degree of stability in the growth environment, (2) affords protection from a wide
range of environmental challenges, and (3) might have catalytic functions through
localizing cells in close proximity (Hall-Stoodley et al. 2004).
3.5.2 EPS Secretion and Interaction with Sediments
Extracellular polymeric substances (EPS) are a set of compounds of various chemical identities that are purposefully produced by microbes (a) as secretions of
biofilms that secure attachment and enhance their local microenvironments; (b) as
metabolic-excess waste products (Decho and Gutierrez 2017); or (c) as the result of
unbalanced growth, when the supply of nutrients lags behind photosynthetic CO 2
fixation (Staats et al. 2000; Staats et al. 2000; Stal 2010). It is important to point out
that EPS are not an essential component to microbial life, implying that individual
cells can survive and grow without them.
Chemically, EPS consists of complex molecules of which highly hydrated polysaccharides make up the larger fraction. Other minor components include sugars,
amino sugars, amino acids, proteins, lipids and lipopolysaccharides, phosphate and
sulfate groups, and uronic acids which confer EPS their acidic nature (Stal and
Brouwer 2005; Stal 2010; Decho and Gutierrez 2017). Depending on the extractive
method applied, diatom EPS can be separated into different (operational) fractions.
Hence, there are soluble and bound EPS. Also, as some microorganisms produce
exopolymers as part of their natural organic coatings (e.g., cyanobacterial sheaths),
there is also capsular EPS in sediments. Finally, some EPS chelate Ca
2+ and Mg
2+
ions through which the EPS matrix gets bound to the sediment by cation bridges with
charged silt and clay particles (Decho 1990).
On the one hand, EPS facilitate microbial attachment to surfaces that leads to the
formation of biofilms; on the other, sediment-EPS interactions have important
sedimentological implications, of which the most immediate is the role of EPS in
sediment stabilization. Sediment stabilization or biostabilization (de Boer 1981; Stal
2010) is one of the most important emergent properties of microbial activity over the
substrates they colonize (Noffke 2000; Noffke et al. 2001; Noffke and Paterson
2008). Sediment stabilization is a direct consequence of the cohesiveness conferred
by microbial EPS and the microbial cells themselves (Decho and Gutierrez 2017).
Individual microorganisms may exert small-scale effects on substrate stability, but
whole microbial communities and their metabolic EPS create scale-dependent positive and negative feedbacks that have consequences for the functioning of whole
ecosystems (Stal 2010).
A variety of sediment microbes produce a variety of EPS. For example, experimental studies have demonstrated that sediment bacteria may produce exopolymeric
capsules as an induced protective mechanism from enzymatic digestion (Plante
2000). Other studies have focused on the role of bacterial EPS with high uronic
acid content in increasing the erosion threshold of intertidal fine-sand beds (Dade
et al. 1990); this EDTA-extractable EPS is tightly bound to the sediment and also
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
77
range of environmental challenges, and (3) might have catalytic functions through
localizing cells in close proximity (Hall-Stoodley et al. 2004).
3.5.2 EPS Secretion and Interaction with Sediments
Extracellular polymeric substances (EPS) are a set of compounds of various chemical identities that are purposefully produced by microbes (a) as secretions of
biofilms that secure attachment and enhance their local microenvironments; (b) as
metabolic-excess waste products (Decho and Gutierrez 2017); or (c) as the result of
unbalanced growth, when the supply of nutrients lags behind photosynthetic CO 2
fixation (Staats et al. 2000; Staats et al. 2000; Stal 2010). It is important to point out
that EPS are not an essential component to microbial life, implying that individual
cells can survive and grow without them.
Chemically, EPS consists of complex molecules of which highly hydrated polysaccharides make up the larger fraction. Other minor components include sugars,
amino sugars, amino acids, proteins, lipids and lipopolysaccharides, phosphate and
sulfate groups, and uronic acids which confer EPS their acidic nature (Stal and
Brouwer 2005; Stal 2010; Decho and Gutierrez 2017). Depending on the extractive
method applied, diatom EPS can be separated into different (operational) fractions.
Hence, there are soluble and bound EPS. Also, as some microorganisms produce
exopolymers as part of their natural organic coatings (e.g., cyanobacterial sheaths),
there is also capsular EPS in sediments. Finally, some EPS chelate Ca
2+ and Mg
2+
ions through which the EPS matrix gets bound to the sediment by cation bridges with
charged silt and clay particles (Decho 1990).
On the one hand, EPS facilitate microbial attachment to surfaces that leads to the
formation of biofilms; on the other, sediment-EPS interactions have important
sedimentological implications, of which the most immediate is the role of EPS in
sediment stabilization. Sediment stabilization or biostabilization (de Boer 1981; Stal
2010) is one of the most important emergent properties of microbial activity over the
substrates they colonize (Noffke 2000; Noffke et al. 2001; Noffke and Paterson
2008). Sediment stabilization is a direct consequence of the cohesiveness conferred
by microbial EPS and the microbial cells themselves (Decho and Gutierrez 2017).
Individual microorganisms may exert small-scale effects on substrate stability, but
whole microbial communities and their metabolic EPS create scale-dependent positive and negative feedbacks that have consequences for the functioning of whole
ecosystems (Stal 2010).
A variety of sediment microbes produce a variety of EPS. For example, experimental studies have demonstrated that sediment bacteria may produce exopolymeric
capsules as an induced protective mechanism from enzymatic digestion (Plante
2000). Other studies have focused on the role of bacterial EPS with high uronic
acid content in increasing the erosion threshold of intertidal fine-sand beds (Dade
et al. 1990); this EDTA-extractable EPS is tightly bound to the sediment and also
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
77