Comprendre les facteurs favorables à l’établissement et au maintien de proliférations d’algues toxiques et nuisibles est essentiel afin de mieux les prévoir. L’allélopathie, c’est-à-dire la libération de composés qui inhibent les compétiteurs, favorise l’espèce allélopathique et contribue ainsi à modifier la communauté planctonique. Les dinoflagellés toxiques du genre Alexandrium produisent des composés allélopathiques, cependant les interactions allélopathiques sont encore mal comprises. L'objectif de cette thèse était de contribuer à la compréhension des mécanismes d’allélopathie en étudiant l'interaction allélopathique entre A. minutum et la diatomée Chaetoceros muelleri. Les résultats de ces travaux ont mis en évidence que les composés allélopathiques d'A.minutum perturbaient le fonctionnement des membranes des diatomées en quelques minutes. La cascade d'événements physiologiques suivant l'interaction allélopathique comprend des effets tels que la perméabilisation de membranes, l'inhibition de la photosynthèse, la production d'espèces réactives de l'oxygène et la modification de la composition des membranes. Ce projet a mis en évidence le fait que l’allélopathie d’A. minutum est sous contrôle environnemental, son activité s’est accrue d’un facteur 4 en présence de concentrations toxiques de Cu. Enfin, le développement d’un bioessai a facilité la purification des composés allélopathiques par fractionnement guidé. Nous avons isolé un groupe de candidats hydrophobes dont l’activité doit être testée après isolement. Cette thèse propose de nouveaux outils et ouvre de nouvelles perspectives pour étudier les conséquences de ces interactions dans les écosystèmes marins. / Understanding the factors that favor the establishment and the persistance of harmful algal blooms is essential for predicting them. Allelochemical potency, i.e. the release of compounds that inhibit competitors, is hypothesized to favor the organisms that produce them and shape plankton community. The toxic dinoflagellates from the genus Alexandrium produce allelochemicals, however, associated interactions are poorly understood. The objective of this thesis is to contribute to our understanding by studying the mechanisms behind the allelochemical interactions between the A. minutum and the diatom Chaetoceros muelleri. The results of this work have highlighted that A. minutum allelochemicals disrupted membrane functioning of the diatoms within minutes. The cascade of physiological events following the allelochemical interaction that was described here, included permeabilisation of membranes, inhibition of photosynthesis, production of reactive oxygen species and modifications of the biochemical composition of membranes. This project has also highlighted that the allelochemical potency of A. minutum can be significantly modulated by environmental parameters, as allelochemical potency increased 4 times when exposed to toxic concentrations of Cu. Finally, a bioassay developed during this PhD eased the partial isolation of the allelochemicals through fractionation. We isolated a hydrophobic candidates whose activity have to be investigated following their isolation. This PhD provides a better understanding of the cellular mechanisms underlying allelochemical interactions and offers new tools and raises new perspectives to study the consequences of these interactions on marine ecosystems.

Diatoms are estimated to contribute 25 percent of the primary production on Earth and therefore they play a significant role in the global carbon cycle. Diatom blooms often terminate with the formation of aggregates that sink rapidly from surface waters, affecting the flux of organic carbon from the surface to deep waters and the sea floor. The role of carbon-rich transparent exopolymeric particles (TEP) in aggregate formation as ocean temperature increases has yet to investigated in continuous cultures. I hypothesize that temperature increase can influence the production of TEP, a fraction of total suspended exopolymers. To test the hypothesis, a laser in situ scattering and transmissometry instrument (LISST-100X, Sequoia Instruments) successfully counted and sized six individual diatom species in batch culture: Chaetoceros muelleri, Coscinodiscus wailesii, Thalassiosira weissflogii, Phaeodactylum tricornutum, Skeletonema costatum, and Skeletonema marinoi and successfully demonstrated its efficacy in detecting diatom aggregates using S. costatum. Four replicate continuous cultures were sampled for particle size distribution (PSD), nutrients, chlorophyll a, total carbohydrates, prokaryote concentration, and TEP at temperatures of 22.5, 27 and then 20 degrees C. While TEP particles were scarce, acid polysaccharide (APS)-coated C. muelleri cells were observed, forming dense webs on the filters. Both carbohydrate per cell and APS area per cell were found to significantly correlate with temperature (p<0.05) while significant difference between APS concentration at each temperature was only found between 27 and 22.5 or 20 degrees C (p<0.05). Net changes in PSDs with increasing temperature showed that distributions of relative volume concentration decreased in the smallest size bins and increased in the largest size bins. Our results show that increasing the temperatures of nitrogen-limited C. muelleri cultures did not cause increased TEP formation but instead resulted in increased cell-surface coating. Increasing concentration of cell coatings and TEP particles will cause diatoms to aggregate more readily, enhancing their sinking rate away from the ocean surface. Increased ocean temperature has great implications for diatom blooms and other microorganisms, causing greater export of carbon out of the surface waters and potentially altering the microbial loop.

Made available in DSpace on 2014-06-12T23:02:30Z (GMT). No. of bitstreams: 2 arquivo8258_1.pdf: 1570642 bytes, checksum: 2b0c8c62eb728b6a2ff6cca67f0a17bb (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2007 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O presente trabalho teve por objetivo determinar a composição bioquímica de Chaetoceros muelleri Lemmerman, uma importante espécie para atividades na aqüicultura, em diferentes meios de cultivo. Os bioensaios foram realizados em duas etapas e mantidos nas mesmas condições (iluminação constante; 22±1°C; pH de 8 a 9 e salinidade 33 ups). O meio controle utilizado foi F/2 Guillard (F/2) e os meios alternativos de cultivo foram extrato de esterco de gado (EEG) e de minhoca (EEM). Na primeira etapa, a diatomácea C. muelleri foi cultivada em 500 mL dos meios de cultivo, em triplicatas, e determinou-se a densidade celular, taxa diária de crescimento, clorofila a e biovolume. A fase exponencial foi definida através da curva de crescimento. A segunda etapa dos bioensaios foi iniciada em garrafão plástico até o volume final de 20 litros, para cada meio. A cada aumento do volume foram determinadas a densidade celular e a concentração de clorofila a. As análises bioquímicas foram realizadas com o material precipitado dos garrafões (20 L), através de centrifugação (4000 rpm, 15 min). Esse precipitado foi seco em estufa até peso constante, à temperatura de 40ºC. Foram analisados nitrogênio total, carboidrato total, proteína total e lipídeo total, em triplicatas. Na primeira etapa dos bioensaio, as maiores densidades celulares foram de 11,05±1,32 x106 céls.mL-1, F/2; no 60 dia; de 8,33±0,83 x106 céls.mL-1, EEG, no 40 dia e de 6,86±0,38 x106 céls.mL-1, EEM, 80 dia. O valor médio da taxa diária de crescimento celular de C. muelleri, na fase log, para o meio F/2 foi de 0,65±0,70, enquanto que, para os meios EEG e EEM foi de 0,97±0,43 e 0,36±0,31, respectivamente. Os teores máximos de clorofila a foram de 0,34±0,03 pg.cél.-1, no 70 dia, meio F/2; de 0,74±0,90 pg.cél.-1, no 90 dia; meio EEG e de 2,26±1,70 pg.cél.-1, no 120 dia, meio EEM. Os maiores valores de biovolume celular foram de 528,55±321,06 &#956;m3, para o meio F/2 e de 515,28±351,87 &#956;m3, para o meio EEG, no 50 dia, em ambos meios; e, de 534,17±454,13 &#956;m3, para o meio EEM, no 110 dia. Na segunda etapa dos bioensaios, para o volume final de 20 L, as concentrações médias da densidade celular para o meio F/2 foi de 21,61±0,24 x106 céls.mL-1, para o meio EEG de 18,53±0,43 x106 céls.mL-1 e para o meio EEM de 23,07±0,44 x106 céls.mL-1. Os teores médios de clorofila a foram de 0,90±0,01 x10-7 pg.cél.-1, para o meio EEG, seguida de 0,79±0,05 x10-7 pg.cél.-1, para o meio EEM e de 0,73±0,02 x10-7 pg.cél.-1, para o meio F/2. Quanto ao nitrogênio total, os valores médios foram 3,35±0.06% de matéria seca, para o meio EEM; 2,30±0,05% de matéria seca, para o meio F/2 e 1,80±0.05% de matéria seca, para o meio EEG. Em relação ao valor médio de lipídio total, no meio EEM foi de 0,112±0,001 pg.cél.-1; e para os meios F/2, de 0,104±0,001 pg.cél.-1 e EEG, 0,100±0,001 pg.cél.-1. Para a proteína total, o valor médio no meio EEM, foi de 0,226±0,012 pg.cél.-1 e nos meios F/2, de 0,140±0,014 pg.cél.-1 e EEG, 0,125±0,001 pg.cél.-1. Para o carboidrato total, o valor médio no meio EEM, foi de 36,22±1,53 pg.cél.-1, seguido dos meios EEG, 29,16±0,81 pg.cél.-1 e F/2, 19,76±0,85 pg.cél.-1. Entre as médias dos dados analisados para os bioensaios (F/2, EEG e EEM), foram encontradas diferenças significativas (p<0,05). Estatisticamente, em relação aos parâmetros analisados para a espécie C. muelleri, o meio extrato de esterco de minhoca demonstrou que pode ser utilizado como fonte nutritiva para o desenvolvimento algal, podendo substituir meios convencionais como o meio F/2 Guillard