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Circadian rhythms and carbon acquisition in the red algae Kappaphycus alvarezii and Eucheuma denticulatum
Stockholm University, Faculty of Science, Department of Botany.
2001 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Circadian rhythms and carbon acquisition in Eucheuma denticulatum and Kappaphycus alvarezii (English)
Abstract [en]

Aspects of carbon and nitrogen metabolism have been studied in two closely related tropical red macroalgae (Rhodophyta) that belong to the commercially important genera Kappaphycus and Eucheuma. These algae are cultivated around the world for their carrageenan content.

Circadian rhythms are controlled by endogenous clocks and express a rhythm with a period of approximately 24 h. They can be studied in constant conditions. Circadian regulation of photosynthesis (measured as O2 evolution) was found in Kappaphycus alvarezii. The rhythm was expressed in all spectral ranges tested, white, blue, green and red light. In blue light the period of the free-running rhythm became shorter, and in red light longer, with higher irradiance. In contrast, in white and green light the period did not change with irradiance. These results suggest that at least two photoreceptors are involved in the light transduction pathway, controlling the circadian rhythm of photosynthesis in K. alvarezii, one sensitive to red light and another one sensitive to blue light.

The enzyme nitrate reductase (NR) is very important in the nitrogen assimilation process. Measurements of NR activity in K. alvarezii were performed in vitro on crude extracts and protein levels estimated by probing Western blots with monoclonal antibodies against NR. Diurnal oscillation of both protein levels and activity of NR was found in K. alvarezii. The data also suggest a circadian regulation of NR which is expressed at a high irradiance (100 µmol photons m-2 s-1) but not at lower irradiances (25 and 55 µmol photons m-2 s-1). Characterization of the NR of K. alvarezii showed that the enzyme is composed of two 100 kDa subunits with a holoenzyme mass of 210 kDa. The enzyme was NADH specific and had high affinity both for nitrate and NADH. Furthermore, the enzyme was characterised by broad pH and temperature optima and high stability. A highly regulated, stable enzyme with high affinity for its substrates is likely to be important for growth in dense algal populations in nutrient-poor tropical waters.

One important constraint of the marine environment on algae is the limited access to inorganic carbon (Ci). The major system that ensures sufficient Ci supply in Eucheuma denticulatum and K. alvarezii was found to be mediated by an external carbonic anhydrase (CA). The suggested function of this enzyme is to speed up the dehydration of HCO3- (the major form of inorganic carbon in natural seawater) to CO2, which can freely diffuse across the algal cellular membrane. Both E. denticulatum and K. alvarezii were also able to make direct use of the HCO3- in seawater via an anion exchange transport protein. This mechanism was found to be a major component of their carbon concentrating system at pH above 8.9 while the external CA was active in a wide pH range from pH 6.5 (start pH of our measurements) to pH 9.3, the pH compensation point. 

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2001. , 43 p.
National Category
Biological Sciences
Research subject
Physiological Botany
Identifiers
URN: urn:nbn:se:su:diva-142308ISBN: 91-7265-386-8 (print)OAI: oai:DiVA.org:su-142308DiVA: diva2:1091931
Public defence
2002-01-11, 10:00 (English)
Opponent
Note

Härtill 5 uppsatser

Stockholms universitetsbiblioteks retrospektiva digitalisering. Avhandlingar 1906-2003.

Available from: 2017-04-28 Created: 2017-04-28 Last updated: 2017-07-10Bibliographically approved

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