Environmental Engineering Reference
In-Depth Information
Some bioilm species attract certain larvae and repel other settlers [8]. Larvae of the
polychaete worms, Hydroides elegans and Phallusia nigra , the barnacle Balanus
amphitrite and other invertebrates require to make contact with the bioilms to detect
settlement stimulating ligands which help to give a strong attachment of the larvae
[32, 33]. The larvae may also use the adhesive properties of the bacteria to enhance
their own attachment strength during settlement. It is more dificult to remove the
larvae that settle and initiate metamorphosis on a bioilmed surface by turbulent shear
forces and lowing water than larvae which settle on a clean surface [33].
Climate change is also an important cue in biofouling. A number of studies have shown
seasonal variation of biofouling loading. Wisely [34] in a study at Sydney harbour
over 10 years, observed that biofouling was heaviest during the summer months
(average surface water temperature midsummer was 24 °C). Underwood and Anderson
[35] observed the highest deployment of barnacles ( Hexaminius sp .) and algae on
submerged panels from October (Spring) to March (Autumn) at Quibray Bay, Botany
Bay in New South Wales, Australia. The panels submerged in January (Summer) for a
period of four to ive months resulted in an oyster-dominated assemblage. Sudhakar
and co-workers [31] observed that biofouling loading and organic matter were
maximal on HDPE, LDPE and PP during the end of the South-West monsoon and
the start of North-East (in August) wind, because of changes in the ocean currents.
Although the cues for settlement are homogeneous, the method of settlement varies
between species and may also depend on the environmental conditions.
The green algae, Enteromorpha is the major biofouling algae. It colonises and settles
on new surfaces by producing microscopic motile zoospores that excrete glycoprotein
adhesives by exocytosis of the contents of membrane bound cytoplasmic adhesive
vesicles [36] ( Figure 2.5 ). Freshly released adhesives are very sticky with strength of
500 mN/m. Within hours of attachment these zoospores germinate and attach to the
substratum by adhesives secreted by rhizoids [29].
The tube dwelling H. elegans , a polychaete, is a common biofouler on PP ( Figure 2.6 ).
The larval settlement depends on the bacterial density on the surfaces, mainly of short,
rod-shaped bacteria [37]. Larva of H. elegans has apical cilia that are used to sense
cues from the bioilm bacteria. The larvae must come in contact with a bioilmed
surface to detect a settlement cue [33]. The bacterial species (both gram negative and
gram positive) in the bioilm stimulates both the settlement and metamorphosis of
the larvae of H. elegans [38].
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