[Coral-List] bacteria, nutrients, diffusion, turbulence

Thomas Goreau goreau at bestweb.net
Wed Feb 25 10:05:03 EST 2009

Forest Rohwer raises some very important points below, but it is  
important to consider BOTH molecular and turbulent diffusion in such  
arguments, and his comments are strictly correct only for the former.

Real world nutrient and bacterial interactions are going to involve a  
mixture of molecular diffusion in totally static zones and on  
surfaces, laminar diffusion in surface boundary layers (typically  
millimeters/centimeters), and fully turbulent diffusion on a larger  
scales (millimeters/centimeters on up). So while I agree with many of  
Rohwer's points about the importance of experimenting on a range of  
scales, there are still mechanisms to transport bacteria, toxins, and  
allelochemicals over the distances in Vu et al's experiments.

This is reminiscent of a huge oceanographic controversy in the early  
1970s in which phytoplankton were found to enormously increase their  
nutrient uptake rates at high concentrations (just like reef algae  
do). Some argued that phytoplankton took up the vast bulk of their  
nutrients when they happen to drift through clouds of fresh  
zooplankton pee/poo (hope that word passes the politically correct  
standard keepers!). McCarthy got tenure at Harvard by making  
calculations of the rate of diffusion of nutrients from pee/poo  
clouds, and concluded that he had turned ocean ecology on its, well,  
rear. But he used molecular diffusion coefficients appropriate to  
totally still fluids. In fact the turbulent eddy diffusion  
coefficients of the real world ocean are a million to a hundred  
million times higher! This means that these drifting clouds of high N  
and high P would be diffused away before phytoplankton or bacteria  
could get to them, unless they lived right ON the source, with lips  
closely appressed, passively mobile bottom-feeders, so to speak.

There has been inadequate consideration of the importance of  
turbulence in coral reef ecology, but in fact this is a crucial  
transport mechanism even on very small scales. The reason that it has  
been ignored is because turbulent eddies could not easily be  
visualized. That limitation has now been removed, and we are now able  
to image turbulence in real time on scales down to microscopic. In a  
recent paper several mathematicians at MIT and I have redeveloped my  
grandfather's method of visualizing turbulence in real time, which had  
not been used for 70 years. This allows the full details of turbulent  
transport to be visualized directly in real time in the laboratory. A  
vast wealth of new information on the role of physics in biology at  
small and intermediate scales will follow its serious application. See:


Thomas J. Goreau, PhD
President, Global Coral Reef Alliance
Coordinator, United Nations Commission on Sustainable Development  
Partnership in New Technologies for Small Island Developing States
37 Pleasant Street, Cambridge MA 02139
goreau at bestweb.net

> Date: Tue, 24 Feb 2009 09:03:36 -0800
> From: Forest Rohwer <frohwer at gmail.com>
> Subject: [Coral-List] Algae, DOC, microbes and corals
> To: Coral-List at coral.aoml.noaa.gov
> Message-ID:
> 	<3decc8d00902240903y2beada31k8c4f371f319d47da at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 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.

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