[Coral-List] Algae, DOC, microbes and corals

[Forest Rohwer]

The recent paper by Vu et al completely misses the scale at which
algae-DOC-microbe-coral dynamics work. Basically, the authors put algae in
bags and tacked them to corals. Then they expected the DOC to diffuse
several centimeters to the corals. In the real world, diffusion only works
over very small distances. Other factors like mass transport rapidly destroy
any diffusional signal. In fact, Vu et al show that the DOC enrichment does
not occur at distances >3 cms from the algae.

The dynamics that need to be investigated at occur in the micron-millimeter
range. If you look at Figure 1 in the Vu et al. paper, you can clearly see
the real/important coral-algal interface is the one that occurs against the
coral (i.e., the bright green line in the photo; not the algae in the bag
several cm away). Even the reference nails for the disease boundary are
covered with algae as the coral-algal interface advances. The algae in the
bag about 3 cms away should not have any influence, which is what the
authors found. In fact, the Vu et al. data more closely matches the proposed
feedback between algae-DOC-microbe-coral (Smith et al. 2006. Ecology
Letter), then their own conclusions.

Similarly, the authors should not expect the algal-associated pathogens (as
described in a very nice paper by Nugues et al.) to easily "hop" from the
bagged algae. From a microbial point-of-view, 3 cm represents about 6,000
body lengths. That would be like a human hopping 10 kms.

There are a couple leasons here: First, think small when you are dealing
with microbes. If you look at coral-algal interfaces use micro-probes and
microscopes, then you see that most of the activity occurs within very small
distances. Second, don't title papers with a conclusion that can easily be
wrong. Many (most?) people aren't going to read the details in the
Discussion.