Microorganisms are capable of growing on most solid surfaces in aquatic environments.
In fact, bacteria in natural aquatic populations have a tendency to adhere to surfaces, eventually forming biofilms. The formation of biofilms on surfaces is a survival strategy for bacteria. Bacterial populations exist in a highly-protected environment within the biofilm matrix, and nutrients are abundant on the water/solid surface interface (Costerton et al. 1987). Clean surfaces that are submerged in natural water soon show evidence of a microbial colony on them. The development of microbial colonies has been observed on glass slides immersed in aquatic systems as well as on sand grains, pebbles, and detritus (Costerton et al. 1987; Marshall 1992).
When nutrients accumulate on a surface, motile microorganisms with chemo tactic responses are attracted to them, and they swim along a concentration gradient of the nutrients toward this surface. Other mechanisms by which microorganisms are attracted to and attach to surfaces in aquatic environments are physicochemical attraction forces.
Reported modes of microbial attachment onto aquatic surfaces are Brownian motion, electrostatic attraction, van der Waals forces, and hydrophobic interactions (Marshall 1992).
Following some form of attraction, microorganisms establish contact with the surface.
Most microorganisms adhere to a surface by means of extracellular polymers that anchor the organisms permanently to the surface. Some of these extracellular materials include pili, flagella, acidic polysaccharides, and specific extracellular proteins (Costerton et al.
1987; Marshall 1992).
Experiments show that the initial colonization of surfaces is a selective process. First, small rods (<0.8 µm long) adhere within an hour of immersion of a surface, followed by adhesion of coccoidal, spiral, and larger rod forms within six to eight hours. Stalked or budding microorganisms appear after 24 hours, and diatoms and protozoa appear around the same time. Eventually biofilms, which are the extensive growth of microorganisms on aquatic surfaces exposed to flowing water systems, develop. The nature of the developed mature films differs depending on the conditions in the aquatic ecosystem. As the thickness of the film increases, some of the microorganisms in the film matrix become nonviable as the nutrient diffusion becomes limited (Marshall 1992).
As many different organisms pile up on the aquatic surface, the biofilm becomes a micro- cosmos with a variety of diverse species. An increase in interspecific and intraspecific
1992; Rauch et al. 1999; Tartakovsky et al. 2005). An illustration of biofilm formation is given in Figure 2.5.1 below. Another illustration shows the steps of biofilm formation in Figure 2.5.2.
Biofilm buildup depends on the nutrient load and biofilm-forming microorganisms.
Activity in the biofilm increases as the biofilm thickness increases to a certain level above, the activity of which is not affected by the biofilm thickness. Diffusion of nutrients into the biofilm from the bulk water occurs over the active depth (Khlebnikov et al. 1998). Similarly, oxygen can only penetrate up to a certain depth in the film, and the amount of oxygen diffusion into the inner portions of the biofilms affects the metabolic efficiency of microorganisms in the film (Marshall 1992).
Gerhardt and Schink (2005) state that the variable oxygen profile formed due to the limits of the oxygen penetration depth provide bacteria with new chances and challenges. These challenges help the formation of mixed cultures with aerobic and anaerobic transformations.
Figure 2.5.1. Biofilm formation (source: http://www.biofilmsonline.com/cgi- bin/biofilmsonline/ed_how_primer.html
Figure 2.5.2. Steps in biofilm formation (source: http://www.erc.montana.edu/CBEssentials- SW/bf-basics-99/bbasics-01.htm
Oxygen is very rapidly consumed in most natural water systems due to the high rate of mineralization. Thus, the oxygen penetration depth is generally less than 10 mm and
heterotrophic bacteria can completely mineralize organic matter into carbon dioxide. In anaerobic sediments, organic matter is decomposed in more complex and less energy efficient pathways that require colonies of physiologically different microorganisms.
Each of the microbial groups participates in the decomposition process, forming an end product used by other groups of microbes until decomposition is complete (Holmer and Storkholm 2001; Tank and Dodds 2003).
The aerobic and anaerobic zones occur due to limitations of oxygen penetration into the biofilm, as depicted in Figure 2.5.3 below:
Figure 2.5.3. Portions of aerobic and anaerobic microorganisms in biofilms. (Source:
http://www.erc.montana.edu/CBEssentials-SW/bf-basics-99/bbasics-01.htm) SOLID SURFACE
After several experiments, (Hibiya et al. 2004) developed some empirical equations that related oxygen penetration depth with biomass concentration and the effective oxygen diffusion coefficient with oxygen penetration depth as follows:
4 5
2.5 10 1.5 10 exp
X = x + x ⎛⎜−175δ ⎞⎟
⎝ ⎠ , (2.38)
where X [M/L3] is the biomass concentration and δ[L] is the biofilm thickness. This empirical equation is used to estimate the effective oxygen diffusion coefficient as
19 21 0.5
(3.5 1.5 )
De = − + − δ , (2.39)
where De [L2T-1] is the effective oxygen diffusion coefficient.
Higashino and Stefan (2005) developed a model that estimates the diffusion of oxygen from the water column to sediments and “sediment oxygen demand,” which is the oxygen in the sediment that the microbial colony requires to continue their activities. Their model is suitable for laminar flow, which causes the formation of a laminar diffusive boundary layer on top of the sediment/water interface. The illustration for their model is given in Figure 2.5.4. below:
Figure 2.5.4. The velocity and dissolved oxygen profiles for a reactive sediment/water interface (Source: (Higashino and Stefan 2005))
They have concluded that the free-stream velocity has very little effect on the oxygen penetration depth and that the oxygen penetration depth depends very strongly on the maximum biomass concentration. Hence, the oxygen penetration depth can be calculated independent of the stream velocity and can be assumed constant over the entire length of a river, given the biomass concentration is also assumed to be constant.