Most people who are familiar with corals–whether admiring them in the wild or displaying
them in a glass box–understand quite well that corals are incredibly complex animals. This
certainly so with the zooxanthellate corals.
Virtually all corals (zooxanthellate corals included) are predators of plankton and other tiny
organisms. But the zooxanthellate types undoubtedly are the most advanced and versatile. This
large and fascinating group, which includes the so-called reef-building corals, harbor symbiotic
algae (i.e. zooxanthellae) that provide the animal host with an additional source of nutriment.
The most common and first discovered of these algal symbiotes are dinoflagellates of the genus
Symbiodinium. Others (e.g. Amphidinium) have been discovered since. It seems there may be
others. Due to the multiplicity, and apparent importance, of these tightly interconnected
organisms, biologists now refer to such assemblages as “coral holobionts.”
As we continue to study the intricate associations that characterize the coral holobiont, we are
coming to realize the extent to which it depends upon bacteria. This should not be so
surprising. Bacteria are the predominate plankton on a tropical coral reef. They accordingly
make up a considerable amount, if not the bulk, of the typical coral planktivore’s diet. Since
most corals are planktivores–that’s pretty significant!
For this reason alone, one could think that a greater emphasis on the microbial component of
the captive coral diet is long overdue. And they arguably would be correct. But there is far more
to it than that. As current research is just starting to elucidate, the dietary contribution of
bacterial symbionts may far outweigh that of other, straight-up-food bacteria.
The feeder within
Mom was on to something when she insisted on you eating your beans. Beans, peas and other
legumes are (at least for plants) rich in protein. This is only possible because of the rich source
of “fixed” nitrogen that is available to them, which is only available in large quantities thanks to
symbiotic rhizobial bacteria that reside in special structures called root nodules.
Most plants (from seaweeds to cacti) rely on a fixed source of nitrogen such as ammonia or
nitrate in order to synthesize amino acids, the building blocks of protein. Though nitrogen gas is
quite abundant, the vast majority of plants simply cannot utilize it; however, nitrogen-fixing
bacteria (alternatively referred to as diazotrophs) can turn it into bioavailable substances quite
literally out of thin air. Those few plants that form associations with these microbes, and draw
from their reserves, are able to survive and grow even in the most nutrient-poor soils.
It is now known that diazotrophic bacteria play a similar role within the coral holobiont. Not
only the zooxanthellae, but also the coral, can directly use nitrogenous compounds exuded by
their bacterial partner. In fact, both the corals and their associated dinoflagellates produce
enzymes that are used to harvest such loose nutrients. Fixed nitrogen is in high demand in coral
reefs which are necessarily nutrient-poor environments. Indeed, corals (and particularly the
stony, reef-building types) cannot survive where dissolved nutrients are abundant in the
surrounding waters due to the resulting competition with algae. Having an internal, symbiotic
source of fixed nitrogen confers a massive nutritional advantage for the holobiont. The
bacterial partner is rewarded for its work with a source of fixed carbon in the form of exudates
released from the coral and the zooxanthellae.
Purple non-sulfur bacteria (PNSB) are important coral holobiont partners. This is remarkable in that
PNSB are closely related to those rhizobia associated with legumes. Though the actual bacterial
assemblage of the holobiont is often complex and may vary between regions or even seasons, it
has been conclusively shown that PNSB such as Rhodopseudomonas palustris are prevalent and
important coral symbionts. Preferring anaerobic environments but also capable of performing
photosynthesis, the low-oxygen but brightly illuminated areas just under the coral’s mucus (and
especially within its gut) are prime microhabitat for PNSB.
Let it snow
Many marine filter feeders (hard and soft corals, cerianthid anemones, feather duster worms,
etc.) exploit suspended organic particulates as a food source. The particle substrate might be
composed of copepod exoskeleton fragments (chitin), dead phytoplankton (mostly cellulose) or
other poorly digestible flotsam. This material becomes nutritious only as it is colonized by bacteria. As the particles become coated in the bacterial secretions, they stick together and
sink out of the water column. These sinking clumps of enriched organic matter are known as
Just a few coral foods simulate a marine snowfall. Even fewer are live. PNS YelloSno™ is a truly
unique live product containing a chitin-based substrate that is enriched with live purple
nonsulfur bacteria. Loaded with protein, B vitamins and carotenoids, PNS YelloSno™ can
significantly improve growth and coloration in a broad range of coral species. What’s more, it
serves as an inoculant, seeding lasting and potentially permanent PNSB populations within the
aquarium substrate as well as within the coral holobiont.
Best of all… When PNSB are floating around in the water column, if ammonia and/or nitrate is
present in appreciable amounts, the bacteria will actually remove theses wastes. So, like live
phytoplankton, this food actually cleans the water as you use it! When you consider its overall
nutritional, probiotic and symbiotic qualities, it’s a real wonder that serious reef aquarists and
coral farmers are only now becoming aware of purple non-sulfur bacteria!