10. Lipids in Water-Surface MicroJayers and Foams
255
posited at the air-water interface (Johnson et ai., 1989; Hardy, 1982; Elzennan
and Annstrong, 1979; Eisenreich et ai., 1978; Szekielda et ai., 1971). The presence of biogenic surfactants in the microlayer, consisting of fulvic and humic
acids draining from land and also from the aquatic microbial origin, enhances the
solubility of hydrophobic compounds. The enrichment factor of lipophilic constituents in the surface micro layer varies enonnously because it depends on
the interplay of many factors such as the proximity of the sources (decaying
organic material, industrial imput), transport agents (advection currents, wind,
drainage), and the suitability of the particular microlayer or foam to sustain
microorganisms. An additional factor contributing to the variability in the
measure of the concentrations of substances in the water micro layer is the disparity of tools and materials used for sampling. The different sampling devices
not only collect a different thickness of the microlayer, but some of the collecting surfaces (e.g., glass, metal, ceramic, PTFE) may have a differential affinity
for the organic matter in natural waters. Due to inertness considerations, PTFE
screens, collecting surfaces, and accessories (wipers and bottles) are recommended for the sampling of lipids and hydrophobic contaminants from surface
waters.
Although foam fonnations are often associated with pollution events, they are a
common occurrence in uncontaminated waters. Foams are constructed of the
materials fonned and accumulated at the surface microlayer, and therefore, they
offer an excellent opportunity to study the chemical composition of the microlayer. Surface microlayers and foams of natural waters present a large list of
organic constituents, which include humic and fulvic acids, carbohydrates, proteinaceous materials, and lipids. All lipid classes found in aquatic and terrestrial
organisms are also found in the surface microlayers and foams (Napolitano et ai.,
1995; Johnson et ai., 1989). The lipid class composition of the water microlayer,
however, differs significantly from that of aquatic organisms. Lipids of the microlayers are, in general, enriched in those fractions resistant to chemical degradation, such as hydrocarbons and saturated free fatty acids. Hydrocarbons nonnally
constitute a very small fraction of the lipids of most organisms (Nevenzel, 1989),
but they can be the major constituent of the water microlayer. In most of these
cases, however, a structural analysis reveals that the hydrocarbons are largely
anthropogenic in origin.
Although foams normally concentrate the most refractory fraction of the biogenic lipids (Simoneit et ai., 1980), some foam formations may also contain
relatively large proportions of labile phospholipids or PUFA of both 0)3 and 0)6
series. The presence of these chemically unstable lipids reflects the capacity of
foam to host microbial life.
Considering the spatial continuity of the freshwater and marine environments,
it is easy to assume that the processes occurring at the water-surface micro layers,
which cover ~ 70% of the area of the planet, have profound implications for a
complete assessment of the cycles of the aquatic organic matter. Unfortunately,
owing to the lack of standard methods for the consistent and reproducible sampling of the surface microlayer, the examination of its properties and chemical
composition is not nonnally included in environmental studies. We look forward
255
posited at the air-water interface (Johnson et ai., 1989; Hardy, 1982; Elzennan
and Annstrong, 1979; Eisenreich et ai., 1978; Szekielda et ai., 1971). The presence of biogenic surfactants in the microlayer, consisting of fulvic and humic
acids draining from land and also from the aquatic microbial origin, enhances the
solubility of hydrophobic compounds. The enrichment factor of lipophilic constituents in the surface micro layer varies enonnously because it depends on
the interplay of many factors such as the proximity of the sources (decaying
organic material, industrial imput), transport agents (advection currents, wind,
drainage), and the suitability of the particular microlayer or foam to sustain
microorganisms. An additional factor contributing to the variability in the
measure of the concentrations of substances in the water micro layer is the disparity of tools and materials used for sampling. The different sampling devices
not only collect a different thickness of the microlayer, but some of the collecting surfaces (e.g., glass, metal, ceramic, PTFE) may have a differential affinity
for the organic matter in natural waters. Due to inertness considerations, PTFE
screens, collecting surfaces, and accessories (wipers and bottles) are recommended for the sampling of lipids and hydrophobic contaminants from surface
waters.
Although foam fonnations are often associated with pollution events, they are a
common occurrence in uncontaminated waters. Foams are constructed of the
materials fonned and accumulated at the surface microlayer, and therefore, they
offer an excellent opportunity to study the chemical composition of the microlayer. Surface microlayers and foams of natural waters present a large list of
organic constituents, which include humic and fulvic acids, carbohydrates, proteinaceous materials, and lipids. All lipid classes found in aquatic and terrestrial
organisms are also found in the surface microlayers and foams (Napolitano et ai.,
1995; Johnson et ai., 1989). The lipid class composition of the water microlayer,
however, differs significantly from that of aquatic organisms. Lipids of the microlayers are, in general, enriched in those fractions resistant to chemical degradation, such as hydrocarbons and saturated free fatty acids. Hydrocarbons nonnally
constitute a very small fraction of the lipids of most organisms (Nevenzel, 1989),
but they can be the major constituent of the water microlayer. In most of these
cases, however, a structural analysis reveals that the hydrocarbons are largely
anthropogenic in origin.
Although foams normally concentrate the most refractory fraction of the biogenic lipids (Simoneit et ai., 1980), some foam formations may also contain
relatively large proportions of labile phospholipids or PUFA of both 0)3 and 0)6
series. The presence of these chemically unstable lipids reflects the capacity of
foam to host microbial life.
Considering the spatial continuity of the freshwater and marine environments,
it is easy to assume that the processes occurring at the water-surface micro layers,
which cover ~ 70% of the area of the planet, have profound implications for a
complete assessment of the cycles of the aquatic organic matter. Unfortunately,
owing to the lack of standard methods for the consistent and reproducible sampling of the surface microlayer, the examination of its properties and chemical
composition is not nonnally included in environmental studies. We look forward