Symbiotic Fungi: Principles and Practice (Soil Biology)

16 Using Stable Carbon Isotope Labelling 281
16.4.5 Quantification of Transferred Carbon
The correspondence between 13 C enrichment in NLFA 16:1o5 and 13 C enrichment
in total mycelium, shown by Olsson et al. 2005, suggests that the flow of C from
host plants to fungal mycelia can be calculated from 13 C enrichment in NLFA
16:1o5. In this specific study it was found that for every C incorporated into NLFA
16:1o5 in mycelia, 2.7 atoms of C was incorporated into the total mycelium mass.
The flow of C from the host plant to the NLFA 16:1o5 can be calculated from
measurements of NLFA 16:1o5 in the mycelia and its 13 C enrichment:
13
NLFA16:1o5-CðmgÞ C-enrichment in NLFA16:1o5ð%=100Þ
¼ C-flow to NLFA16:1o5
Similarly, the flow of total C to the mycelia can be calculated by:
Mycelium biomass-CðmgÞ total 13 C-enrichmentð%=100Þ
¼ C-flow to mycelium
The flow of plant C to the AM fungal mycelia can be estimated based on measurements
of the signature NLFA 16:1o5:
C-flow to mycelium ¼ C-flow to NLFA16:1o5 2:7
Thus, for every C atom incorporated in NLFA 16:1o5, 2.7 C atoms are incorporated
into the fungal mycelia. This calculation can be used when pure mycelium
cannot be extracted, such as from colonised roots or mycelium-containing
soil.
16.5 Sensitivity and Specificity of the Method
Natural abundance of 13 C(d 13 C) is generally around d -30 (corresponds to 1.15%
13 C) in C3 plants, and the 13 C abundance in AM fungi is similar to that of their host
plant (Nakano et al. 1999, 2001; Staddon et al. 1999). Abraham et al. (1998)
provided various substrates to different microorganisms and observed that the 13 C
abundance in the fatty acid 16:0 extracted from the microorganisms resembled that
of the substrate. With our method, using a 13 C-labelled substrate, we reach a much
higher sensitivity when tracing the C metabolism than by just using the natural
differences in natural 13 C abundance between different substrates.
The dominance of the fatty acid 16:1o5 in AM fungi and its rareness in other
fungi (Müller et al. 1994; Stahl and Klug 1996) make it a useful biomarker probably
for all Glomus species (Graham et al. 1995) and also for Scutellospora species