[Annelida] Carbon cycle and earthworms

[Geoff Read]

Hi all,

Remember that news item last year "Gas from worm farms" which reported
worms produced nitrous oxide?
So you stopped making compost, or you were slightly downhearted about
the side-effects? 

""Are worm farms bad for the environment?  Hamburg, DPA  German 
researchers have found that worm composting systems produce more 
greenhouse gases than landfill sites."
http://www.bio.net/bionet/mm/annelida/2007-July/002789.html

You can cheer up and start again. Because there's new research.

Briones, M. J. I.; Ostle, N. J. ; Piearce, T. G. 2008:  Stable isotopes
reveal that the calciferous gland of earthworms is a CO2-fixing organ. 
Soil Biology and Biochemistry 40(2): 554-557. 
http://dx.doi.org/10.1016/j.soilbio.2007.09.012

Abstract: "Since they were first described in 1829, earthworm
calciferous glands have intrigued invertebrate anatomists and
physiologists alike. These organs are present in all species of the
family Lumbricidae, occurring in a range of morphological forms. A
common feature of the glands is that constituent secretory cells produce
a concentrated suspension of calcium carbonate. A number of possible
biological roles have been suggested for the secretion (i.e. egg
formation, pH buffering of the blood and ingested food, excretion and
respiration) but the true function has not yet been demonstrated
satisfactorily. Here, we investigated the putative respiratory function
of these organs by exposing the worms to 13C-labelled CO2 and glucose
and measuring tracer incorporation into the body wall, the gland tissues
and the calcareous secretion. Our results support the view that these
organs provide a mechanism of CO2 regulation in their tissues and that
both environmental and metabolic CO2 can be fixed in this way."

The article concludes: "This production and release of CaCO3
concretions could potentially have an important role to play in
soil-carbon dynamics. Individual worms can produce 1–3 concretions
(>250 μm diameter) a week (Canti, 1998), many of which survive for
significant time below ground, with examples commonly found in
quaternary and archaeological deposits (e.g. Canti, 1998).  By taking an
average concretion weight of 21 mg, and assuming that 96% of this mass
is CaCO3 (Robertson, 1936), it is possible to estimate that an earthworm
population of 100 individuals m−2 would fix 12.5 g C m−2 y−1. Although
densities of 100–500 earthworms m−2 have been recorded (Lavelle and
Spain, 2001) in neutral and high-pH soils, an equivalent of 10–20
large L. terrestris per square metre is often recorded. This average
population density would yield 12.5–25 kg C ha−1 y−1, suggesting that
this mechanism of C sequestration could be extremely important."

Actually biologists seemed to know a fair amount about the functions of
the glands a long time ago.

See: http://jeb.biologists.org/cgi/reprint/13/3/279.pdf 

Robertson, J. D. 1936:  The function of the calciferous glands of
earthworms.  
Journal of Experimental Biology 13: 279–297.

Conclusions: 
"1. In the Lumbricidae the secretion of the calciferous glands consists
mainly of
calcium carbonate, the percentage of carbonate in the calcite
concretions being
95-97 per cent.
5. The amount of carbon dioxide bound as carbonate by the glands was
measured
in a series of experiments with earthworms kept in different calcium
salts. 
The percentage of carbon dioxide excreted as carbonate never exceeded
10 per cent,
 of the total metabolic carbon dioxide.
7. The true function of the calciferous glands is excretion, calcium
carbonate
being passed into the gut as crystals of calcite which are chemically
inactive in the
gut.



-- 

 Geoff Read <g.read from niwa.co.nz>
   http://www.annelida.net/
  http://www.niwascience.co.nz/ncabb/