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Old 01/04/2016, 08:46 AM   #2514
34cygni
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Join Date: Mar 2013
Posts: 59
Quote:
Originally Posted by Unseen players shape benthic competition on coral reefs
Benthic algae on coral reefs are conventionally grouped into functional groups, including the crustose coralline algae (CCA), macroalgae, and turf algae. Each of these groups contains many different species, each with their own species-specific subtleties. ... CCA are commonly associated with healthy reefs and are generally thought to have positive interactions with corals. ...

Macroalgae are the most commonly studied type of algae regarding coral–algae competition. A variety of different macroalgae species (mostly fleshy algae) have been shown to inhibit coral growth and cause bleaching, hypoxia, and lower photosynthetic efficiency and chlorophyll-a content of symbiotic zooxanthellae along the edge of the coral colony. ...

Turf algae (i.e., 'turfs') are heterogeneous assemblages of short filamentous algae, juvenile macroalgae, and cyanobacteria. Turfs are also home to diverse and essentially uncharacterized eukaryotic and prokaryotic microbial communities, as well as to viruses. The heterogeneity of turf algal assemblages means that they have different effects on corals, although the majority of interactions studied have been negative. Turf algae inhibit coral growth and negatively influence adjacent coral tissue integrity, physiology, and fecundity. ...

Initial studies of coral–algae competition were focused on the physical mechanisms that corals and algae use to damage each other (reviewed by McCook in 2001). Algae employ tactics such as shading and abrasion, and corals respond with mesentery and nematocyst attack. More recent studies have shown that other biological factors change the relative competitive advantage in specific ways. ...

Benthic algae harbor rich microbiota, including a large number of potential pathogens and coral disease-associated microbes. These pathogens may be transmitted to corals during competitive interactions, but different groups of algae have distinct effects on the coral holobiont. Turf algae, for example, are associated with major shifts in the bacterial communities along the coral border, including more potential pathogens and virulence genes. ... By contrast, coral interactions with CCA have a distinct community of bacteria at the interface, but these are not pathogen-like.

One common physiological signature that separates coral–CCA interactions from coral–turf and coral–macroalgae interactions is hypoxia. Both experimentally-initiated and naturally-occurring interactions between corals and turf or macroalgae are hypoxic, whereas coral tissues in contact with CCA remain superoxic. Low oxygen along the coral–algae interaction zone can be alleviated by removal of the alga or by treatment with antibiotics, showing in all experiments to date that hypoxia is the result of microbial activity. Although hypoxia may be the cause of coral mortality, it is highly probable that other so far unidentified pathogenicity and chemical factors are the actual lethal factors in most of the interaction zones, and that hypoxia is a secondary effect of the microbes eating the decaying tissue. ...

The DDAM model is based on experimental and ecological evidence showing that algae (i) release DOM, which (ii) facilitates microbial growth and respiration on the benthos and the water column, particularly that of opportunistic pathogens, which in turn (iii) causes morbidity and mortality of corals. This effect can be mitigated by antibiotics, implicating microbes as a significant factor in algae-mediated coral death. ...

Physical interactions between corals and algae can inflict damage directly on the competitor, potentially freeing space for the attacker to advance. However, physical mechanisms alone, typically tested through the use of plastic mimics, play a relatively minor role in coral–algae competition when compared to the effects of live organisms. This difference is due to the transfer of chemicals and microbes to the competitor. Direct contact between algae and corals, for example, delivers DOM, potential pathogens, and hydrophobic organic matter (including allelochemicals) to the tissue of the competitor. ...

Given that direct and indirect contact between algae and corals can elicit negative influences, it will be important to determine how organic matter (OM) and microbes move between holobionts. Coral reefs are complex physical structures that have a significant influence on the movement of water. Despite often high flow and wave action on coral reefs, net water transport is slow within and directly above the reef. This water also has structure. There is a misconception that flow and advection homogenize the reef water landscape, when in fact the water over a coral reef is a varying, complex landscape that is shaped by the structure of the benthos and the flow of the water interacting with it. From the microbial perspective, every drop of seawater is a heterogeneous mix of gels, strings of organic matter, microscopic particles, and discrete hotspots of microbial and viral activity. It is within these water masses that most coral–algae interaction dynamics occur, but this layer of connectivity on coral reefs is only beginning to be described and visualized.
It's worth noting that carbon dosing has been examined in light of the DDAM model...


Quote:
Originally Posted by Advanced Aquarist Feature Article: Total Organic Carbon (TOC) and the Reef Aquarium: an Initial Survey, Part I
Any discussion on the relationship between DOC levels and coral health would be remiss without a digression into the currently popular practice of dosing reef tanks with carbon sources, specifically vodka (= ethanol), sugar, and/or vinegar (see http://glassbox-design.com/2008/achi...perimentation/ for a timely discussion). The logic behind this husbandry technique stems from the speculation that the increase in DOC provided by these chemicals will promote bacterial growth, and this increase in bacterial growth will in turn boost the removal of nitrogen and phosphorus-containing nutrients from the water column. The increased bacterial mass can then be removed by efficient skimming, leading to a net export of undesirable nutrients (N, P) from the aquarium. A standard recipe has been developed by Eric of Glassbox-Design: 200 mL of 80-proof vodka, 50 mL of vinegar, and 1.5 tablespoons sugar, mixed together. The dosing recommendation with this mixture involves starting with 0.1 mL/20-gal per day, and gradually increasing to a maintenance dose of 0.5 mL/20-gal per day. How do these carbon input values compare to the carbon (via carbohydrate) input values of Rohwer? In fact, the Eric/Glassbox-Design protocol is equivalent to raising the aquarium water by about 1.1 ppm of C at the maintenance dose. The Rohwer carbon dosing values that led to coral mortality over a 30-day exposure were in the range 2 - 10 ppm of C. So, it appears that the Eric/Glassbox-Design recipe does not leave much margin for error in dosing levels; overdosing by 2-3X might lead to coral mortality.
Recall that one of the ways that organisms can influence the community composition of bacteria living on them is by controlling the composition of organic carbon that they release. Algae release excess photosynthate when nutrient limited rather than shut down their photosynthesis machinery; but I did not know algae have evolved to release labile DOC constantly, much as I did not know wild corals release mucus more or less constantly, in order to influence the population of their microbiomes. It's a safe bet algae recruits potentially pathogenic (to us and animals we care about) proteobacteria and helps them outcompete potentially pathogenic (to the algae) Bacteroidetes such as cytophaga, which as noted earlier is discouraged from settling on macro by DMSP (...and this may explain why ostis don't reach high population densities when growing epiphytically: their main food bacteria can't grow, so unless they can infect the algae with cytophaga and melt it down, they have to make do with eating vibrio or some other proteobacteria). This is a good example of how host organisms can work the problem from both ends, using the organic carbon they release to both attract and repel bacteria to keep themselves healthy.

The DOC primary producers put into the water has downstream effects on reef microbial communities just as coral mucus does, and causing coralline to die back by shifting its epiphytic community away from coral-friendly bacteria may be one of those effects. The impact of algal DOC on aquarium corals has also been seen -- there was an interesting thread about low-level DDAM effects from algae scrubbers a few years back on Santa Monica's web site (...if the link doesn't work, here's the URL: algae scrubber dot net /forums/archive/index.php/t-1327.html). Primary producers are powering up algae-friendly bacteria with labile DOC just as corals use their mucus to tip the balance of the microbial population in their favor. In other words, the battle for dominance between algae and corals isn't only taking place when individual organisms are bumping up against one another and competing for light and nutrients in the same physical space; they're both trying to remake the entire reef ecosystem from the bottom up.


Quote:
Originally Posted by Influence of coral and algal exudates on microbially mediated reef metabolism
Coral reefs, although generally located in oligotrophic environments, are one of the most biodiverse ecosystems on the planet, due largely to their high productivity and efficient nutrient recycling mechanisms. ... Organic material supplied to the ecosystem by benthic primary producers as exudates is thought to play a pivotal role in community-wide transitions on coral reefs. Exudates may serve different ecological functions depending on their origin. Coral exudates may keep valuable resources in oligotrophic reef systems by trapping particles from the water column, which are remineralized by the benthic microbial communities. In contrast, algae derived exudates have been shown to stimulate rapid growth of planktonic microbies and community shifts towards copiotrophic and potentially pathogenic microbial communities in the water column. ...

Previous studies of tropical reef-associated primary producers have shown that all primary producers release a significant portion of their photosynthetically fixed carbon immediately into their environment. It has further been established that fleshy macroalgae and especially small ( < 2 cm) filamentous algal turfs generally have noticeably higher DOC release rates than calcifying primary producers including hermatypic corals. ...

However, counter to expectations, Nelson et al. (2011) demonstrated that in a backreef system dominated by algae rather than corals, DOC concentrations were significantly lower than in the surrounding offshore waters. Other studies incorporating multiple islands in the central Pacific have shown similar patterns where fleshy algal abundance is inversely related to DOC concentrations in the water column. This surprising inverse correlation may be explained by a significantly more heterotrophic microbial metabolism following initially higher availability of algae derived bio-available DOC. A system wide decrease in DOC concentrations could then be the result of (a) increases in the abundance of heterotrophic microbes and, (b) a co-metabolism, which occurs when microbes are given an initial surplus of labile carbon, enabling this bacterial community to utilize refractory carbon sources.

Recent research has shown that macroalgae derived exudates, enriched in the dissolved combined neutral sugar components Fucose and Galactose, facilitate significantly higher rates of bacterioplankton growth and concomitant DOC utilization than coral exudates or untreated seawater. Further, microbial communities growing in different exudates selectively remove different dissolved combined neutral sugar (DCNS) components, whereby the bacterial communities growing on algal exudates have significantly higher utilization rates of the sugar components which were enriched in the respective algal exudates. Analysis of microbial community composition identifies clear differentiation between the communities selected for by algae exudates and those growing on coral exudates or seawater controls. Macroalgae fostered rapid growth of less diverse communities and selected for copiotrophic bacterial populations with more opportunistic pathogens -- so-called "super-heterotrophic" communities. In contrast coral exudates engendered a smaller shift in bacterioplankton community structure and maintained relatively high diversity.

The microbial landscape on tropical reefs, however, is not only restricted to the water column directly adjacent to the reef benthos ( ~ 10^5 - 10^6 / cm^3 ). In addition to microbes associated with benthic macro-organisms ( > 10^7 / cm^2 of surface area), those associated with calcareous reef sands ( ~ 10^9 / cm^3 ) and the vast porous reef structures in the reef matrix may also play a significant role in biogeochemical cycling. Surface associated microbes may carry out multiple ecological functions, such as nitrogen fixation or inhibition of potential pathogens for their host organisms. The benthic microbial communities, living in the reef structure or reef sands, on the other hand have been recognized as important components for the reef community, as they are capable of rapidly reallocating nutrients in the otherwise oligotrophic tropical reef environments. They also may constitute an essential food source for protists and invertebrates, forming the base of benthic food webs. Next to remineralization and redistribution of nutrients, recent studies have emphasized the role of the benthic microbial communities as important primary producers in these ecosystems. ...

In the present study, over a full diurnal cycle, benthic primary producers released about 10% of their daily fixed carbon as DOC in the surrounding waters.

Responses of the associated microbial communities to these exudates varied widely and were dependent on the source of the exudates as well as the habitat that the microbes originated from. ... Further, our results suggest that, with shifts from coral to algae dominated systems, dissolved organic carbon concentrations in the water column will decrease as a result of an elevated heterotrophic microbial community metabolism, congruent with demonstrated DOC depletion in shallow reefs.

Results from the beaker incubations containing either benthic or planktonic microbes and seawater only showed that while the planktonic microbial community was consistently net heterotrophic the benthic microbial community metabolism was net autotrophic due to daytime photosynthesis, producing significantly higher amounts of oxygen during the daylight hours than it consumed over a 24 h period. Scaled volumetrically to the scale of a 3 m deep reef ecosystem, the effects of the respective net autotrophic benthic and net heterotrophic planktonic microbial communities had comparable magnitudes, resulting in a combined neutral net microbial community metabolism with no significant change of DOC and DO values over a whole diurnal cycle.

The introduction of exudates, however, had noticeable and significantly diverging influences on this balanced community metabolism. Coral exudates increased the net planktonic microbial community production, changing the net oxygen production towards an average positive balance during daylight hours. Coral exudates also enhanced the inherently autotrophic character of the microphytobenthos, such that at the reef scale coral exudates overall stimulated net ecosystem productivity...by an increase in bioavailable inorganic nutrients, supplied by heterotrophic remineralization of coral exudates in the biocatalytic reef sands. In contrast, addition of algal exudates, most noticeably exudates derived from turf algae, stimulated heterotrophic oxygen and organic carbon consumption rates by the planktonic and benthic microbial community, mediating an overall shift toward a significantly more heterotrophic microbial community metabolism. ... Our previous study conducted in this reef system demonstrated that exudates from fleshy macroalgae were enriched in specific carbohydrate components and were more labile than exudates derived from corals, fostering rapid but inefficient growth of primarily copiotrophic bacterioplankton in the surrounding water column. By facilitating the remineralization of semi-labile DOC inputs from the open ocean the high carbon demand of inefficient copiotrophic "super-heterotrophs" may be a mechanism fueling the excessive carbon consumption rates estimated here and the subsequent depletion of DOC on reefs dominated by fleshy algae such as the backreef of Mo'orea.

In contrast, the shift towards a net autotrophic metabolism of the collective microbial community stimulated by coral exudates likely compensates for the initially lower photosynthetic oxygen production rates of corals compared to algae. In our estimates this resulted in comparable net oxygen fluxes of the combined community metabolism in coral compared to algae dominated locations. Coral exudates facilitated changes in the microbial community metabolism towards higher primary production rates and led to an overall increase in DOC concentrations (resulting from net coral and microbial DOC release). Together these results suggest that reefs dominated by corals, by stimulating microbial primary production, may maintain comparable net ecosystem productivity to those dominated by fleshy algae, but additionally may maintain elevated levels of potentially labile DOC available for remineralization and recycling by microbial communities.
Benthic microbial primary production is limited to the uppermost couple of mm in reef sands because light doesn't penetrate very far, but on the other hand, what applies to heterotrophic bacteria applies to single-celled primary producers, as well: there's a very large surface area available in that thin layer of sand. Added up, benthic primary production turns out to be quite substantial.


Quote:
Originally Posted by Microbial photosynthesis in coral reef sediments (Heron Reef, Australia)
We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 um in diameter, surrounded by a hard transparent wall) [...note that this sounds a bit like what Quiet_Ivy described as "harder brown circular spots on the glass"] were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. ... An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals.
That's a lot of production. From our perspective as hobbyists, that's the engine powering a DSB -- and corals are trying to rev it up with the mucus they shed. Bumping up primary production levels in the sediments would help corals outcompete algae on a broad, reef-wide level because as we hobbyists learned from Dr. Shimek, a healthy and diverse benthic food web releases coral chow into the water column.

While I was looking for papers addressing how organic carbon influences microbial populations on coral reefs, papers on sponges kept coming up. Sponges are weird and ancient creatures, so I couldn't resist checking them out. When I read some recent papers, I discovered that a completely new aspect of reef biology has come to light in the last couple of years, finally bringing the role of sponges into focus. The original paper is behind a paywall, but fortunately for us, Google Scholar found a follow-up paper from another scientist chiming in with an important refinement of the theory...


Quote:
Originally Posted by Sponge waste that fuels marine oligotrophic food webs: a re-assessment of its origin and nature
Sponges are prominent members of coral reefs, where they mediate the transfer of energy and matter through the fluxes of organic carbon and dissolved inorganic nutrients. A new perspective on their trophic role comes from the recent finding by de Goeij et al. (2013) that reef sponges take up most of the dissolved organic matter (DOM) available in the water column before it is transferred away from a reef. The fate of that DOM carbon used by sponges has been a mystery, as respiration requires only about 40% of the total carbon taken up, and the remainder is not converted into detectable growth. de Goeij et al. proposed that DOM energy may be invested in renewing the entire cell layer of choanocytes (monociliated filtration cells) every few hours. The choanocyte renewal would produce a significant outflow of particulate organic matter (POM) rich in carbon and nitrogen that would be rapidly assimilated by a variety of invertebrates, thereby fueling the reef food chain. By this mechanism, sponges are proposed to play a crucial trophic role, fueling food chains of not only coral reefs but also many other oligotrophic marine communities, including caves, varied deep-sea habitats, etc. ...

The TEM [Transmission Electron Microscope] approach reveals that the outgoing POM through which sponges fuel oligotrophic food webs results from more complex cellular processes than mere choanocyte renewal. The squeezing of entire cells with inclusions (spherulous, granular and archaeocyte-like cells) into the excurrent canals and the extrusion of membrane-bound inclusions mediated by the endopinacocytes appears to contribute notably to the outgoing POM. ...

The detected migration of mesohyl cells into the canals appears to be related to the elimination of digestive leftovers (egestion and defecation) and metabolic by-products (excretion), two basic physiological functions only rarely investigated in sponges. As sponges lack organ systems to collect and evacuate products from intra-cellular digestion and metabolism in the deep mesohyl, these waste products are stored in cells that subsequently enter into the outgoing flow, contributing to the POM that exits the sponge. Archaeocyte-like cells, known to have intense phagocytic activity, appear to be engaged in digestion and elimination of refractory leftovers, while spherulous and granular cells appear to be involved in excretion of metabolic by-products. Although many aspects of the physiology of sponges still remain poorly understood, it is clear that many physiological processes of the sponges are based on the ability of these organisms to maintain substantial cell and metabolite traffic through their simple epithelia. Extrusion of spherulous cells through the epithelia of the aquiferous canals of A. cavernicola has previously been documented by Vacelet (1967), who first suggested that it could be a way to eliminate excretory products. Likewise, spherulous cells heavily charged with inclusions have been reported to leave the body of the non-feeding larva of the sister sponge species Aplysina aerophoba. The larva is a lecithotrophic life-cycle stage unable to incorporate particulate food but able to generate metabolic excreta. Therefore, spherulous cells are concluded to be involved in elimination of metabolic by-products that are not related to the digestive process. ...

In the absence of detailed studies on vesicle content, it is assumed that the energetic content of these mesohyl cells -- charged with excretion by-products and digestive leftovers -- is lower than that of the choanocytes. It is worth noting that many of the discarded choanocytes and some archaeocyte-like cells were charged with phagosomes containing undigested food. Consequently, these cells are expected to contribute greatly to the POM transfer of energy to the following steps in the trophic chain. As water pumping and food ingestion are energetically costly processes, it is intriguing that choanocytes that are about to be discarded keep engulfing and start digesting pieces of particulate food that will never contribute to the sponge energy balance because these cells will readily be discarded as POM.
So coral reefs run on sponge poop... I've long had a vague suspicion that a heterotrophic element was missing from a system consisting of a mixotrophic reef tank and an autotrophic sump, but I did not see that coming.

CONTINUED...


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