145
concentrations (Lin et al. 2016b). In addition to antioxidative systems, the heavy
metal- binding ligands, phytochelatins (PCs) and metallothioneins (MTs), have been
proposed to play a role in metal homoeostasis in plants (Cobbett and Goldsbrough
2002). Accumulation and synthesis of PCs following heavy metal exposure (cadmium and lead) have been observed in the seagrasses T. testudinum (AlvarezLegorreta et al. 2008) and T. hemprichii (Tupan et al. 2014). Transcript-encoding
metallothioneins (including metallothionein-like protein) and putative type II
metallothioneins have been identified in Z. marina and P. oceanica, respectively
(Giordani et al. 2000; Cozza et al. 2006; Kong et al. 2014; Olsen et al. 2016). In P.
oceanica, an increase in transcript level of putative type II metallothionein after
exposure to copper and cadmium has been reported (Giordani et al. 2000; Cozza
et al. 2006). These results point to the importance of ROS scavenging and heavy
metal detoxification in seagrass metal tolerance. Uptake and accumulation of pesticides, herbicides, and trace metals into the seagrass biomass and the possibility of
using seagrasses as a bioindicator of such contaminants have been explored (Ferrat
et al. 2003; Govers et al. 2014; Lin et al. 2016a; Bonanno and Di Martino 2016).
Much more research effort is still needed to establish the relationship between duration and dose of exposure and rapid physiological responses associated with photosynthetic mechanisms, antioxidative systems, and heavy metal homoeostasis which
tend to precede those that manifest at the whole plant or higher levels.
In sediments of seagrass habitats, microbial reduction of sulfate in anoxic condition generates sulphide which is a potent phytotoxin (Lamers et al. 2013). Generally,
seagrass releases oxygen to the rhizosphere via aerenchyma which then oxidizes
sulfide to nontoxic form (Hasler-Sheetal and Holmer 2015; Brodersen et al. 2015a,b,
2016). A recent study also suggested that once sulfide enters plant cells, it is detoxified into thiols and further metabolized in sulfur metabolic pathways (Hasler-Sheetal
and Holmer 2015). Nevertheless, following an event of organic matter load, excessive organic matter breakdown by microbes can lead to an increase in concentrations of sulfide in porewater (Pérez et al. 2007; Govers et al. 2014). Additionally,
limiting light brought about by sedimentation, overgrowth of epiphytes, or algal
blooms might lower seagrass tolerance to sulfide by decreasing photosynthetic
rates, thereby reducing photosynthetically derived oxygen available for transport to
belowground tissue (Goodman et al. 1995; Brodersen et al. 2015b). Sulfide exposure was found to decrease maximum photosynthetic rates (calculated as O 2 evolution), and chlorophyll a concentration and increase light compensation point of Z.
marina (Goodman et al. 1995; Holmer and Bondgaard 2001). When exposed to
high sulfide concentrations (>100 mM), the photosynthetic activity of Z. marina
was fully inhibited after 6 days (Holmer and Bondgaard 2001). Sulfide has been
proposed to act as an inactivator of metalloenzymes such as oxygen-evolving complex of PSII (Goodman et al. 1995; Armstrong et al. 1996; Fürtig et al. 1996) and
inhibitor of cytochrome c oxidase in mitochondrial respiratory electron transport
chain (Goodman et al. 1995; Holmer and Bondgaard 2001). Nevertheless, the
mechanisms of sulfide toxicity and plant strategies to cope with sulfide intrusion at
the molecular level remain to be further investigated.
7 Photobiology of Seagrasses: A Systems Biology Perspective
concentrations (Lin et al. 2016b). In addition to antioxidative systems, the heavy
metal- binding ligands, phytochelatins (PCs) and metallothioneins (MTs), have been
proposed to play a role in metal homoeostasis in plants (Cobbett and Goldsbrough
2002). Accumulation and synthesis of PCs following heavy metal exposure (cadmium and lead) have been observed in the seagrasses T. testudinum (AlvarezLegorreta et al. 2008) and T. hemprichii (Tupan et al. 2014). Transcript-encoding
metallothioneins (including metallothionein-like protein) and putative type II
metallothioneins have been identified in Z. marina and P. oceanica, respectively
(Giordani et al. 2000; Cozza et al. 2006; Kong et al. 2014; Olsen et al. 2016). In P.
oceanica, an increase in transcript level of putative type II metallothionein after
exposure to copper and cadmium has been reported (Giordani et al. 2000; Cozza
et al. 2006). These results point to the importance of ROS scavenging and heavy
metal detoxification in seagrass metal tolerance. Uptake and accumulation of pesticides, herbicides, and trace metals into the seagrass biomass and the possibility of
using seagrasses as a bioindicator of such contaminants have been explored (Ferrat
et al. 2003; Govers et al. 2014; Lin et al. 2016a; Bonanno and Di Martino 2016).
Much more research effort is still needed to establish the relationship between duration and dose of exposure and rapid physiological responses associated with photosynthetic mechanisms, antioxidative systems, and heavy metal homoeostasis which
tend to precede those that manifest at the whole plant or higher levels.
In sediments of seagrass habitats, microbial reduction of sulfate in anoxic condition generates sulphide which is a potent phytotoxin (Lamers et al. 2013). Generally,
seagrass releases oxygen to the rhizosphere via aerenchyma which then oxidizes
sulfide to nontoxic form (Hasler-Sheetal and Holmer 2015; Brodersen et al. 2015a,b,
2016). A recent study also suggested that once sulfide enters plant cells, it is detoxified into thiols and further metabolized in sulfur metabolic pathways (Hasler-Sheetal
and Holmer 2015). Nevertheless, following an event of organic matter load, excessive organic matter breakdown by microbes can lead to an increase in concentrations of sulfide in porewater (Pérez et al. 2007; Govers et al. 2014). Additionally,
limiting light brought about by sedimentation, overgrowth of epiphytes, or algal
blooms might lower seagrass tolerance to sulfide by decreasing photosynthetic
rates, thereby reducing photosynthetically derived oxygen available for transport to
belowground tissue (Goodman et al. 1995; Brodersen et al. 2015b). Sulfide exposure was found to decrease maximum photosynthetic rates (calculated as O 2 evolution), and chlorophyll a concentration and increase light compensation point of Z.
marina (Goodman et al. 1995; Holmer and Bondgaard 2001). When exposed to
high sulfide concentrations (>100 mM), the photosynthetic activity of Z. marina
was fully inhibited after 6 days (Holmer and Bondgaard 2001). Sulfide has been
proposed to act as an inactivator of metalloenzymes such as oxygen-evolving complex of PSII (Goodman et al. 1995; Armstrong et al. 1996; Fürtig et al. 1996) and
inhibitor of cytochrome c oxidase in mitochondrial respiratory electron transport
chain (Goodman et al. 1995; Holmer and Bondgaard 2001). Nevertheless, the
mechanisms of sulfide toxicity and plant strategies to cope with sulfide intrusion at
the molecular level remain to be further investigated.
7 Photobiology of Seagrasses: A Systems Biology Perspective