Seaweeds that occupy the intertidal zone grow in distinct vertical bands. Elucidating the causes of zonation of intertidal seaweeds has been a key question for over a century, and the prevailing paradigm is that species occupying the higher shore position are limited by abiotic factors associated with exposure to atmospheric conditions, whereas for the lower-shore species competition is considered to limit a macroalga�s distribution. The main objective of this study was to assess the effect of desiccation, and to a lesser extent nutrient limitation, on the distribution of Stictosiphonia arbuscula, a red intertidal seaweed that grows in a wide vertical band in the mid - high shore region. A multi-disciplinary approach was used to investigate the effect of desiccation at different functional levels, from individual cells to the whole organism. Ecophysiological comparisons were made on high-shore and low-shore populations to determine whether the upper and lower shore distributions have different abilities to tolerate and recover from desiccation stress during summer and winter. The effect of desiccation was visually assessed, using electron microscopy, at the cellular and whole organism level. Carbohydrate and pigment concentrations were determined to provide information on how S. arbuscula allocates its resources between winter and summer.
Investigations into the ability of Stictosiphonia arbuscula to tolerate and recover photosynthetic capacity after desiccation events (relative humidity (R.H.) 5% and 40%; ranging from 30 minutes - 24 hours) unexpectedly revealed no differences between high- and low-shore specimens. However, there were seasonal differences, with summer specimens better able to recover both tissue water content and photosynthetic capacity after desiccation compared to winter specimens, especially after desiccation for 24 hours. In contrast high-shore specimens had increased rates of nitrogen uptake after mild desiccation treatments, compared to low-shore specimens during summer, and specimens from the low-shore had increased variability in their uptake rates, indicating disruption to the uptake mechanisms, compared to the high-shore specimens. S. arbuscula was able to recover photosynthetic capacity faster than nitrogen and phosphorus uptake during re-immersion after desiccation, which indicates that the ability to regain photosynthetic function may be more important for this species than regaining the ability to take up nitrogen and phosphorus. As S. arbuscula can photosynthesise in air (providing there is sufficient tissue water content) and water, the �window of opportunity� to assimilate carbon is greater than for nitrogen and phosphorus uptake, which is only available to S. arbuscula during immersion at high tides. Despite this, no evidence was found of severe nutrient limitation in either high- or low-shore specimens.
The disruption and recovery of cellular organisation during rehydration, assessed visually using electron microscopy, showed that the rate and duration of desiccation also affected the extent of recovery. Extremely desiccating conditions (R.H. 5%) damaged the cells and no recovery was observed during rehydration, compared to moderate desiccation treatments (R.H. 40%) where the time taken to recover during rehydration increased with increasing desiccation periods. The disruption of cellular organisation observed in Stictosiphonia arbuscula cells, was found to match the reduction and / or disruption to photosynthesis and the uptake of nitrogen and phosphorus under similar desiccation regimes. The formation of ridges and depressions on the surface of the tissue was also observed during desiccation, which is thought to help reduce the rate of evaporation by trapping �pockets� of air.
Finally, Stictosiphonia arbuscula is extremely well suited to the mid - high region in the intertidal zone and it can maximise its competitive ability within this niche during immersion by fully rehydrating within 10 minutes, regaining maximum photosynthetic capacity within 20 minutes, and stability of nutrient uptake rates within 90 minutes. Further, S. arbuscula has higher rates of photosynthesis in winter, associated with increased photosynthetic pigment concentrations at this time. In contrast, during summer, the photosynthetic rates, chlorophyll a and phycobiliprotein concentrations are reduced and S. arbuscula increases its allocation of resources into protective mechanisms such as UV-absorbing compounds.
| Identifer | oai:union.ndltd.org:ADTP/221508 |
| Date | January 2004 |
| Creators | Loughnan, Abigale Ella, n/a |
| Publisher | University of Otago. n/a |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Abigale Ella Loughnan |
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