I was looking up refs. to the crusts seen at the Opportunity site when
I came upon this:

Bacterial Communities Found to Follow Water - Implications for Mars?
Tempe - Sept. 26, 2001
http://www.spacedaily.com/news/life-01zh.html

The caption to the figure accompanying the article says this:

"Vertical distribution of cyanobacterial community in desert crusts
from the Colorado Plateau as seen through fluorescence laser confocal
microscopy. Cyanobacteria are naturally fluorescent due to their
photosynthetic pigments. This allows to observe them upon laser
excitation without the background disturbance of mineral matrix and
other microorganims. The top picture shows the cyanobacterial
community at the surface with short, thick filaments of Scytonema sp.
and round colonies of Nostoc sp. A different community, dominated by
thin, long filaments of Microcoleus vaginatus can be found 0.3-0.5 mm
deep into the crusts."

I looked up some refs. to Microcoleus vaginatus:

Cryptobiotic Soils: Holding the Place in Place
Jayne Belnap
Soil Ecologist
U.S. Geological Survey
"Figure 4. Crust in sandy soils. The visible fibers are Microcoleus
vaginatus. Note how Microcoleus connects the otherwise loose sand
grains together, thus preventing wind and water erosion. Microcoleus
is important in enhancing water and nutrient relations within the
soil, as well. Scale bar is 100 micrometers."
http://geochange.er.usgs.gov/sw/impacts/biology/crypto/

Further Adventures with a Field Microscope:
Cryptobiotic Soil
By Wayne Lanier, USA
"Although this soil community also contains lichen and mosses, it is
initiated by and dominated by cyanobacteria. On the Colorado Plateau,
the species of cyanobacteria commonly found in cryptobiotic soil is
Microcoleus vaginatus."
...
"M. vaginatus is a large, filamentous green cyanobacterium. Each
segment in a filament is an individual bacterium. Filaments range
considerably in length. Shown below is a short filament, but I found
some filaments as long as 300 µm."
...
"As they grow, the bacterial filaments produce a sticky mucilaginous
polysaccharide sheath, usually covering a number of filaments."
...
"This sticky mucilaginous sheath binds the filaments to soil
particles, producing a matrix that is sufficiently strong to make the
soil surface crusty. As the soil dries out, the filaments return to a
cryptobiotic state, and the sheath around them remains connected to
the soil grains, pulling the grains tightly together."
http://www.microscopy-uk.org.uk/mag/artaug02/wlfield2.html

This article has a link to a close-up image of the threadlike
features at the Opportunity site:

Rover Microscope Detects Puzzling ‘Thread-Like' Features.
By Leonard David
Senior Space Writer
posted: 01:05 pm ET
20 February 2004
http://www.space.com/missionlaunches/opportunity_threads_040220.html

Raw images here:

Microscopic Imager :: Sol 019 (8 images)
http://marsrovers.jpl.nasa.gov/gallery/all/opportunity_m019.html

Explanations given for these threads have been that they may be
threads from the airbags or perhaps they are due to grains strung
together by electrostatic forces.

This page gives the resolution of the Microscopic Imager as about 30
microns:

Mars Exploration Rovers Project
------------------------------------------------------------------------
Microscopic Imager.
http://astrogeology.usgs.gov/Projects/MER-AthenaMI/microscopic_imager.html

The threadlike features seen in the Opportunity images appear to be
just barely within the resolution limit, so perhaps are 30 to 60
microns wide.

Microcoleus vaginatus filaments are about 5 to 10 microns wide.
However, the filaments are sometimes contained within a mucilagenous
sheath about 30 to 50 microns wide during dry conditions, so perhaps
these would be within visibility of the microscopic imager.

Bob Clark

cf:

From: Robert Clark (rgclark@my-deja.com)
Subject: Dark spots in Mars gullies - evidence of cyanobacteria?
Newsgroups: sci.astro, sci.astro.seti, alt.sci.planetary, sci.bio.misc
Date: 2001-11-29 14:05:07 PST
http://groups.google.com/groups?th=b4c2374dcdf153f6