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6.3.3 Temperature
The somewhat hidden thermal tolerance of seagrasses allows them to inhabit a wide
range of habitats in the marine environment. Out of tolerance temperature variations
or abrupt temperature shifts would probably introduce stress. Depending on the
severity of the stressor, it may lead to irreparable disturbance of homeostasis and
death. Long-term exposure may result in habitat degradation and loss.
Differential gene expression is an excellent approach to analyze cellular disturbance at the level of transcription. Franssen et al. (2011) used this technique to study
different thermal adaptations as a response to heat stress. In Zostera marina subjected to heat stress, the consistently upregulated genes included pectinesterases,
proteins involved in the synthesis of ribosomal chloroplast proteins, and proteins
involved in protein folding, which contain immunophilins. Pectinesterases are
enzymes implicated in cell wall modification. In Arabidopsis, 66 ORFs have been
annotated as pectinesterases, some of which are ubiquitously expressed and others
are expressed during specific developmental stages.
Several temperature-responsive pathways include photosynthesis-related (photosystems I and II) and carbon fixation through photosynthesis such as RuBisCO
(Reusch et al. 2008). Indeed, Campbell et al. (2006) suggested that photosynthetic
condition of seagrasses is likely to suffer irreparable effects in seawater temperatures as high as 40–45°C.
6.3.4 Salinity
High salinity causes both hyperionic and hyperosmotic stress effects; the results
of these potentially cause death. Most commonly, stress is caused by high ionic
content in the medium. There are growing concerns on increased salinities in
marine estuarine areas worldwide. Although increased salinity can disturb carbon and O 2 balance in cells, Thalassia testudinum, Halodule wrightii, and
Ruppia maritima seem to actively tolerate a wide range of salinities (35–50 psu)
(Koch et al. 2007).
Salinity stress may cause ion channel gating, which leads to signal transduction pathway activation. In response to salinity fluctuation, many plants accumulate organic solutes to achieve osmotic adjustment. Sucrose and proline appear to
be the principle organic osmoprotectants in Zostera marina (Ye and Zhao 2003)
and Ruppia maritima (Murphy et al. 2003). Khalafallah et al. (2013) suggested
the same mechanism of tolerance to increased salinity for the seagrass Halodule
uninervis. Apart from sucrose and proline, free amino acids were significantly
increased in the vegetative parts of Halodule uninervis. The recorded data showed
that with increasing salinity for a few days, soluble sugars, free amino acids, and
proline were significantly increased in the vegetative parts of Halodule uninervis,
while long-term effect of salinities (60 and 65 psu) resulted in a highly significant
6 Abiotic Stress of Seagrasses
6.3.3 Temperature
The somewhat hidden thermal tolerance of seagrasses allows them to inhabit a wide
range of habitats in the marine environment. Out of tolerance temperature variations
or abrupt temperature shifts would probably introduce stress. Depending on the
severity of the stressor, it may lead to irreparable disturbance of homeostasis and
death. Long-term exposure may result in habitat degradation and loss.
Differential gene expression is an excellent approach to analyze cellular disturbance at the level of transcription. Franssen et al. (2011) used this technique to study
different thermal adaptations as a response to heat stress. In Zostera marina subjected to heat stress, the consistently upregulated genes included pectinesterases,
proteins involved in the synthesis of ribosomal chloroplast proteins, and proteins
involved in protein folding, which contain immunophilins. Pectinesterases are
enzymes implicated in cell wall modification. In Arabidopsis, 66 ORFs have been
annotated as pectinesterases, some of which are ubiquitously expressed and others
are expressed during specific developmental stages.
Several temperature-responsive pathways include photosynthesis-related (photosystems I and II) and carbon fixation through photosynthesis such as RuBisCO
(Reusch et al. 2008). Indeed, Campbell et al. (2006) suggested that photosynthetic
condition of seagrasses is likely to suffer irreparable effects in seawater temperatures as high as 40–45°C.
6.3.4 Salinity
High salinity causes both hyperionic and hyperosmotic stress effects; the results
of these potentially cause death. Most commonly, stress is caused by high ionic
content in the medium. There are growing concerns on increased salinities in
marine estuarine areas worldwide. Although increased salinity can disturb carbon and O 2 balance in cells, Thalassia testudinum, Halodule wrightii, and
Ruppia maritima seem to actively tolerate a wide range of salinities (35–50 psu)
(Koch et al. 2007).
Salinity stress may cause ion channel gating, which leads to signal transduction pathway activation. In response to salinity fluctuation, many plants accumulate organic solutes to achieve osmotic adjustment. Sucrose and proline appear to
be the principle organic osmoprotectants in Zostera marina (Ye and Zhao 2003)
and Ruppia maritima (Murphy et al. 2003). Khalafallah et al. (2013) suggested
the same mechanism of tolerance to increased salinity for the seagrass Halodule
uninervis. Apart from sucrose and proline, free amino acids were significantly
increased in the vegetative parts of Halodule uninervis. The recorded data showed
that with increasing salinity for a few days, soluble sugars, free amino acids, and
proline were significantly increased in the vegetative parts of Halodule uninervis,
while long-term effect of salinities (60 and 65 psu) resulted in a highly significant
6 Abiotic Stress of Seagrasses