigment absorbance spectra. The concentrations of chlorophyll-a and -b, phycocyanin and phycoerythrin could be obtained from direct absorbance measurements and were correlated with the concentration obtained after pigment extraction (R2 ≥ 0.96 for all pigments). A systematic test of every possible two-band and three-band normalized difference between optical indices was then performed, and the coincidental correlation with chlorophyll-b (absent in cyanobacteria) was used as an indicator of non-specificity. Two-band indices were shown to suffer from non-specificity issues and could not yield strong and specific relationships with phycocyanin or phycoerythrin (maximum R2  0.8)

Unusual cohabitation and competition between Planktothrix rubescens and Microcystis sp (cyanobacteria) in a subtropical reservoir (Hammam Debagh) located in Algeria

Succession in bloom-forming cyanobacteria belonging to distant functional groups in freshwater ecosystems is currently an undescribed phenomenon. However in the Hammam Debagh reservoir (Algeria), P. rubescens and Microcystis sp. co-occur and sometimes proliferate. With the aim of identifying the main factors and processes involved in this unusual cohabitation, water samples were collected monthly from February 2013 to June 2015 at the subsurface at four sampling stations and along the entire water column at one sampling station. In addition, the composition of the cyanobacterial communities was estimated by Illumina sequencing of a 16S rRNA gene fragment from samples collected over one year (October 2013-November 2014). This molecular approach showed that the Hammam Debagh reservoir displays high species richness (89 species) but very low diversity due to the high dominance of Microcystis in this community. Furthermore, it appears that Planktothrix rubescens and Microcystis sp. coexisted (from September to January) but proliferated alternately (Spring 2015 for P. rubescens and Spring 2014 and Autumn 2014/2015 for Microcystis). The main factors and processes explaining these changes in bloom-forming species seem to be related to the variation in the depth of the lake during the mixing period and to the water temperatures during the winter prior to the bloom season in spring

The phyto-bacterioplankton couple in a shallow freshwater ecosystem: Who leads the dance?

International audienceBloom-forming phytoplankton dynamics are still unpredictable, even though it is known that several abiotic factors, such as nutrient availability and temperature, are key factors for bloom development. We investigated whether biotic factors, i.e. the bacterioplankton composition (via 16SrDNA metabarcoding), were correlated with phytoplankton dynamics, through a weekly monitoring of a shallow lake known to host recurrent cyanobacterial blooms. We detected concomitant changes in both bacterial and phytoplankton community biomass and diversity. During the bloom event, a significant decrease in phytoplankton diversity, was detected, with a first co-dominance of Ceratium, Microcystis and Aphanizomenon, followed by a co-dominance of the two cyanobacterial genera. In the same time, we observed a decrease of the particle-associated (PA) bacterial richness and the emergence of a specific bacterial consortium that was potentially better adapted to the new nutritional niche. Unexpectedly, changes in PA bacterial communities occurred just before the development the emergence of the phytoplanktonic bloom and the associated modification of the phytoplanktonic community composition, suggesting that changes in environmental conditions leading to the bloom, were first sensed by the bacterial PA community. This last was quite stable throughout the bloom event, even though there were changes in the blooming species, suggesting that the association between cyanobacterial species and bacterial communities may not be as tight as previously described for monospecific blooming communities. Finally, the dynamics of the freeliving (FL) bacterial communities displayed a different trajectory from those of the PA and phytoplankton communities. This FL communities can be viewed as a reservoir for bacterial recruitment for the PA fraction. Altogether, these data also highlight s that the spatial organization within these different microenvironments in the water column is a relevant factor in the structuring of these communities

Role of bacteria in the production and degradation of Microcystis cyanopeptides

International audienceThe freshwater cyanobacteria, Microcystis sp., commonly form large colonies with bacteria embedded in their mucilage. Positive and negative interactions between Microcystis species and their associated bacteria have been reported. However, the potential role of bacteria in the production and degradation of cyanobacterial secondary metabolites has not been investigated. In this study, a Microcystis-associated bacterial community was isolated and added to the axenic M. aeruginosaPCC7806 liquid culture. After 3 years of cocultivation, we studied the bacterial genetic diversity adapted to the PCC7806 strain and compared the intra- and extracellular concentration of major cyanopeptides produced by the cyanobacterial strain under xenic and axenic conditions. Mass spectrometric analyses showed that the intracellular concentration of peptides was not affected by the presence of bacteria. Interestingly, the produced peptides were detected in the axenic media but could not be found in the xenic media. This investigation revealed that a natural bacterial community, dominated by Alpha-proteobacteria, was able to degrade a wide panel of structurally varying cyclic cyanopeptides

Linking phytoplankton pigment composition and optical properties: A framework for developing remote-sensing metrics for monitoring cyanobacteria

International audienceThis study has been performed in the framework of a research program aiming to develop a low-cost aerial sensor for the monitoring of cyanobacteria in freshwater ecosystems that could be used for early detection. Several empirical and mechanistic remote-sensing tools have been already developed and tested at large scales and have proven useful in monitoring cyanobacterial blooms. However, the effectiveness of these tools for early detection is hard to assess because such work requires the detection of low concentrations of characteristic pigments amid complex ecosystems exhibiting several confounding factors (turbidity, blooms of other species, etc.). We developed a framework for performing high-throughput measurements of the absorbance and reflectance of small volumes (~= 20 mL) of controlled mixtures of phytoplankton species and studied the potential of this framework to validate remote-sensing proxies of cyanobacteria concentration. The absorption and reflectance spectra of single and multiple cultures carried a specific signal that allowed for the quantitative analysis of culture mixes. This specific signal was shown to be related to known pigment absorbance spectra. The concentrations of chlorophyll-a and -b, phycocyanin and phycoerythrin could be obtained from direct absorbance measurements and were correlated with the concentration obtained after pigment extraction (R2 ≥ 0.96 for all pigments). A systematic test of every possible two-band and three-band normalized difference between optical indices was then performed, and the coincidental correlation with chlorophyll-b (absent in cyanobacteria) was used as an indicator of non-specificity. Two-band indices were shown to suffer from non-specificity issues and could not yield strong and specific relationships with phycocyanin or phycoerythrin (maximum R2  0.8)

An automatic monitoring and modelling system of cyanobacteria dynamics in urban water bodies with high-frequency measurements

International audienceThe hydrodynamics and ecological status of water bodies have been affected during the last decades by anthropogenic stressors. While climate change threatens lakes globally, urbanization can enhance symptoms of eutrophication locally by increasing nutrient loads. These anthropogenic pressures need to be carefully assessed through relevant monitoring, and generalized via modelling approaches.The scarcity of available data limits the knowledge and the transfer of research outcomes to decision makers. During the last decade, the availability of a wide range of sensors allowed to collect high-frequency measurements of variables relevant to the functioning of the water bodies. Nevertheless, a limited number of monitoring systems has been developed for measuring the cyanobacteria specific photosynthetic pigment, phycocyanin, as well as for designing warning systems for preventing public health risks from cyanobacteria blooms.This paper will present outcomes obtained in the framework of the OSS-CYANO project (funded by ANR), regarding: (i) a full-scale experimental site for high-frequency monitoring of cyanobacteria biomass in an urban lake; (ii) a data transfer platform for real-time management of harmful blooms; (iii) the automatic design for data analysis and three dimensional model initialization, and (iv) how the short term predictions of the cyanobacteria biomass can be used for supporting decision-making

Vertical variation in <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. densities and the mixing depth (Z_m) in the center (St3) of the Hammam Debagh reservoir.

<p>Interpolated cyanobacteria densities were obtained using the Surfer Software (v. 7.0, Golden Software Inc.).</p

Spatio-temporal variation in chlorophyll-<i>a</i> concentrations under the surface and in <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. cell densities over the first 1 m in the subsurface of the lake at the four sampling stations.

<p>Interpolation was obtained with Surfer (v. 7.0, Golden Software Inc.).</p

Temporal variation in the abundance of <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. estimated by microscopic cell counting and by the number of reads in our Illumina dataset (from October 2013 to November 2014).

<p>(a) Indicates the number of reads of <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. performed by Illumina MiSeq sequencing. DNA extracts from 4 sampling points were pooled before sequencing. Reads were normalized to the smallest sample (n = 67,012 reads) on the basis of bacterial sequences. (b) Indicates the temporal dynamics in <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. cell counts. Cell counts are expressed as the median values of the data collected at the four sampling points. * No sample available.</p

Principal component analysis (PCA) performed on the dataset collected from the surface layer during the whole study period.

<p>(a) Correlation of environmental parameters (blue arrows) with the first factorial plane defined by the two first axes. <i>P</i>. <i>rubescens</i> and <i>Microcystis</i> sp. cellular abundances were included in the analysis as supplemental variables. (b) Dispersion of the observations (black points) on the first factorial plane defined by the two first components of the analysis; samples are grouped by months in ellipses. Z_max: maximum depth; %O₂: oxygen saturation; Evap: evaporation; WT°C: water temperature; AT°C: air temperature; Z_eu: depth of the euphotic zone; Cond: conductivity; Z_m: mixing depth; Turb: water turbidity; TDS: total dissolved solids.</p