Nick,

Maybe Makay and team didn't present at the 6th Mars Conference about Mars life issue but they do at the Lunar and Planetary Science XXXIV (2003):

http://www.lpi.usra.edu/meetings/lpsc2003/pdf/2060.pdf

MN CARBONATES IN THE MARTIAN METEORITE NAKHLA: POSSIBLE EVIDENCE OF BRINE EVAPORATION

Mn Carbonates as Indicators of Microbial Activity: The
poss i bi li t i es for ei t her bi ogeni c or abi ot ic origi ns , or perhaps
a com bi nat i on of the two, need furt her inves t i gat i on
On Eart h, low-t em perat ure Mn carbonat e preci pi t at i on is
often mi crobi al ly medi at ed. Si gni fi cant am ounts of bi ot ic
m anganous carbonate have been di scovered in the Bal t i c
S ea [14]. Addi t i onal l y, bacteri all y preci pit at ed rhodochros
i t e has been produced under l aborat ory condi t i ons by
Geobact er metal l i reducens [15]. F ut ure st udi es us i ng ion
m i croprobe anal ys is of 13C / 12C and 34S / 32S may be abl e to
di fferent i at e bet ween thes e two pos s i bi l i t ies . S t able is ot
ope fract i onat i on on Eart h can be used as an indi cat or of
m i crobi al acti vi t y becaus e met aboli c proces ses can lead to
enri chm ent in the li ghter isot opes of carbon and sulfur.
The mi cros copi c text ures of the carbonates in this study
al so exhi bi t a compl ex morphol ogy. The possibility that
t his text ure coul d have been produced by eit her abiotic
processes or through biological activity unders cores the
need to develop good cri t eria for identification of microbi
al precipitates .