The organic matter released from corals is well known to be utilized by bacteria and stimulate the bacterial growth. However, degradability of the organic matter has not been investigated in a long-term perspective and therefore it is still not understood what % of the released organic matter can actually be mineralized by bacteria. To investigate the long-term degradability, the reef-building corals Acropora pulchra and Porites cylindrica were first incubated in a normal submerged condition or a stressful condition (air exposure), and released organic matter to the ambient seawater. After taking out of the corals, the incubation seawater containing organic matter was put under dark over 1 year to follow bacterial decomposition of dissolved and particulate organic matter (DOM and POM). The results showed that the concentrations of DOM and POM rapidly decreased within the first 1 week and the remaining organic matter was gradually decomposed much more slowly, suggesting that the coral-derived organic matter had two different fractions in bacterial degradability. The labile organic matter (L-OM) had turnover time of 3.2-9.1 d (average 6.2 d) for DOM and 5.6-9.1 d (average 7.4 d) for POM. The L-OM could be easily mineralized within or around the reef ecosystem. The stressful condition increased the ratio of degradable to bulk organic matter. On the other hand, a part of the coral-derived organic matter was not mineralized even after 1 year, suggesting that it was very recalcitrant to bacterial decomposition. The corals might routinely release such refractory organic matter to the ambient seawater for defense against pathogens.

It is known that corals exude slime and sugars and that this feeds bacteria. They do so naturally, even when not under stress. In fact, up to 50% of their metabolic energy is 'wasted' this way. Note that these scientists understood the necessity of excluding light to prevent algal growth in their vials. They allowed decomposition to proceed for a whole year, whereas decomposition of the labile component completed within two weeks. They discovered two fractions, the non-labile recalcitrant organic matter corresponds to our slush. The recalcitrant fraction did not mineralise, even after waiting for a whole year. What these scientists failed to discover, is symbiotic decomposition where bacteria continue to decompose slush once fed a small amount of energy. They also failed to discover that the recalcitrant fraction is found in high concentrations in all oceans.

 Bulk of dissolved organic matter in deep outer ocean is highly resistant against microbial decomposing attacks and known to have a very long lifetime (> 1,000 yr). This is also a quantitatively important reservoir in the global carbon cycle. There are several hypotheses that explain the mechanisms of preservation of organic carbon in seawater as well as in marine sediment; however, the molecular structure, the origins and the mechanism of formation of RDOC still remain unknown. We are now studying the interactions between various natural organic matter and marine microorganisms as a possible agent for the production of RDOC.

In other words, our discovered slush has a very long lifetime and exists in huge quantities. We agree. Their name for it is RDOC, Recalcitrant/Refractory Dissolved Organic Carbon. We discovered that slush originates from ordinary bacterial decomposition that ends prematurely due to an energy deficit. The same bacteria that stopped decomposition prematurely can be encouraged to continue provided a small amount of energy food is given, to bridge their energy deficit.

....Both production and decomposition processes on coral reefs are exquisitely tied to their structural organization at all levels (in the physiological, physiographic and community sense to underline the holistic principles of community stability). The energetic pools within a reef seem so large, but considering that they are spread over huge areas across the circum-tropical belt in an oligotrophic environment attributes a relativistic momentum to this abundance. Close to the trophic base of this abundance are hermatypic corals living in a symbiotic relationship with endosymbiotic dinoflagellates and exosymbiotic microbial associations.  ....

....Symbiotic relationships are primarily responsible for the success of benthic reef communities in the tropics. Reef corals in particular not only rely on the important relationship between them and their autotrophic endosymbionts (dinoflagellates of the genus Symbiodinium sp., commonly, but incorrectly referred to as zooxanthellae), corals also acquire a substantial amount of their energetic and nutrient requirements by heterotropy (the in/direct ingestion of zooplankton and other organic particles from the water column and the Muco-Polysaccharide Layer (MPSL). The endosymbionts reside within vacuoles in the cells of the host gastrodermis where they serve as primary producers and supply their coral host with up to 95% of their photoassimilates, such as sugars, amino acids, carbohydrates and small peptides making corals autotrophic with respect to carbon. The bacterial exosymbionts cultivated by the coral’s own MPSL are likewise used for nutritional requirements. More important though, is their important function in shielding the coral’s soft tissue against opportunistic microbial settlers. A third source of resource allocation is the coral’s endolithic community that may satisfy 55-65% of the coral’s nitrogen requirements. Together these energetic pathways enable the coral to perform its metabolic needs for growth, reproduction, and the deposition of its CaCO3 skeleton.  .....

See how close these scientists came to discovering the real function of exobacteria, and that they guess at their possible role to ward off invading bacteria?

ABSTRACT: Mucus release by hard corals of the genus Acropora under submersed and naturally occurring air exposure was quantified at Heron Island/Great Barrier Reef. These measurements were conducted with beaker and in situ container incubation techniques. Mucus release rates for A. millepora, normalized to the coral surface area, were 10 ± 5 mg C and 1.3 ± 0.8 mg N m-2 h-1 for submersed corals, and 117 ± 79 mg C and 13 ± 8 mg N m-2 h-1 after exposure to air at low tide. This corresponds to increases by factors of 12 for C and 10 for N. The main monosaccharide components of freshly released Acropora mucus were arabinose and glucose, accounting for 14 to 63% and 13 to 41% of the carbohydrates. A protein content of 13 to 26 mg l-1 caused a low C:N ratio of 8 to 14. The chlorophyll content of 7 to 8 µg l-1 in the mucus compared to 0.6 ± 0.004 µg l-1 in the surrounding seawater revealed mucus contamination with zooxanthellae. A low pH value of 7.7 compared to 8.3 in the surrounding seawater indicates the existence of acidic components in fresh coral mucus. Concentrations of most measured inorganic nutrients were highly increased in coral mucus, reaching values of 3 to 4 µM for silicate, 19 to 22 µM for phosphate and 20 to 50 µM for ammonium concentration. Phosphate concentrations were 130-fold higher in coral mucus compared to the surrounding seawater, underlining the role of coral mucus as a carrier of nutrients. Addition of coral mucus to stirred benthic chambers resulted in a shift of phosphate, ammonium and nitrate/nitrite fluxes towards the sediments, confirming the transport of nutrients via coral mucus into permeable reef sands.

All findings point to decomposition inside coral mucus: a low pH for hydrogen ions and CO2, inorganic nutrients in high concentrations, but where did these come from? They guess that coral mucus must be a carrier of nutrients but fail to discover that it is a bacterial substrate with plenty of high-calory sugars to help bacteria decompose the invisible but abundant substance we call slush (Dissolved Organic Matter that could not be decomposed further). See also the publications below. The study fully supports our discoveries.

Wild, C., et al. 2004. Coral mucus functions as an energy carrier and particle trap in the reef ecosystem. Nature 428(March 4):66-70.
http://www.nature.com/nature/journal/v428/n6978/abs/nature02344.html;jsessionid=F1F9492CFC75952CFC2F90DE4B5044BB Not freely available

Knowlton, N., and F. Rohwer. 2003. Multispecies microbial mutualisms on coral reefs: The host as a habitat. American Naturalist 162(October):S51-S62. Abstract available at link. http://dx.doi.org/10.1086/378684

Abstract
Decomposition of 13C-labeled dissolved organic carbon (DOC) produced in two marine diatom blooms was followed for 2.5 yr with large volume (20 liter) incubations performed in the dark. The 13C tracer was used to partition decomposition dynamics of the fresh diatom-derived DOC and the turnover of background DOC from Woods Hole Harbor. DOC from Woods Hole Harbor proved largely refractory, with DOC concentrations falling from 122 to ~ 100 µM C in 2.5 yr. DOC from the diatom blooms was more labile, but was also incompletely mineralized, with 25-35% remaining after 2.5 yr. Neither nutrients nor labile carbon (dextrose) added at 1.5 yr significantly stimulated DOC mineralization. The experiments indicate that DOC produced in short-term blooms can be surprisingly resistant to microbial attack.

Dissolved organic C (DOC) is the most abundant form of organic C in the marine environment and has apparently accumulated because compounds comprising DOC have chemical structures that resist microbial attack. . . . . .

This study confirms our discovery of slush, although it dates back to 1996. By using radioactive carbon, these researchers were able to establish that about 2/3 decomposes fully while about 1/3 remains as slush. Their samples did not react to the addition of dextrose (a sugar) because it was done far too late.

The accumulation and fate of dissolved organic carbon (DOC) were followed during an experimental diatom bloom in freshwater mesocosms with and without macrozooplankton.
The biodegradability of DOC was assayed in independent degradation experiments with bacteria from the mesocosms. Algal biomass was allowed to accumulate 14 days before macrozooplankton was added to one of the mesocosm (+).
Chlorophyll a peaked at 103 and 148 µg chl/l after 17 days in + and - grazing, respectively. The concentration of DOC started to increase after 5 days and when the blooms collapsed after 22 days 235 and 284 µM had accumulated in + and - grazing, respectively. Prior to the peak in algal biomass about 100 µM DOC had accumulated and the degradation experiments showed this DOC to be completely biodegradable. During the decline of the bloom the rate of DOC production increased and the degradability changed. About 25 to 30% of the accumulated DOC was now recalcitrant (RDOC). The degradation experiments also showed the DOC degradation kinetics to differ between + and - zooplankton. The presence of macrozooplankton apparently lowered the initial rate of degradation. However, after 230 days of decomposition all experiments with post bloom water reached the same DOC concentration. Grazing by macrozooplankton did not change the concentration of RDOC. Autochthonous DOC produced during the decline of a diatom bloom apparently included a significant fraction of recalcitrant DOC.

The experiment shows that introduced zooplankton digested about 50 µM out of 284 µM food and once all life was decomposed, the resulting RDOC was the same for the grazed and non-grazed samples. About 25-30% remained in RDOC (slush).What this implies is that slush accumulates at a fixed rate of the total biomass (25-30%).
Autochthon = the original inhabitants, the ones living here