Geoscience Reference
In-Depth Information
And there's still more because I simply cannot help but explain about
the ecology of
Prochlorococcus
in the oceans. I do this partly because it's
fascinating, and partly because when thinking about the distant past,
we can also imagine that ancient organisms must have lived with similar,
and equally fascinating, adaptations to their environment. So to begin,
Prochlorococcus
species contain only about 2000 genes, making them among
the most gene-deicient oxygen producers known. You would think
more genes would be good, giving an organism more flexibility and a
better chance to adapt to changing environmental conditions.
Prochloro-
coccus
, however, uses a different strategy. Rather than covering all even-
tualities with a huge genome, these cyanobacteria are lean and mean,
and tune their genome to specific environmental conditions.
The story continues by recognizing that
Prochlorococcus
isn't just
Prochlo-
rococcus
. A number of different
Prochlorococcus
strains have been isolated
from the environment, and while some are better adapted to high-light
conditions, others prefer low light, and others still are adapted to the
particulars of nutrient availability at different depths in the ocean. These
different
Prochlorococcus
ecotypes, as Penny Chisholm calls them, stratify
in the marine water column, and this stratification also changes with
latitude and longitude relative to their preferred light and chemical
environment. An example of such stratification is shown in
igure 4.4.
If we begin with the distribution of
Prochlorococcus
near the equator, we
see that the ecotypes adapted to high-light conditions prefer the upper
water column, and lower-light ecotypes prefer the deeper waters. If we
move to higher latitudes (48°N) where less light hits the sea surface, the
intermediate-light ecotypes populate the surface layers and dominate
overall. Here the ecotype adapted to the highest light conditions is
greatly reduced in abundance, and the lowest-light adapted ecotype is
almost completely missing, where, apparently, there isn't enough light
for it to compete with the other ecotypes.
We can get some sense for what an organism does, and how it adapts
to the environment, by looking at its genes; this is because genes repre-
sent a blueprint of all the different processes that an organism conducts.
To date, the genomes have been fully sequenced for 12 different
Prochlo-
rococcus
types. If these are compared, a total of about 1270 genes are
shared in all of the different genomes. These represent the core of what
Prochlorococcus
does for a living, and they include the genes controlling
