Heterotrophic bacterioplankton plays a pivotal role in marine food webs and biogeochemical cycling. However, their temporal dynamics and underlying factors are still poorly understood in many regions, including the tropical waters of the Red Sea. The main goal of the MS project was to describe the seasonality and assess the impact of top-down controls (viruses and heterotrophic nanoflagellates) in parallel to bottom-up controls (substrate availability) on coastal bacterioplankton on a weekly basis. To that end, we monitored the abundance of the different planktonic groups by flow cytometry together with a set of environmental variables including temperature, salinity, dissolved organic carbon (DOC) and nitrogen (DON) and chlorophyll a concentration. We analyzed a weekly dataset collected over 2017 at the surface water of KAUST Harbor. The abundance of heterotrophic bacteria ranged from 1.55 to 4.97 x 105 cells ml-1, with that of autotrophic bacteria 4 to 14 fold less on average and presents 1 x 104 to 1.19x105 cells ml-1, while viruses ranged from 1.30 x 106 to 1.59 x 107 particles ml-1, and heterotrophic nanoflagellates (HNF) ranged from 8.62 x 10 to 1.63 x 103 cells ml-1. We distinguished between five groups of heterotrophic bacteria depending on the relative nucleic acid content, membrane state and cell-specific metabolic activity, two groups of Synechococcus, as well as three groups of viruses based on relative nucleic acid content. We found unexpected inverse relationship between viruses and HNFs. Based on a strong negative correlation, the results suggest that viruses controlled heterotrophic bacteria during summer until early winter period. HNFs showed a selective grazing behavior based their apparent preference to prey on both high (HNA) and low nucleic acid bacteria (LNA). Our results demonstrates that top-down control are key agents of heterotrophic bacterioplankton mortality and more important than bottom-up control in governing heterotrophic bacterioplankton abundances in the coastal tropical waters of the Red Sea.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19960号 / 理博第4227号 / 新制||理||1607(附属図書館) / 33056 / 京都大学大学院理学研究科生物科学専攻 / (主査)教授 中野 伸一, 教授 木庭 啓介, 教授 沼田 英治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Made available in DSpace on 2016-06-02T19:32:00Z (GMT). No. of bitstreams: 1 4173.pdf: 1140916 bytes, checksum: 20dfe79f655af716f14bb7ff0f256252 (MD5) Previous issue date: 2012-02-13 / Universidade Federal de Minas Gerais / Planktonic bacteria play important role in the organic matter decomposition, nutrient cycling and as food for organisms of higher trophic levels such as flagellates and ciliates, thus representing fundamental components in planktonic food webs. This study analyzed the microbial community structure and evaluated the importance of heterotrophic nanoflagellates (HNF) and ciliates bacterivory in a subtropical environment predominantly mesotrophic (Lobo-Broa reservoir, Brazil) by the quantification of their ingestion rates during one year period. The in situ experiments using fluorescently labelled bacteria (FLB) were conducted on the surface of two sampling points that have different trophic degrees. In parallel, physical, chemical and biological water variables were also evaluated. The ciliates showed a higher specific ingestion rate of bacteria (3,3 x 103 bact ind-1 h-1) than HNF (79,7 bact ind-1 h-1) due to differences in size and food acquisition among the two protozoans. The ingestion rates for both ciliates and HNF were always higher in the point 2 (P2) probably due to the higher water temperatures, which accelerate the protozoans metabolism, and higher bacterial densities. Concerning total protozoan bacterivory, the HNF populations had the greatest grazing impact (60%) on the reservoir bacterial community, especially the HNF smaller than 5 μm. The data showed that HNF grazing, in addition to regulation the bacteria abundance, probably also induced changes in the bacterial community structure, such as increasing bacterial filaments numbers and size. The ciliates contributed significantly to the bacterivory, especially in P2, where high densities and ingestion rates were verified. The main ciliate consumers of bacteria were from Spirotrichea, Peritrichia, Haptoria and Scuticociliatia groups. The bottom-up control was more important in the dry season and the top-down control was more important in the rainy one. Thus, in an annual basis, the resource availability and protozoan predation are equally important to the bacterial abundance regulation in the Lobo reservoir. / Bactérias planctônicas desempenham importante papel na decomposição da matéria orgânica, na reciclagem de nutrientes e como alimento para organismos de níveis tróficos superiores tais como flagelados e ciliados, representando, assim, componentes fundamentais nas teias alimentares. Este estudo analisou a estrutura da comunidade microbiana e avaliou a importância da bacterivoria por nanoflagelados heterotróficos (NFH) e ciliados em um ambiente subtropical predominantemente mesotrófico (reservatório do Lobo-Broa, Brasil) pela quantificação de suas taxas de ingestão durante o período de um ano. Os experimentos in situ utilizando bactérias marcadas fluorescentemente (FLB) foram realizados na superfície de dois pontos de coleta que apresentam graus de trofia diferentes. Em paralelo foram avaliadas variáveis físicas, químicas e biológicas da água. Os ciliados apresentaram taxa de ingestão individual de bactérias maior (3,3 x 103 bact ind-1 h-1) do que a dos NFH (79,7 bact ind-1 h-1) devido às diferenças de tamanho e forma de obtenção de alimento entre os dois protozoários. As taxas de ingestão, tanto para os ciliados quanto para os NHF, foram sempre maiores no ponto 2 (P2) devido provavelmente as maiores temperaturas da água, que aceleram o metabolismo dos protozoários, e as maiores densidades de bactérias. Em relação à bacterivoria total por protozoários, as populações de NFH causaram maior impacto de predação (60%) sobre a comunidade bacteriana do reservatório, principalmente os NFH menores que 5μm. Os dados indicaram que a predação por NFH, além de regular a abundância de bactérias, provavelmente também induziu modificações na estrutura da comunidade bacteriana, como o aumento do tamanho e da quantidade de filamentos bacterianos. Os ciliados contribuíram significativamente para a bacterivoria, principalmente no P2, onde apresentaram altas densidades e taxas de ingestão. Os principais grupos de ciliados consumidores de bactérias foram os Spirotrichea, Haptoria, Peritrichia e Scuticociliatia. O controle bottom-up teve maior importância no período seco e o controle top-down maior importância no período chuvoso. Assim, em uma base anual, a disponibilidade de recursos e a predação são igualmente importantes na regulação da abundância bacteriana no reservatório do Lobo.

Ce travail est une contribution à la connaissance de l’importance des particules de nature polysaccharidique, les TEP (Transparent Exopolymer Particles) ou particules d’Exopolymères Transparentes, en milieu pélagique lacustre.Les différentes études présentées dans ce mémoire se sont essentiellement focalisées sur la distribution de ces particules et leur relation avec le phytoplancton et les autres microorganismes du réseau trophique aquatique en milieu naturel (le lac Pavin, oligo-mésotrophe et le réservoir hypereutrophe de Grangent) et en conditions semi contrôlées(enclos limniques installés sur le lac de Créteil). Les résultats de l’étude printanière au lac Pavin indiquent que la majorité des TEP sont colonisées par les bactéries et que l’intensité de colonisation est fortement liée à la température et diminue avec l’augmentation en taille des particules. La distribution des nanoflagellés hétérotrophes (HNF) est fortement liée à la densité des TEP mais pas à l’intensité de colonisation de ces particules. L’abondance et la surface cumulée de TEP sont significativement plus élevées dans le lac oligo-mésotrophe que dans le réservoir hypereutrophe de Grangent. Les abondances et les concentrations élevées de particules dans le lac Pavin coïncident avec la présence de diatomées de grande taille au printemps et en automne et avec les chlorophycées à la fin de l’été.Dans le réservoir de Grangent les valeurs maximales de TEP coïncident avec le développement de la cyanobactérie Microcystis aeruginosa. Si les TEP augmentent avec la productivité de l’écosystème, la production de ces particules par unité de chlorophylle a dépend de la composition algale et tend à diminuer avec l’augmentation du niveau trophique du milieu. Les résultats issus de la biomanipulation en enclos limniques indiquent que la structure du réseau trophique aquatique (par la présence ou l’absence de poissons planctonophages) influence fortement la distribution,la dynamique et le spectre de taille des TEP. Dans le traitement poisson, l’abondance des TEP, la chlorophylle a et la biomasse des chlorophycées sont fortement corrélées. De par son broutage sur le phytoplancton, le zooplancton a un effet négatif sur les TEP dans le traitement sans poissons mais il contribue sans doute à la formation de TEP e tinfluence le spectre de taille de ces dernières dans ce traitement. Ce travail souligne l’importance des particules de nature polysaccharidique en milieu pélagique lacustre qui doivent être considérées comme une part importante du carbone organique qui transite des producteurs primaires vers les décomposeurs et vers le sédiment. / This work adds to the knowledge of the significance of polysaccharidic detrital particles or TEP (= Transparent Exopolymer Particles) in freshwater pelagic environments. Studies in this thesis have mainly focused on the distribution of TEP and their relationships with phytoplankton and other microorganisms in natural environments (the oligo-mesotrophic Lake Pavin and the hypereutrophic reservoir of Grangent) and in limnetic enclosures (lake of Créteil). The intensity of bacterial colonization during spring in Lake Pavin was strongly related to temperature and decreased with particle size. The abundance of heterotrophic nanoflagellates (HNF) in this lake was more significantly related to the density of the particles than to the density of total bacteria and the intensity of bacterial colonization of TEP, suggesting that TEP is a more important factor for HNF development than attached and free bacteria. The abundance and the total surface area of the particles were significantly higher in the hypereutrophic Lake Grangent than in the mesotrophic Lake Pavin. Maximum TEP density in Lake Pavin was recorded during the spring diatom bloom, while TEP concentration peaked in late summer when the phytoplankton community was largely dominated by small-size chlorophytes with an abundant polysaccharide cell coating. In the hypereutrophic Lake Grangent,maximum values of TEP appeared during the summer development of the cyanobacterium Microcystis aeruginosa. Per cell production of TEP, expressed by the ratio between TEP concentration and chlorophyll a concentration, was significantly higher in the less productive lake and the analysis of the size spectra of the particles indicated a greater proportion of small particles in this lake. TEP therefore appear as more significant for microbial development and aggregates formation in the less productive environment. Results from limnetic enclosures (either dominated by planktivorous fish or fishless) indicated that food-web structure strongly influences the distribution and size spectra of TEP. TEP abundances were related to chlorophyll a concentrations and the biomass of chlorophytes in the fish treatment. As expected by the trophic cascades theory, zooplankton had an indirect negative effect on TEP abundance. Our results suggest, however, that metazoan probably influence the formation and the size spectra of the particles in the fishless treatment. TEP must be regarded as a major part of the organic carbon which is transferred from the primary producers to the microbial food web and the sediments in freshwater ecosystems.

<p>Heterotrophic microorganisms are important for the flow of carbon and nutrients in the sea. Bacteria, nanoflagellates and ciliates are relevant components of the pelagic food web. In order to be able to predict the outcome of e.g. eutrophication or climate change we need to know how the different components of the pelagic food web are regulated. With the focus on the northern Baltic Sea food web, this thesis deals with limitation and control of heterotrophic protists, the effect of resource heterogeneity on food web efficiency and diversity of nanoflagellates.</p><p>In-situ microcosm experiments showed that the net growth of heterotrophic flagellates were resource limited throughout the year. Field data confirmed that the abundance of flagellates was bottom-up controlled. Furthermore, field data also showed that the annual average biomass of protists, flagellates and ciliates increased with primary productivity. On a smaller seasonal scale temperature and bacterial biomass were able to explain most of the variation in flagellate biovolume. The temporal variation in ciliate biovolume could not be explained by any bottom-up factors like bacterial biomass, flagellate biomass or chlorophyll a. This and an in-situ microcosm experiment implied that the seasonal dynamics of ciliates were more regulated by predators like mesozooplankton.</p><p>The food web efficiency i.e. how much of production at the resource level is converted to production at the top trophic level, may be affected by specific size or type of resource. Indoor mesocosms revealed that the food web efficiency was 11 times lower when heterotrophic bacteria dominated basal production instead of nano- and micro-sized phytoplankton. This was due to a lengthening of the food web when pico-sized bacteria constituted the main resource.</p><p>The PCR-DGGE molecular biological method was used to study the diversity of heterotrophic or mixotrophic chrysomonads. The focus was set on chrysomonads due to their relatively large contribution to the nanoflagellate community. Group-specific PCR primers were optimized for the target group. A field survey in the northern Baltic Sea showed that a handful of chrysomonad sequences were present throughout the year. Significantly more chrysomonads were recorded in the basin with higher primary productive and salinity. In total 15-16 different chrysomonad sequences were recorded. Most of them matched uncultured chrysomonad clones.</p>

Heterotrophic microorganisms are important for the flow of carbon and nutrients in the sea. Bacteria, nanoflagellates and ciliates are relevant components of the pelagic food web. In order to be able to predict the outcome of e.g. eutrophication or climate change we need to know how the different components of the pelagic food web are regulated. With the focus on the northern Baltic Sea food web, this thesis deals with limitation and control of heterotrophic protists, the effect of resource heterogeneity on food web efficiency and diversity of nanoflagellates. In-situ microcosm experiments showed that the net growth of heterotrophic flagellates were resource limited throughout the year. Field data confirmed that the abundance of flagellates was bottom-up controlled. Furthermore, field data also showed that the annual average biomass of protists, flagellates and ciliates increased with primary productivity. On a smaller seasonal scale temperature and bacterial biomass were able to explain most of the variation in flagellate biovolume. The temporal variation in ciliate biovolume could not be explained by any bottom-up factors like bacterial biomass, flagellate biomass or chlorophyll a. This and an in-situ microcosm experiment implied that the seasonal dynamics of ciliates were more regulated by predators like mesozooplankton. The food web efficiency i.e. how much of production at the resource level is converted to production at the top trophic level, may be affected by specific size or type of resource. Indoor mesocosms revealed that the food web efficiency was 11 times lower when heterotrophic bacteria dominated basal production instead of nano- and micro-sized phytoplankton. This was due to a lengthening of the food web when pico-sized bacteria constituted the main resource. The PCR-DGGE molecular biological method was used to study the diversity of heterotrophic or mixotrophic chrysomonads. The focus was set on chrysomonads due to their relatively large contribution to the nanoflagellate community. Group-specific PCR primers were optimized for the target group. A field survey in the northern Baltic Sea showed that a handful of chrysomonad sequences were present throughout the year. Significantly more chrysomonads were recorded in the basin with higher primary productive and salinity. In total 15-16 different chrysomonad sequences were recorded. Most of them matched uncultured chrysomonad clones.