Browsing by Subject "microbial loop"

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  • Kangas, Jonna (Helsingin yliopisto, 2022)
    Climate change is expected to cause salinity change in the Baltic Sea and therefore may affect organisms living in the Baltic such as plankton. The microbial loop is an important part of the plankton food web. It consists of heterotrophic bacteria, nanoflagellates and ciliates and is connected with the classic plankton food chain through interactions with primary producers and mesozooplankton. Therefore, salinity affects the functioning of the microbial food web not only directly, but also through salinity induced changes on primary producers and mesozooplankton. In this master’s thesis I studied the effects of salinity change on microbial loop components bacteria, nanoflagellates and ciliates in an outdoor mesocosm experiment containing four salinity treatments with salinities of 3.5, 5.5, 7.5 and 9.5, three replicas each. The experiment took place offshore at the Tvärminne Zoological Station. Bacteria were sampled from the mesocosms every other day and nanoflagellates and ciliates every 6th day. Bacteria were analysed with the flow cytometer, nanoflagellates with epifluorescent microscopy and ciliates using an inverted microscope. The effects of salinity on microbial loop components were statistically tested using linear mixed effects models. Results of the experiment show that salinity had an indirect effect on microbial loop components through changes in mesozooplankton composition. There were significant differences between high and low salinity treatments in bacteria abundance and composition, the interaction strength between HNFs and bacteria and in the mean cell size of ciliate communities. These were mainly caused by differences in mesozooplankton community structure between salinity treatments, which had cascading effects on the strength of top-down and bottom-up control on the trophic levels of the microbial loop, leading to changes in bacteria abundances and composition. Based on the results of this thesis, more studies are needed to detect the effects that changes in the composition and functioning of the microbial loop might have on the ecosystem. Further research should also focus on the significance of the structure and diversity of the communities within the microbial loop as well as the functional roles of different species in the microbial food web.
  • Evans, Claire; Brandsma, Joost; Meredith, Michael; Thomas, David Neville; Venables, Hugh; Pond, David; Brussaard, Corina (2021)
    The relative flow of carbon through the viral shunt and the microbial loop is a pivotal factor controlling the contribution of secondary production to the food web and to rates of nutrient remineralization and respiration. The current study examines the significance of these processes in the coastal waters of the Antarctic during the productive austral summer months. Throughout the study a general trend towards lower bacterioplankton and heterotrophic nanoflagellate (HNF) abundances was observed, whereas virioplankton concentration increased. A corresponding decline of HNF grazing rates and shift towards viral production, indicative of viral infection, was measured. Carbon flow mediated by HNF grazing decreased by more than half from 5.7 μg C L−1day−1 on average in December and January to 2.4 μg C L−1 day−1 in February. Conversely, carbon flow through the viral shunt increased substantially over the study from on average 0.9 μg C L−1day−1 in December to 7.6 μg C L−1 day−1 in February. This study shows that functioning of the coastal Antarctic microbial community varied considerably over the productive summer months. In early summer, the system favors transfer of matter and energy to higher trophic levels via the microbial loop, however towards the end of summer carbon flow is redirected towards the viral shunt, causing a switch towards more recycling and therefore increased respiration and regeneration.