30
G.E. Napolitano
and branched-chain fatty acids (Lechevalier, 1982). Although 20:50)3 and 22:60)3
have been found in deep-sea bacteria (DeLong and Yayanos, 1986) and in the
intestinal t10ra of certain fish (Ringo et aI., 1992), PUFA and fatty acids with a
chain length longer than 18 carbon atoms are typically absent in these procaryotes.
These fatty acid structures contrast sharply with those of the eucaryotic algae, that
typically contain large proportions of fatty acids with 20-22 atoms of carbons and
three to six cis double bonds (Table 2.1).
2.4.4. Fatty Acids as Trophic Markers of Bacteria
Although there are a number of methods used for the qualitative and quantitative
analysis of algal biomass (e.g., chlorophyll a, biovolume, dry mass), it is much
more difficult to measure bacterial biomass in natural samples. A major obstacle
to the determination of microbial biomass is that traditional enrichment and
isolation techniques do not ensure a quantitative retrieval of natural bacterial
populations (Findlay and Dobbs, 1993a,b). An additional problem in microbial
ecology is that bacterial morphology is of little use in taxonomic determinations.
Therefore. the use of specific biochemical markers acquires primary importance
in the identification and quantification of bacteria (White, 1995; Tunlid and
White, 1992). Microbial lipid markers have been broadly used in the biogeochemistry of marine (Saliot et aI., 1991; Gillan and Johns, 1986) and lacustrine
(Meyers and Ishiwatari, 1993) sediments, and more recently, these techniques
have been applied to the microbiology of soils (Cavigelli et al.. 1995). The
number of studies of bacterial lipid markers in streams, rivers, and lake waters is
much more limited (Findlay and Dobbs, 1993a).
Phospholipid fatty acids (PLFA) are considered an excellent surrogate for
bacterial biomass and are an effective tool in bacterial taxonomy (White, 1995). A
chemotaxonomic study using PLFAs of organism lipids in the euphotic zone in
Ace Lake (Antarctic) presented clear evidence for a dramatic shift in the taxonomic status of the dominant primary producers (Volkman et aI., 1988). PLFA
signatures in Ace Lake indicated that although eucaryotic phytoplankton dominated the surface waters (as evidenced by the high concentrations of 16:40)3,
18:20)6, 18:30)3, and 18:40)3), photosynthetic procaryotes (leading to high concentrations of 14:0, 15:0, 16:0, 18:0, 16:10)7, and 18:10)9) dominated in deeper
waters. These fatty acid profiles. together with the pigment analysis. suggested
that the deepest layers of the euphotic zone of Ace Lake were populated by
photosynthetic bacteria belonging to the genus Chlorobium (Kenyon, 1972). The
results of a similar study performed in the same lake a few years later confirmed
the clear differences in the microbial communities at each depth, although the
presence of a different bacterial community was indicated (Mancuso et aI., 1990).
This later study corroborated that lipids in the POM of the upper layers of the
euphotic zone were dominated by PUFA-producing microeucaryotes. but the
lower layers presented high concentrations of 10-methyl-16:0 and iso 17: I 0)7,
characteristic of the sulfur-reducing bacteria Desulfobacter and Desulfovibrio,
respectively. A fatty acid ratio defined as (2: iso 15:0 + 2: anteiso 15:0)116:0 was
G.E. Napolitano
and branched-chain fatty acids (Lechevalier, 1982). Although 20:50)3 and 22:60)3
have been found in deep-sea bacteria (DeLong and Yayanos, 1986) and in the
intestinal t10ra of certain fish (Ringo et aI., 1992), PUFA and fatty acids with a
chain length longer than 18 carbon atoms are typically absent in these procaryotes.
These fatty acid structures contrast sharply with those of the eucaryotic algae, that
typically contain large proportions of fatty acids with 20-22 atoms of carbons and
three to six cis double bonds (Table 2.1).
2.4.4. Fatty Acids as Trophic Markers of Bacteria
Although there are a number of methods used for the qualitative and quantitative
analysis of algal biomass (e.g., chlorophyll a, biovolume, dry mass), it is much
more difficult to measure bacterial biomass in natural samples. A major obstacle
to the determination of microbial biomass is that traditional enrichment and
isolation techniques do not ensure a quantitative retrieval of natural bacterial
populations (Findlay and Dobbs, 1993a,b). An additional problem in microbial
ecology is that bacterial morphology is of little use in taxonomic determinations.
Therefore. the use of specific biochemical markers acquires primary importance
in the identification and quantification of bacteria (White, 1995; Tunlid and
White, 1992). Microbial lipid markers have been broadly used in the biogeochemistry of marine (Saliot et aI., 1991; Gillan and Johns, 1986) and lacustrine
(Meyers and Ishiwatari, 1993) sediments, and more recently, these techniques
have been applied to the microbiology of soils (Cavigelli et al.. 1995). The
number of studies of bacterial lipid markers in streams, rivers, and lake waters is
much more limited (Findlay and Dobbs, 1993a).
Phospholipid fatty acids (PLFA) are considered an excellent surrogate for
bacterial biomass and are an effective tool in bacterial taxonomy (White, 1995). A
chemotaxonomic study using PLFAs of organism lipids in the euphotic zone in
Ace Lake (Antarctic) presented clear evidence for a dramatic shift in the taxonomic status of the dominant primary producers (Volkman et aI., 1988). PLFA
signatures in Ace Lake indicated that although eucaryotic phytoplankton dominated the surface waters (as evidenced by the high concentrations of 16:40)3,
18:20)6, 18:30)3, and 18:40)3), photosynthetic procaryotes (leading to high concentrations of 14:0, 15:0, 16:0, 18:0, 16:10)7, and 18:10)9) dominated in deeper
waters. These fatty acid profiles. together with the pigment analysis. suggested
that the deepest layers of the euphotic zone of Ace Lake were populated by
photosynthetic bacteria belonging to the genus Chlorobium (Kenyon, 1972). The
results of a similar study performed in the same lake a few years later confirmed
the clear differences in the microbial communities at each depth, although the
presence of a different bacterial community was indicated (Mancuso et aI., 1990).
This later study corroborated that lipids in the POM of the upper layers of the
euphotic zone were dominated by PUFA-producing microeucaryotes. but the
lower layers presented high concentrations of 10-methyl-16:0 and iso 17: I 0)7,
characteristic of the sulfur-reducing bacteria Desulfobacter and Desulfovibrio,
respectively. A fatty acid ratio defined as (2: iso 15:0 + 2: anteiso 15:0)116:0 was