125
6.3 Abiotic Stress of Seagrasses
In terrestrial plants the sensing of stress induces signaling pathways that include
kinase cascades, ROS production, ion channel activation, and production of hormones ethylene (ET), abscisic acid (ABA), and jasmonic acid (JA) (Delker et al.
2006). These pathways activate genes responsible for stress allostasis. Marine plants
have been adapted to cope with stress accordingly. Seagrasses have evolved mechanisms to survive in different environments, through modifying their physiological
functions. They are able to cope with conditions of light limitation, activate mechanisms, and change light spectrum through metabolic adjustments such as downregulation of RuBisCO and enhanced proteolysis. Noteworthy for marine and also
freshwater plants is the oxygen transport, due to its low solubility. Oxygen is transported to whole plant tissues, when photosynthesis releases oxygen into a special
tissue which is presented in all seagrasses, aerenchyma. In most seagrass meadows,
an ancient symbiosis takes place between seagrasses, bivalves, and their sulfideoxidizing gill bacteria which decay organic matter and reduce sulfide stress on plant
tissues (Papenbrock 2012). Zostera marina plants from a southern European population (Italy) managed to overcome a simulated heat wave by returning gene expression profile in contrast to plants acquired from Denmark (Franssen et al. 2011). In
the other hand, Winters et al. (2011) concluded that both populations suffered heat
wave equally, although the photochemical activity fully recovered in the southern
population. Noteworthy is the fact that the northern population demonstrated
reduced ability to recover its photo-physiological functions. Seagrasses, due to their
cosmopolitan extent, can help understand the adaptation to wide range of temperatures, salinity tolerance, ocean acidification, and light gradients.
6.3.1 Acidification
Rising carbon dioxide levels in the atmosphere are acidifying the world’s oceans
and threatening the survival of seagrasses among other marine organisms. Over the
past years, there has been much focus on studying the potential impacts of ocean
acidification (Boyd 2011). Underwater hydrothermal vents can serve as natural
laboratories for ocean acidification, due to the higher CO 2 concentrations.
Lauritano et al. (2015) studied the gene expression of 35 stress-related genes in
Posidonia oceanica exposed to volcanic vents in southern Italy. Among them, notable
was the induction of the antioxidant enzymes peroxiredoxin Q (PRXQ) and glutathione peroxidase (GPX). Both are reactive oxygen species (ROS) scavenging enzymes,
which require the donation of electrons from thioredoxin and glutathione, respectively. Nevertheless, responses to elevated CO 2 are affected by nutrient availability,
mostly nitrogen, because carbon and nitrogen metabolisms affect each other (Touchette
and Burkholder 2007). Ow et al. (2016) observed that respiration in Halodule uninervis with high levels of CO 2 was reduced by nitrate enrichment. On the other hand,
6 Abiotic Stress of Seagrasses
6.3 Abiotic Stress of Seagrasses
In terrestrial plants the sensing of stress induces signaling pathways that include
kinase cascades, ROS production, ion channel activation, and production of hormones ethylene (ET), abscisic acid (ABA), and jasmonic acid (JA) (Delker et al.
2006). These pathways activate genes responsible for stress allostasis. Marine plants
have been adapted to cope with stress accordingly. Seagrasses have evolved mechanisms to survive in different environments, through modifying their physiological
functions. They are able to cope with conditions of light limitation, activate mechanisms, and change light spectrum through metabolic adjustments such as downregulation of RuBisCO and enhanced proteolysis. Noteworthy for marine and also
freshwater plants is the oxygen transport, due to its low solubility. Oxygen is transported to whole plant tissues, when photosynthesis releases oxygen into a special
tissue which is presented in all seagrasses, aerenchyma. In most seagrass meadows,
an ancient symbiosis takes place between seagrasses, bivalves, and their sulfideoxidizing gill bacteria which decay organic matter and reduce sulfide stress on plant
tissues (Papenbrock 2012). Zostera marina plants from a southern European population (Italy) managed to overcome a simulated heat wave by returning gene expression profile in contrast to plants acquired from Denmark (Franssen et al. 2011). In
the other hand, Winters et al. (2011) concluded that both populations suffered heat
wave equally, although the photochemical activity fully recovered in the southern
population. Noteworthy is the fact that the northern population demonstrated
reduced ability to recover its photo-physiological functions. Seagrasses, due to their
cosmopolitan extent, can help understand the adaptation to wide range of temperatures, salinity tolerance, ocean acidification, and light gradients.
6.3.1 Acidification
Rising carbon dioxide levels in the atmosphere are acidifying the world’s oceans
and threatening the survival of seagrasses among other marine organisms. Over the
past years, there has been much focus on studying the potential impacts of ocean
acidification (Boyd 2011). Underwater hydrothermal vents can serve as natural
laboratories for ocean acidification, due to the higher CO 2 concentrations.
Lauritano et al. (2015) studied the gene expression of 35 stress-related genes in
Posidonia oceanica exposed to volcanic vents in southern Italy. Among them, notable
was the induction of the antioxidant enzymes peroxiredoxin Q (PRXQ) and glutathione peroxidase (GPX). Both are reactive oxygen species (ROS) scavenging enzymes,
which require the donation of electrons from thioredoxin and glutathione, respectively. Nevertheless, responses to elevated CO 2 are affected by nutrient availability,
mostly nitrogen, because carbon and nitrogen metabolisms affect each other (Touchette
and Burkholder 2007). Ow et al. (2016) observed that respiration in Halodule uninervis with high levels of CO 2 was reduced by nitrate enrichment. On the other hand,
6 Abiotic Stress of Seagrasses