caulerpa taxifolia in moreton bay - Centre for Marine Science ...

Caulerpa taxifolia in Moreton Bay
reduction in the amount of oxygen transported by seagrasses to the rhizosphere would
allow anaerobic processes to dominate in this area. Second, the increased primary
production from macroalgae and phytoplankton that is often associated with
eutrophication leads to a greater input of organic matter to the sediment which
enhances anaerobic microbial activity.
Sediments colonised by C. taxifolia in Moreton Bay had typical anaerobic
characteristics, ie. black colour and sulphide smell (pers. obs.). High sulphide
concentrations in sediments colonised by C. taxifolia have also been observed in the
Mediterranean (Chisholm et al., 1997; Fernex et al., 2001) which implies that
sulphide is not toxic to C. taxifolia as it is to some seagrasses (Carlson et al., 1994;
Goodman et al., 1995; Azzoni et al., 2001). The leakage of photosynthetically-fixed
dissolved organic carbon (DOC) from C. taxifolia rhizoids into the surrounding
sediment further stimulates anaerobic bacterial metabolism, including nitrogen
fixation and sulphate reduction (Thomas, 2002b; Chisholm and Moulin, 2003).
5.2.2.4 Caulerpa taxifolia toxins
The toxins in C. taxifolia represent another potential mechanism for competing with
seagrass. This was explored with planthouse experiments, and no significant effect on
growth and physiology of the seagrasses Z. capricorni and Cymodocea serrulata was
demonstrated. The main toxin in Caulerpa, caulerpenyne, is apparently degraded in the
presence of light, oxygen and chlorophyll. However, it is unknown if the degradation
products are also toxic (Guerriero et al., 1994). A wound-activated transformation of
caulerpenyne into potentially more toxic forms has also been observed (Jung and
Pohnert, 2001). However, the toxins may still be active in the sediment as marine
sediments are typically anoxic and dark. The methodology used in the planthouse
experiments may not have exposed the seagrass to the potentially toxic compounds, as
the mechanical damage to C. taxifolia during production of the extract and the
application of it to the light-exposed sediment surface may have reduced the amount of
toxins in the extract. Additionally, the application of the C. taxifolia extract to the
sediment surface may have prevented the toxins from reaching the seagrass roots. In
situ, the C. taxifolia rhizoids and seagrass roots are in close enough proximity for
exposure to the toxins (pers. obs.). The role of C. taxifolia toxins in allelopathic
interactions has been explored using seawater containing C. taxifolia extract, however
Jane Thomas -43-
Discussion