1 Introduction

Geosiphon pyriformis– a Glomeromycotan Soil Fungus 273
◭ Fig. 1. 1–11 Positive influence of charcoal on Geosiphon symbiosis formation. Bladders are
nearly exclusively formed where charcoal is scattered onto the substrate. 1 Culture on coarse,
quartz sand. 2 Culture on fine, white quartz sand. 3 Cultureon2%agar(arrowheads indicate
young bladders). 4 Viewfromthetopontotwoculturesonfine,whitequartzsand(without
charcoal layer), only a part of the culture vessels is shown. Nostoc growth is much lower
(after 12 weeks) in medium with 5 µM phosphate (right) comparedto1mM phosphate
(left). 5 A typical Geosiphon habitat in the Spessart Mountains. 6 Schemes comparing the
symbiotic interfaces of the Geosiphon and AM symbioses. 7 Spores formed in coarse quartz
sand culture. In contrast to culture on fine substrates where spores are regularly round,
spores formed in the coarse substrate can be irregularly shaped (arrows), showing the
potential influence of the substrate on spore morphology. 8 Two Geosiphon bladders with
newly formed hyphae. At the tips new young bladders develop (arrows). Left inset One-weekold
bladder. Right inset Endosymbiotic Nostoc arranged at the periphery of the bladders; the
light green cells are heterocysts. 9 Earlystagesofrecognition(arrowheads). Upon contact
with a Nostoc primordium, the fungus forms several ‘projections’, eventually engulfing the
Nostoc cells. 10 Incorporation of a Nostoc primorium. Confocal laser scanning microscopy
(CLSM); excitation 488 nm, maximum projection (23 optical sections) through 9.5 µm.
Green indicates ConA-Alexa488 fluorescence (detection: 505–550 nm; stains mannose), red
indicates Nostoc pigment autofluorescence (detection: 620–730 nm). Pictures were taken 27
(left), 41 (centre)and60h (right) after start of green light illumination (see text). The Nostoc
cells deform and bleach during incorporation (arrow). 11 CLSM of a very young Geosiphon
bladder. Maximum projection (11 optical sections) through 5.5 µm in the middle part of
the bladder. Simultaneous 2-photon (780 nm) and 1-photon (488 + 568 nm) excitation.
Detection: blue (405–480 nm) = Calcofluor White (stains chitin), green (515–560 nm) =
ConA-Alexa488 (stains mannose), red (600–775 nm) = Nostoc pigment autofluorescence.
The endosymbionts begin to recover. The EPS of free-living Nostoc as well as the surface of
the hyphae contain mannose. From a part of the hypha, cytoplasm was retracted and septae
were formed
invagination (see 2.2, Figs. 9–11, and retains the capability to synthesize
chitin. This is interesting in the context that the fungus belongs to the
Glomeromycota (see Sect. 6.1), and the symbiotic interface is similar in
structure and function to that formed in AM fungi (Fig. 6: Schüßler et al.
1996). The endosymbionts are located mainly in the apical three-fourths of
the bladder, while the basal part acts as a storage region, containing many
lipid droplets. The Geosiphon bladders, hyphae and spores contain another
endosymbiont, the ‘bacteria-like organisms’ (BLOs), which are bacteria of
unknown phylogenetic affiliation with an ultrastructure identical to those
frequently found in AM fungi (for a review, see Schüßler and Kluge 2001).