Much of the world’s aquatic food webs and nutritional relationships have been blurred by the ever-increasing evidence that many phytoplankton are not exclusively heterotrophic or autotrophic, but instead mixotrophic. Mixotrophy is a continuum of different energy and carbon-acquisition mechanisms utilizing both autotrophy and heterotrophy which distorts the concept of single trophic tier modality. This makes mixotrophs flexible to adapt to environmental pressures and is becoming more the rule than the exception in many aquatic ecosystems. One unique environmental setting where mixotrophy could be highly beneficial to food web stability is in seasonally occurring ephemeral pools – aka vernal pools. Mid-Atlantic vernal pools are biodiverse biogeochemical hotspots and critical breeding habitats for a diverse number of endemic taxa including many endangered amphibian species. Vernal pools are not permanent standing bodies of water and have fluxes in hydrology, temperatures, nutrients, and irradiance to name a few. These extremes make vernal pools an ideal setting for mixotrophic phytoplankton, however it’s never been investigated. Our survey found mixotrophy in every vernal pool sampled, as well as elevated grazing rates in pools experiencing nontypical seasonal conditions. From these small-scale forest pools to the world’s oceans mixotrophy is a widespread nutritional strategy. The Southern Ocean is essential for powering worldwide ocean circulation, regional biogeochemical cycles, and global climate. One of the major hurdles with understanding mixotrophy is identifying the phytoplankton capable of shifting nutritional strategies. While many Southern Ocean plankters have been properly identified as mixotrophic, one such keystone species has gone mislabeled until now. Phaeocystis antarctica is a well-studied Haptophyte algae that plays major roles in the global carbon and sulfur cycles. This species has been historically labeled as an obligate phototroph, but contradictory survives the long dark Antarctic winter without any known evidence of encystment. We suspect that this highly abundant species is in fact mixotrophic, capable of phagocytosis to supplement the irradiance shortcomings of the Antarctic dark. We experimented with varying degrees of light and nutrient limitations to determine possible triggers for P. antarctica grazing. Our results showed P. antarctica ingesting in every treatment, but its highest grazing rates corresponded with limitations to its primary photosynthetic mode. Apart from the newly realized complexity P. antarctica brings to the Southern Ocean food web, it is an environment that suffers from microplastic pollution that can impede these mixotrophic species. Mismanaged plastic waste around the world, especially microfiber discharge from laundered synthetic textiles, escape into the natural environment, and eventually concentrate in the oceans. The Southern Ocean can become disproportionately polluted in regions due to microfibers becoming sequestered once crossing the Antarctic circumpolar current and even becoming trapped in sea-ice formations. While it is easy to see the devastation plastic waste has on megafauna (i.e. turtles, fish, birds, and whales), its microscopic devastation is less obvious. Plastic waste comes in many forms and one less researched form is buoyant polyester microfibers <1mm that interact with colony forming algae. We utilized different concentrations of polyester microfibers and mixing speeds to determine if microfiber interactions with colony formations increases or decreases overall colony buoyancy. Smaller concentrations of polyester microfibers can impart a positively buoyant effect onto P. antarctica colonies regardless of mixing speed, however larger concentrations negatively affected colony buoyancy regardless of mixing speed. / Biology

Environmental heterogeneity in both space and time has significant repercussions for community structure and ecosystem processes. Dimictic lakes provide examples of vertically structured ecosystems that oscillate between stable and mixed thermal layers on a seasonal basis. Vertical patterns in abiotic conditions vary during both states, but with differing degrees of variation. For example, during summer thermal stratification there is high spatial heterogeneity in temperature, nutrients, dissolved oxygen and photosynthetically active radiation. The breakdown of stratification and subsequent mixing of the water column in fall greatly reduces the stability of the water column to a vertical gradient in light. Nutrients and biomass that were otherwise constrained to the depths are also suspended, leading to a boom in productivity. Freshwater lakes are teeming with microbial diversity that responds to the dynamic environment in a seemingly predictable manner. Although such patterns have been well studied for nanoplanktonic phototrophic and heterotrophic populations, less work has been done to integrate the influence of mixotrophic nutrition to the protistan assemblage. Phagotrophy by phytoplankton increases the complexity of nutrient and energy flow due to their dual functioning as producers and consumers. The role of mixotrophs in freshwater planktonic communities also varies depending on the relative balance between taxon-specific utilization of carbon and energy sources that ranges widely between phototrophy and heterotrophy. Therefore, the role of mixotrophy in the microbial food web is difficult to predict because functional types of mixotrophs along a gradient of nutritional strategies contribute differently to nutrient cycling and carbon sequestration. The overall objective of this work was to advance existing knowledge of the abundance and activity of phagotrophy phytoplankton in lacustrine systems. The incorporation of mixotrophy into the microbial food web requires the complement of physiological studies in culture (as described in chapter 2) and quantification of activity (including abundance and bacterivory) in relation to strict phototrophs and heterotrophs in situ (as described in chapter 3 and 4). Information on the physiological ecology of mixotrophic protists is crucial to understanding their role in planktonic food webs and influence on the dynamic microbial community structure in lake ecosystems. An understanding of the ecological functioning of lakes has ultimate consequences for management of water resources, particularly in the face of global climate change. / Biology

Polar regions impose harsh conditions, including low temperatures, and prolonged periods of darkness on resident microbial communities. Despite these challenges, the conditions in these environments can also create opportunities for organisms utilizing combined trophic strategies (Mixotrophy). Only a limited number of studies have identified mixotrophic behavior in polar microbial eukaryotes, and even fewer studies have quantified the response of mixotrophs to likely environmental drivers of trophic behavior (light and nutrients). The goal of this work is to provide an identification of mixotrophic behavior and elucidate of some of the factors that influence algae isolated from polar environments. First, a study of the Arctic prasinophyte, Micromonas pusilla is presented in the first species-specific identification of mixotrophy in a eukaryotic phytoflagellate of this size class. M. pusilla grazed on bacteria under all experimental conditions, responding to nutrient limitation with increased rates of bacterivory. M. pusilla also showed evidence of prey selection. In contrast to the phagotrophic response, photosynthetic production was decreased under low-nutrient conditions. In an additional study of microbial eukaryotes from the Antarctic environment, identification of phagotrophy in photosynthetic nanoflagellates representing multiple evolutionary lineages: Cryptophyceae (Geminigera cryophila) and Prasinophyceae (Pyramimonas tychotreta and Mantoniella antarctica), showed that mixotrophy is more widespread in the Southern ocean that previously thought. G. cryophila and M. antarctica increased ingestions in dark treatments, but did not respond to difference in nutrient concentrations. In contrast, no significant grazing activity was observed in P. tychotreta under high nutrient conditions. When nutrients were reduced, ingestion of bacteria by P. tychotreta was observed and grazing increased in dark as compared to illuminated treatments. Finally, through a series of experimental assays, the competitive advantages of mixotrophic flagellates as opposed to monotrophic specialists were evaluated, using organisms isolated from the Southern Ocean. In these experiments, G. cryophila is emerged as a dominant competitor against two solely autotrophic diatoms (Fragilaria sp. and Fragilariopsis sp.). In contrast, P. tychotreta was outcompeted by the solely heterotrophic chrysophyte Paraphysomonas antarctica. These results show that mixotrophic ability can confer advantages to organisms in some cases, while in other interactions the cost associated with maintenance of multiple trophic strategies results in competitive exclusion by a specialist. These results present novel identification as well as rigorous investigation of mixotrophic behaviors in phototrophic flagellates from both polar (Arctic and Antarctic) environments representing two evolutionary lineages. This work provides a significant contribution to our understanding of the versatile nature of the physiology and trophic ecology of microbial eukaryotic organisms occupying polar marine ecosystems. / Biology

Made available in DSpace on 2016-06-02T19:32:00Z (GMT). No. of bitstreams: 1 4171.pdf: 1651530 bytes, checksum: 8003a7d4f21e79f409398cd7ec50f25d (MD5) Previous issue date: 2012-02-13 / Universidade Federal de Minas Gerais / Bacteria, found in terrestrial and aquatic environments, besides their important role in organic matter decomposition and nutrient cycling, are a food source for organisms of the higher trophic levels in the food chain, such as protozooplanktonic and metazooplanktonic organisms. This study aimed to evaluate the relative importance of protozooplanktonic (ciliates and heterotrophic nanoflagellates) and metazooplanktonic organisms (rotifers, cladocerans and copepods) as bacterial consumers throughout a year, which included a rainy and a dry season, in a eutrophic environment (Monjolinho reservoir-SP) by quantifying their ingestion rates. For this purpose, grazing experiments were carried out bimonthly during one year at the sub-surface of a central station of the reservoir using fluorescently labeled bacteria (FLB) dyed with DTAF. In each survey, besides the experiments, limnological (pH, dissolved oxygen, temperature, conductivity, water transparency and nutrients) and biological (chlorophyll-a and phaeophytin concentrations, bacterioplankton, protozooplankton and metazooplankton density) water parameters were also sampled. The heterotrophic nanoflagellates (HNF) showed higher population ingestion rates than the ciliates in dry and rainy seasons, and the HNF smaller than 5&#956;m were the main consumers of bacteria. Among the metazooplanktonic organisms, the rotifers dominated the bacterial ingestion, and the organisms belonging to the genus Filinia and Keratella and to the families Testudinellidae and Brachionidae were the main bacterial consumers. Besides ingesting bacteria, however, protozoa and rotifers population also seemed to be controlled by cladocerans and copepods predation, which, in turn, seemed to be regulated by predation of larger organisms, such as small vertebrates. Thus, the predation control (top-down) occured both on bacterial communities and protozooplanktonic and metazooplanktnic communities and bacteria were regulated both by direct and indirect predation of zooplanktonic population. The resources control (bottom-up) semmed to be higher during the dry season, while in the rainy season bacterial communities were more controlled by predation. The greatest impact on bacterial communities was caused by HNF and ciliates during the rainy season and by HNF and rotifers during the dry one. / As bacterias, encontradas em ambientes terrestres e aquaticos, alem de possuirem importante papel na decomposicao da materia organica e na ciclagem de nutrientes, sao fonte de alimento para organismos de niveis troficos superiores, tais como protozoarios e organismos do metazooplancton. O presente trabalho avaliou a importancia relativa de organismos protozooplanctonicos e metazooplanctonicos (rotiferos, cladoceros e copepodes) como consumidores de bacterias em um ambiente eutrofico (Reservatorio do Monjolinho-SP) pela quantificacao das taxas de consumo durante o periodo de um ano, que englobou uma estacao seca e uma chuvosa. Para isso, na superficie de um ponto central no reservatorio, foram realizados experimentos de bacterivoria (grazing) bimestralmente durante um ano, utilizando bacterias marcadas fluorescentemente (FLB) com o corante DTAF. Nos dias dos experimentos foram tambem analisados parametros limnologicos (pH, oxigenio dissolvido, temperatura, condutividade, transparencia da agua, e nutrientes) e biologicos (concentracoes de clorofila-a e feofitina, densidade do bacterioplancton, protozooplancton e metazooplancton). Os nanoflagleados heterotroficos (NFH), quando comparados aos ciliados, apresentaram maiores taxas populacionais de ingestao de bacterias tanto no periodo seco quanto no chuvoso, sendo os NFH menores que 5 &#956;m os maiores consumidores de bacterias. Dentre os organismos metazooplanctonicos, os rotiferos dominaram o consumo de bacterias, sendo os representantes dos generos Filinia e Keratella e das familias Testudinellidae e Brachionidae os maiores consumidores de bacteria. Os dados obtidos indicaram que, alem de predadores de bacterias, as populacoes de protozoarios e rotiferos pareceram ser reguladas pela predacao de cladoceros e copepodes, os quais, por sua vez, parecem sofrer pressao de predacao por organismos maiores, como provavelmente pequenos vertebrados. Assim, o controle por predacao (top-down) ocorreu tanto sobre as comunidades bacterianas quanto sobre as protozooplanctonicas e metazooplanctonicas, sendo as comunidades bacterianas reguladas tanto pela predacao direta quanto indireta das populacoes zooplanctonicas. O controle por disponibilidade de recursos (bottom-up) pareceu ter maior influencia no reservatorio no periodo seco, enquanto no periodo chuvoso, as comunidades bacterianas pareceram mais controladas por predacao, sendo que maiores impactos sobre as comunidades bacterianas foram causados pelos NFH e ciliados no periodo chuvoso, e pelos NFH e rotiferos no periodo seco.

Over the period of the spring phytoplankton bloom (March-May 2009), an intensive sampling program was conducted at 2 sampling depths (0.5 m and 1% PAR) located at lacustrine zone of the canyon-shaped, meso-eutrophic Římov reservoir (Czech Republic). Changes in the production and community dynamics of epilimnetic bacterioplankton, studied by means of group-specific rRNA-targeted oligonucleotide probes, were related to shifts in composition and activity of phytoplankton, and to remarkable changes in total protistan bacterivory. It was documented that particular groups of bacterioplankton responded differently to: (i) major shifts in phytoplankton composition and its activity and to (ii) changes in overall protozoan grazing pressure.