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Effects of ageing on the photosynthetic capacity of the seagrass Zostera marina Linnaeus
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect of tissue age and light stress on seagrass productivity was examined on three scale levels by comparisons among: (i) different parts of a leaf (i.e. the base middle and top), (ii) leaves of different ages, and (iii) shoots of different development stages on the same genet, in the temperate species Zostera marina L. Rapid light curves were performed to estimate chlorophyll fluorescence and to determine the maximal photosynthetic rate (ETRmax), the photosynthetic efficiency (the alpha slope) and the maximal quantum yield (both as Fv/Fm and as the more sensible Fv/F0). The plants were also exposed to inhibiting light stress with measurements performed during the following recovery and non-photochemical quenching (NPQ). The results suggest that seagrass tissue age has an effect on the photosynthetic performance of the plant and that the level of strength varies among different parts of a single shoot and also among shoots of different development stages along the same genet. Younger and senescing tissues generally had lower photosynthetic capacity than mature tissues. It appeared that very young tissues tolerate light stress better than mature and senescing tissues, as the NPQ values of the very young tissue were higher, and they also showed a lower recovery to initial Fv/F0 values. A clear difference was also found in photosynthetic performance and recovery capacity of the youngest shoot compared to the rest of the shoots belonging to the same genet. The leaves of young shoots appeared to better tolerate light stress than leaves of old shoots. These findings provide new insight on seagrass ageing and expand the understanding of ageing effects on photosynthesis on a population level.

National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-155869OAI: oai:DiVA.org:su-155869DiVA, id: diva2:1202695
Available from: 2018-04-30 Created: 2018-04-30 Last updated: 2018-04-30Bibliographically approved
In thesis
1. Seagrass productivity: from plant to system
Open this publication in new window or tab >>Seagrass productivity: from plant to system
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrasses form one of the most productive habitats on earth and are recognized as very efficient carbon sinks. The levels and patterns of productivity within and across different seagrass systems vary widely due to natural or human-induced factors. Seagrass plants, being the foundation species of seagrass meadows, have a substitutional role as primary producers to the overall productivity of their habitat. Clarifying the variation in the carbon capture potential of these plants on physiological and ecological levels is essential to understand of the whole system’s carbon balance. In this thesis, the photosynthetic performance and productivity of seagrass plants were studied in relation to factors that have large impact on productivity, such as tissues age, season and water depth. Furthermore, the seagrass response, in terms of capacity to capture and sequester carbon, to human-induced stress factors such as shading and simulated grazing was evaluated in a tropical seagrass meadow. The research has included a multitude of seagrass productivity assessments from plant- to system level.

The results showed that age has a significant effect on the photosynthetic performance of the temperate seagrass Zostera marina L., both within a single shoot and between shoots. When comparing leaves among the same shoot, the photosynthetic capacity and efficiency were highest in mature tissues and significantly reduced in very young tissues as well as in tissues undergoing senescence. In response to high light stress, very young tissues seemed to cope better with dissipating excess light energy, which was demonstrated by the higher values of non-photochemical quenching (NPQ) observed compared to mature and senescent tissues. Such an effect was also observed when comparing the oldest and youngest shoots from the same genet; the youngest shoot showed higher ability to dissipate excess light energy compared to the oldest one, and might thus be able to better withstand light stress.

On a larger spatiotemporal scale, the areal productivity of seagrass plants was significantly affected by light availability and temperature, leading to a strong seasonal variation. In addition, depth had a strong site-specific effect on plant productivity in terms of biomass. On a yearly basis, productivity rates varied substantially, reaching up to 20 g C m-2 24h-1 in the summer months. This high carbon capture potential was, however, outbalanced by the high respiration rates of the benthic community. Overall, the whole system had a low but positive yearly carbon balance.

Both shading and simulated grazing negatively affected seagrass plants and the whole habitat after five months of experimental disturbance. On the plant level, photosynthesis, productivity and growth were all reduced. On the system level, a reduction in community productivity was recorded. The long-term refractory carbon was, however, not affected although erosion was observed in treatments subjected to simulated grazing.

In summary, this thesis has established that age, season, depth and exposure are factors highly responsible for natural variation in seagrass plant- and habitat productivity, and that seagrasses respond to human-induced stress by significantly reducing their productivity. Even though seagrass plants are generally capable of surviving stress periods, these results suggest that prolonged deteriorating stress conditions will lead to serious harm on the plants as well as the entire habitat, and thereby compromising the carbon burial capacity of the seagrass system.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2018. p. 51
Keywords
Seagrass, Chlorophyll a fluorescence, Ageing, Productivity, Seasonality, Stress response
National Category
Botany Ecology
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-155497 (URN)978-91-7797-116-0 (ISBN)978-91-7797-117-7 (ISBN)
Public defence
2018-06-08, Vivi Täckholmssalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 2: Manuscript.

Available from: 2018-05-16 Created: 2018-04-23 Last updated: 2018-05-09Bibliographically approved

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