Large- and Microscale Community Structure and Abundance of Microalgae in the Coorong Lagoons, South Australia

Author: Eloise Prime

Prime, Eloise, 2017 Large- and Microscale Community Structure and Abundance of Microalgae in the Coorong Lagoons, South Australia, Flinders University, School of Biological Sciences

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Microbial communities are composed of prokaryotic and eukaryotic organisms and form the basis of aquatic food webs. These microbial communities show seasonal patterns on spatial and temporal scales in the tens of metres to kilometres, known as the large scale. However, community interactions between these organisms occur on the micrometre to centimetre scale, known as the microscale. The dominant eukaryotes are phototrophic microalgae, commonly referred to as phytoplankton or, if on the bottom, the microphytobenthos. Of particular interest in this thesis are the diatoms, a ubiquitous and the most abundant group of eukaryotic phytoplankton which are characterised by their species specific shell-like structures, called frustules. Diatoms are of particular importance globally because they are estimated to produce up to 20% of global oxygen. Additionally, changes in species composition and abundances provide a reliable indication of ecosystem structure and have been used to assess changes in water quality on time scales, from seasonal to decadal.

The primary focus of this thesis is to investigate the variation in phytoplanktonic communities at the large scale and microphytobenthic communities at the microscale in the Coorong Lagoons, South Australia. In Chapter 2, the environmental parameters that influence phytoplankton communities are determined across large scale temporal and spatial distances, taking into account the impact of wind. It was shown that within the south lagoon phytoplankton composition were influenced by wind speed and in the north lagoon were influenced by water parameters such as pH and concentration of dissolved nutrients. Microscale variation of the microphytobenthos was also investigated. In Chapter 3, taxonomic analysis of the microphytobenthos was compared to chlorophyll concentrations at microscale distances. Here, it was found that haptophytes showed the only positive and significant relationship with chlorophyll a, suggesting that haptophytes are contributing to the chlorophyll biomass. However, it is shown that it is the diatoms which show the strongest interactions with the ciliates and a group of unknown microeukaryote. Therefore, suggesting that the diatoms are a keystone group within the microbial community. These interactions in the microphytobenthos was further explored in Chapter 4 using metagenomics in order to take into account both the prokaryote and eukaryote populations to determine the key taxa influencing the communities. Network analysis was used to demonstrate that while bacteria show relative abundances that are up to one order of magnitude greater than the microeukaryotes, it is in fact the diatoms that provide crucial links between the anaerobic bacteria and other eukaryotes. A final study, Chapter 5, investigated how the frustule lengths of two diatom species vary over microscale distances. Using the concept of body length as an indicator of life stage or life cycle, variation in distribution frustule length distribution was assessed for the pennate diatoms Amphora hyalina and Cocconeis costata. The results for this chapter suggested that there is not only microscale variability of frustule length within populations of diatoms, but the changes in skewness of these populations indicate that this variability may be explained by predation, which is supported by the importance of ciliates and diatoms observed in Chapter 3.

The findings of this thesis provide insight to large- and microscale variation within eukaryotic communities. In the large scale study the importance of local environmental factors such as wind speed and nutrient concentration is revealed for the abundance and diversity of diatoms and dinoflagellates. Furthermore, microscale interactions in microbial eukaryotes show the impact of diatoms within microbial food webs, which are grazed upon by ciliates and other predatory protists, when assessing the taxa present. These findings were further supported by the heterogeneous distribution of diatom frustule lengths, showing patches that favour larger cell sizes which is suggested to be a direct result of predation.

Keywords: Microalgae, Microeukaryotes, Phytoplankton, Diatoms, Chlorophyll, Abundance, Diversity, Microscale, Large scale, The Coorong, River Murray Mouth

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Biological Sciences
Supervisor: Jim Mitchell