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Seawater pH as a Controlling Factor in Macroalgal Calcification and Photosynthesis
Stockholm University, Faculty of Science, Department of Botany.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biological calcification and photosynthesis are important processes with a great influence on both structure and function of oceanic ecosystems. The pH of the seawater has a strong influence on both these processes and therefore the impacts of different pH levels on calcareous macroalgae were investigated in laboratory and field experiments at Chwaka and Fumba Bays in Zanzibar, Tanzania. The levels of pH were manipulated, first in the laboratory by adding HCl, NaOH or by bubbling seawater with CO2 enriched air. Secondly, pH was allowed to fluctuate naturally as a consequence of marine photosynthetic carbon uptake or release through respiration by mussels. The effects on both photosynthesis and calcification were then analyzed on a seagrass (Thalassia hemprichii), and the calcareous red and green algae Mesophyllum sp., Hydrolithon sp., Amphiroa fragilissima and Halimeda renschii, as well as on the mussel Pinna muricata. The laboratory studies revealed a significant decrease in calcification rates in Hydrolithon sp. with decreasing pH, while photosynthesis showed an opposite trend. Also, increased dissolved CO2 lowered pH from 8.1 to 7.8 and caused a ~20% decline in calcification rates. In the field, seagrasses raised pH to ~9, increasing calcification rates significantly in the calcareous algae while photosynthetic rates showed no significant differences with changes in pH expect in Mesophyllum sp., in which rates increased at elevated pH caused by the presence of seagrasses. Conversely, seagrass photosynthesis increased significantly in the presence of mussels. Based on these findings, we conclude that pH is important in shaping biological processes that determines ecological interactions within shallow tidal areas by modifying seawater carbon composition and, thus, influencing calcification and photosynthesis processes.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2009. , 47 p.
Keyword [en]
Calcareous macroalgae, CO2, mussels, pH, seagrasses, tropical lagoons
National Category
Botany
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-27479ISBN: 978-91-7155-887-9 (print)OAI: oai:DiVA.org:su-27479DiVA: diva2:214430
Public defence
2009-06-04, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2009-05-14 Created: 2009-05-05 Last updated: 2009-05-05Bibliographically approved
List of papers
1. Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow
Open this publication in new window or tab >>Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow
2009 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 382, 41-47 p.Article in journal (Refereed) Published
Abstract [en]

Diel fluctuations in seawater pH can be >1 pH unit (7.9 to >8.9) in the seagrass meadows of Chwaka Bay (Zanzibar, Tanzania). The high daily pH values are generated by the photosynthetic activity of the bay’s submerged seagrasses and macroalgae, and maintained by the relatively low, tide-dominated, water exchange rate. Since pH in principle can affect rates of both calcification and photosynthesis, we investigated whether diel variations in pH caused by photosynthesis could affect rates of calcification and photosynthesis of the calcareous red (Hydrolithon sp. and Mesophyllum sp.) and green (Halimeda renschii) algae growing within these meadows. This was done by measuring rates of calcification and relative photosynthetic electron transport (rETR) of the algae in situ in open-bottom incubation cylinders either in the natural presence of the rooted seagrasses or after the leaves had been removed. The results showed that seagrass photosynthesis increased the seawater pH within the cylinders from 8.3–8.4 to 8.6–8.9 after 2.5 h (largely in conformity with that of the surrounding seawater), which, in turn, enhanced the rates of calcification 5.8-fold for Hydrolithon sp. and 1.6-fold for the other 2 species. The rETRs of all algae largely followed the irradiance throughout the day and were (in Mesophyllum sp.) significantly higher in the presence of seagrasses despite the higher pH values generated by the latter. We conclude that algal calcification within seagrass meadows such as those of Chwaka Bay is considerably enhanced by the photosynthetic activity of the seagrasses, which in turn increases the seawater pH.

Keyword
calcareous algae, calcification, Halimeda sp., Hydrolithon sp., Mesophylum sp., photosynthesis
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-27477 (URN)10.3354/meps07973 (DOI)000266474900004 ()
Available from: 2009-05-05 Created: 2009-05-05 Last updated: 2017-12-13Bibliographically approved
2. Alterations in seawater pH and CO2 affect calcification and photosynthesis in the tropical coralline alga, Hydrolithon sp. (Rhodophyta)
Open this publication in new window or tab >>Alterations in seawater pH and CO2 affect calcification and photosynthesis in the tropical coralline alga, Hydrolithon sp. (Rhodophyta)
2009 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 84, no 3, 337-341 p.Article in journal (Refereed) Published
Abstract [en]

Calcification in the marine environment is the basis for the accretion of carbonate in structures such as coral reefs, algal ridges and carbonate sands. Among the organisms responsible for such calcification are the Corallinaceae (Rhodophyta), recognized as major contributors to the process world-wide. Hydrolithon sp. is a coralline alga that often forms rhodoliths in the Western Indian Ocean. In Zanzibar, it is commonly found in shallow lagoons, where it often grows within seagrass beds and/or surrounded by green algae such as Ulva sp. Since seagrasses in Zanzibar have recently been shown to raise the pH of the surrounding seawater during the day, and since calcification rates are sensitive to pH, which changes the saturation state of calcium carbonate, we measured the effects of pH on photosynthetic and calcification rates of this alga. It was found that pH had significant effects on both calcification and photosynthesis. While increased pH enhanced calcification rates both in the light and in the dark at pH >8.6, photosynthetic rates decreased. On the other hand, an increase in dissolved CO2 concentration to ~26 µmol kg-1 (by bubbling with air containing 0.9 mbar CO2) caused a decrease in seawater pH which resulted in 20% less calcification after 5 days of exposure, while enhancing photosynthetic rates by 13%. The ecological implications of these findings is that photosynthetically driven changes in water chemistry by surrounding plants can affect calcification rates of coralline algae, as may future ocean acidification resulting from elevated atmospheric CO2.

Keyword
calcification, CO2, coralline algae, Hydrolithon sp., pH, photosynthesis
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-27474 (URN)10.1016/j.ecss.2009.03.038 (DOI)000270122400007 ()
Available from: 2009-05-05 Created: 2009-05-05 Last updated: 2017-12-13Bibliographically approved
3. Mussel, Seagrass and Calcareous Algal Interactions: Influence of CO2 and pH on Photosynthesis and Calcification in a Tropical Bay
Open this publication in new window or tab >>Mussel, Seagrass and Calcareous Algal Interactions: Influence of CO2 and pH on Photosynthesis and Calcification in a Tropical Bay
Show others...
(English)Manuscript (Other academic)
Abstract [en]

Changes in seawater pH and thus, the inorganic carbon (Ci) composition brought about by photosynthetic carbon uptake have been shown to influence the productivity of marine plants. In this work, we enclosed different combinations of seagrasses Thalassia hemprichii, the mussel Pinna muricata and the calcareous coralline algae Amphiroa fragilissima in open plastic cylinders in a tropical seagrasses meadow at Fumba Bay, Zanzibar, Tanzania, while measuring pH and rates of photosynthesis and calcification. The results showed that the photosynthetic production of seagrasses increased in the presence of mussels (probably due to their supply of respiratory CO2) while the presence of seagrasses supported an increase in calcification of the coralline algae (probably by the higher pH generated by the former). Photosynthetic rates of the seagrasses and the coralline algae were ~20% and ~13% higher, respectively, in the presence of mussels than in their absence. Also, the rate of calcification of the coralline algae was significantly higher (by 11%) in the presence of seagrasses than when alone. Calcification by the mussels, on the other hand, was not affected by the presence of algae or seagrasses. These results illustrate how pH changes induced by fluxes in Ci can act as a factor controlling both productivity and calcification in densely populated shallow marine ecosystems.

Keyword
Coralline algae, Calcification, Mussels, pH, Photosynthesis, Seagrasses
National Category
Botany
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-27478 (URN)
Available from: 2009-05-05 Created: 2009-05-05 Last updated: 2011-04-13Bibliographically approved

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