While competition undoubtedly plays a huge role in shaping the biological community in any ecosystem (certainly including aquarium microbiomes), there are certain instances where two or more species actually team up for their mutual benefit. One example of this sort of mutualism is syntrophy.
Syntrophy refers to an oftentimes complex teamwork between different microorganisms where one species produces waste or byproducts as part of its metabolism, and another species uses those byproducts as food or energy. This partnership helps both organisms grow while keeping their shared environment balanced. For example, one microbe might break down fish waste into simpler compounds, and another microbe might use those compounds to support its own growth, reducing harmful substances in the water and keeping it safe for the aquarium's inhabitants (including yet other microbial species the syntrophic group).
Syntrophic interactions between photosynthetic microorganisms such as Rhodovolum sp., Marichromatium purpuratum, and Prosthecochloris sp. likely involve multiple complementary metabolic exchanges and nutrient cycling that enhance the survival and growth of the consortium. Here's a breakdown of the mechanisms that could contribute to their syntrophy and resilience:
1. Cross-Feeding of Metabolites
Rhodovolum sp. (a purple nonsulfur bacterium):
Facultative phototroph that can use organic acids (e.g., acetate) as carbon and energy sources under anaerobic conditions. It also excretes small amounts of organic acids or reduced compounds that other bacteria may use.
Marichromatium purpuratum (a purple sulfur bacterium):
Oxidizes sulfide or elemental sulfur to sulfate while fixing CO2 through the Calvin cycle. It produces sulfur intermediates like polysulfides, which may be consumed by other bacteria.
Prosthecochloris sp. (a green sulfur bacterium):
Strict anaerobe that uses sulfide or thiosulfate as an electron donor for photosynthesis, producing elemental sulfur as a byproduct. This sulfur can be recycled by purple sulfur bacteria.
2. Redox Balancing
In closed systems, redox imbalances can accumulate. These species likely share a role in maintaining redox equilibrium:
Rhodovolum can utilize electron acceptors like fumarate or nitrate if available and generate reduced compounds that other microbes oxidize.
Marichromatium and Prosthecochloris contribute by oxidizing reduced sulfur species, preventing toxic sulfide accumulation.
3. Light and Spatial Partitioning
In mixed phototrophic communities, different bacteria often occupy specific light niches:
Prosthecochloris: Efficiently absorbs far-red and infrared light due to bacteriochlorophyll c, allowing it to thrive in deeper or shaded regions.
Rhodovolum and Marichromatium: Utilize visible and near-infrared light wavelengths.
This light partitioning minimizes competition and ensures cooperative utilization of light energy for photosynthesis.
4. Nutrient/Vitamin Recycling
Tight recycling of key nutrients helps to stabilize the syntrophic group:
Sulfur cycling: The byproducts of sulfur oxidation by Marichromatium and Prosthecochloris may be reused by each other or by Rhodovolum.
Carbon cycling: Organic acids or fixed carbon from Rhodovolum may support the growth of the other two species.
Nitrogen cycling: Rhodovolum may fix nitrogen under specific conditions, contributing bioavailable nitrogen (e.g., ammonia) to the consortium.
Vitamin and Cofactor Exchange:
Some phototrophs, like Prosthecochloris, rely on external sources of vitamins such as B12, which may be synthesized and excreted by Rhodovolum or Marichromatium.
5. Resilience Through Waste Mitigation
Waste products from one microorganism (e.g., sulfide from organic degradation) become substrates for others (e.g., sulfide oxidation by Marichromatium or Prosthecochloris), preventing the accumulation of inhibitory compounds.
This interdependence stabilizes the system and allows persistence in closed cultures.
6. Quorum Sensing and Cooperative Behavior
Communication through quorum sensing molecules may coordinate metabolic activities, ensuring balanced growth and avoiding overproduction of toxic intermediates.
Cooperative biofilm or aggregate formation may protect the community from environmental stresses, such as fluctuations in substrate or light availability.
Summary of Resiliency Mechanism
The syntrophic interactions between Rhodovolum sp., Marichromatium purpuratum, and Prosthecochloris sp. involve a tightly coupled network of nutrient cycling (carbon, sulfur, nitrogen), light niche partitioning, and mutual detoxification. This interdependence makes the consortium more robust and resilient in relatively closed habitats such as deep sand beds, ensuring efficient utilization of resources and prevention of toxic compound buildup.
These microbial communities are particularly important in small systems like reef aquaria, where unmitigated organic wastes, sulfides and other potentially harmful substances can accumulate and eventually challenge corals, clams and other high-value livestock. While Rhodovolum, Marichromatium, Prosthecochloris, Roseospira and other highly beneficial anaerobic reef microbes are often absent from the aquarium microbiome, they can be replenished with the use of bottled concentrations of these syntrophic cultures such as PNS Deep Cycle™.