University of Tasmania
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The photosynthetic response and extracellular carbohydrate production of tropical and temperate microphytobenthos

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posted on 2023-05-28, 09:55 authored by Lee, SH
Microphytobenthos (MPB) is a community of unicellular microorganisms that inhabit the photic zone of intertidal and subtidal areas of benthic zones. MPB contributes up to half of the total coastal primary productivity, as well as providing a major food source for many invertebrates, small fish and wading birds. Additionally, MPB plays an important role in stabilising the sediment by producing extracellular carbohydrates that form biofilms, which bind sediment particles together. Intertidal zones experience extreme spatial and temporal fluctuations in both their physical, i.e. temperature, irradiances, and tidal position, and chemical, i.e. salinity and nutrient, conditions. MPB, therefore, needs to employ a variety of mechanisms to cope with these changes; these include behavioural changes, such as positioning themselves at the optimum position within the sediment, and physiological changes to their photosynthetic machinery. Extracellular polymeric substances (EPS) are one of the major components of the extracellular carbohydrates produced by MPB when they are undertaking these mechanisms. Much of this extracellular carbohydrate is produced for vertical migration but some is also produced as a product of photosynthetic overflow, when the MPB is experiencing photoinhibition. This study focuses on the photosynthetic performance of MPB in different regions. As extracellular carbohydrate production and photosynthesis are often correlated, this relationship is examined in more detail here. A Pulse Amplitude Modulated (PAM) Fluorometer was used to measure the photosynthetic performance of the MPB, while several different approaches were used to investigate the extracellular carbohydrate production. MPB species composition varies between regions and so the response mechanisms would also be expected to differ. The second chapter of this study focused on the investigation of the relationship between photosynthesis and extracellular carbohydrate production in a southern temperate region of greater Hobart area, Tasmania, at Penna Beach and Kings Beach, of different sediment grain size composition. The MPB was found to be well-adapted to its changing environment and did not experience photoinhibition during environment extremes, although its photosynthetic performance to change to cope with its changing environment. The performance varied between sampling sites and significant differences were found between biomass, photosynthetic performance, and extracellular carbohydrate production. Penna Beach was found to have significantly higher chlorophyll ˜í¬± (chl.˜í¬±), extracellular carbohydrate concentrations and photosynthetic performance than Kings Beach. The analysed parameters also varied significantly seasonally, with both chl.˜í¬± and extracellular carbohydrate concentrations higher during spring and summer, while photosynthesis was higher during winter. Monosaccharides concentrations were found to be higher than polysaccharide concentrations at both sampling sites. However, there was no seasonal variations in mono- and polysaccharide concentrations. The photosynthetic performance and carbohydrate production of MPB in a tropical region, Penang, Malaysia, was examined in Chapter 3. The chl.˜í¬± and extracellular carbohydrate concentrations varied between sampling sites, where they were mainly affected by sediment temperature, salinity, and irradiance. Nitrate was the only nutrient that had a significant impact on the chl.˜í¬± concentration and none of the major nutrient had a significant effects on the total carbohydrates (TCHO), monosaccharides (MCHO), or polysaccharides (PCHO). PCHO concentrations were significantly higher than TCHO at all sampling sites. Measured variable fluorescence showed that the MPB were not experiencing optimal conditions at any site, consistently having low photosynthetic efficiency (F\\(_v\\)/F\\(_m\\)). Unlike chl.˜í¬± and extracellular carbohydrate, the photosynthetic performance was not affected by sediment temperature, salinity or irradiance. Photosynthesis was mostly affected by nutrient levels. The colloidal monosaccharide component of carbohydrate was not able to be measured by the techniques used in earlier chapters (Chapters 2 and 3). In Chapter 4, however, using HPLC, the composition of the monosaccharide component was further differentiated into the key sugars, i.e. glucose, galactose, mannose, arabinose, rhamnose, and xylose. Glucose, which can be produced as photosynthetic overflow at high irradiances and low nutrients, was the major sugar in all samples at both sites. Furthermore, the glucose production varied significantly between sampling sites, with glucose concentrations at Penna Beach much higher (>0.1 mg/mL) than those of Kings Beach (<0.05 mg/mL). Glucose concentrations also varied significantly between seasons, being highest during spring and lowest during winter at Penna Beach, but this was not observed at Kings Beach. Additionally, the chl.˜í¬± normalised glucose concentration did not vary between sampling depths at either site. Real-time production of glucose was determined in the fifth chapter of this study, using an innovative experimental approach that adopted glucose biosensors to measure real-time changes in the concentration. Results showed that in natural communities, glucose production increased under high light and nutrient deficient conditions, although the single species cultures had a more variable response. In a bacterial consumption experiment, glucose was rapidly consumed by bacteria in the dark. However, when the culture was treated with an antibiotic, consumption in the dark was minimal. This study showed that MPB are able to rapidly adapt to changes in their environment. Extracellular carbohydrate, particularly glucose, was excreted mainly as a photosynthetic overflow product. Additionally, glucose biosensors have proven to be a reliable and innovative tool to measure the targeted sugar exudation in microphytobenthic biofilms and have considerable potential in future studies of MPB.


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