Marine Biofouling of Polymers - Polymers in a Marine Environment - page 25

enters other cells of either the same species or a different species, by active or passive
transport and binds to the cytoplasmic LuxR-type receptor to form active dimers
( Figure 2.2 ). The dimer later binds to Lux regions within the promoter sequence and
activates the transcription of QS genes [20, 22].

Quorum sensing
signal molecules

2. Signal
diffusion

3. Binding of
signal molecules
and receptors

1. Signal
production

Receptors

Bacterial cell

5. Gene
expression

4. Signal-receptor
binds to promoter

Bacterial attachment, motility and biofilm formation

Figure 2.2 AHL-mediated QS in gram negative marine bacteria

The polysaccharide ibrils are attached to their chemical counterparts in the bioilm
by lectins or divalent cations (Ca ++ , Mg + ) [23]. The cells are pulled towards the
surface polysaccharides by enzymic shortening of the ibrils. This creates the covalent
bonds between the bacterial glycocalyx and the bioilm [4, 24]. The physical and
biological characteristics and requirements of the bacteria affects its colonisation
which in turn affects the physical, chemical and biological properties of the surface
and also determines the types of bacteria to be employed as secondary colonisers
[25]. Generally the rod shaped microorganisms are the irst colonisers, followed by
coccoid and inally the stalked and ilamentous forms [24].

The eukaryotic microorganisms in biofouling are diatoms, fungi, protozoans and yeast.
Diatoms are found in abundance and appear after the formation of the primary ilm
[4]. Diatoms attach to the surface at any time during the development of the bioilm
and grow when light is suficient. Although there are about 10,000 diatom species
in the ocean, only about 8-10 genera have been documented as biofoulers. The most
commonly reported species include: Amphora coffeaeformis , Achnanthes longipes ,
Craspedostauros australis , Toxarium undulatum and Navicula perminuta [26].
Because the diatoms lack lagella, they land randomly on the substratum [8].