Updated: Mar 7, 2022
Not too long after we released our flagship product PNS ProBio™ (a live culture of purple non-sulfur bacteria) in 2018, we introduced its companion product PNS YelloSno™ (a food for corals and other filter-feeding invertebrates). While more than a few coral foods had just entered the market around that time, PNS YelloSno is markedly different. Thus, we’ve gotten a lot of inquiries about it over the last couple of years. And still get a lot of questions! Thus, we here present a short article describing the content and suggested use of this unusual product.
For sure, the best way to begin is to explain what marine snow is, how it is formed and where it fits into reef food webs.
Marine snow is a heterogeneous organic aggregate of mostly biogenic origin. Aggregates form from the colloidal fraction. Usually comprised of particles no larger than 1 nm to a few micrometers, most of the organic matter in marine colloidal fraction is not readily available to grazers. Indeed, while this fraction has a substantially higher total mass than either bacterioplankton or phytoplankton, its miniscule particle sizes make it mostly unavailable potential consumers. Therefore, to become more bioavailable, the colloidal fraction must aggregate.
The remains of dead phytoplankton (e.g. cellulose-based cell walls) and zooplankton (e.g. chitin-based exoskeleton fragments), fecal matter or even inorganic dust can make up the bulk of marine snow. Mucus and other sticky exudates (i.e. extrapolymeric substances(EPS)) secreted by bacteria, algae, corals, etc. help to promote aggregation. Particles and small organisms floating through the water column can become trapped within aggregates. As marine snow aggregates are porous, some particles are able to pass through them. Aggregates grow over time (often >0.5 mm in diameter with porosities>95%).
Trapped in the EPS, particulate organic matter is much more available to grazers than it is when drifting freely. Marine snowflakes begin to accumulate bacteria as particles aggregate to several micrometers in diameter (large enough to feed and reproduce on). It is also at this size that the flakes are large enough to flocculate and begin sinking.
Marine snow characteristically originates in shallow, sunlit waters where algal and bacterial productivity is high (corals reefs, plankton-rich oceanic surface waters, etc.). A marine snowfall begins as primarily organic detritus falling down from the upper layers of the water column; it might travel for weeks, supporting a growing and changing community of microbes, before reaching the ocean floor.
In marine environments, downward transport of organic carbon is largely governed by the sinking and settlement of marine snow. Sedimentation out of the surface layer (at approximately 100 meters depth) is referred to as export flux whereas sedimentation out of the mesopelagic zone (at approximately 1,000 meters depth) is referred to as sequestration flux. This process of exporting energy from the sunlit photic zone to the aphotic zone below is referred to as the biological pump.
Most of the organic stuff in marine snow is consumed by bacteria, protists, zooplankton and filter-feeding animals within the first 1,000 meters of the descent. Export production (typically measured in units of carbon (e.g. mg C/m2/day)) is the amount of organic matter produced in the ocean by primary production that is not recycled before it falls into the aphotic zone.
Being rich in labile organic matter, marine snow is subject to unusually high levels of microbial activity and metabolic turnover. Bacteria are the most abundant organisms on marine snow (followed by cyanobacteria and nanoflagellates). And they are quite abundant; flocs can host bacterial densities of around one thousand times more than the surrounding seawater.
With respect to marine snow formation, coral reefs are interesting because they are at the terminus of a two-way highway. That is, they both produce and receive marine snow drifts. Incoming drifts originate from oceanic environments via upwelling and tidal action; outgoing drifts originate right over the reef itself. In this latter case, much of the EPS are supplied in the form of coral mucus.
Coral mucus traps organic matter from the water column. And there is plenty of mucus to go around; on some shallow reefs, stands of Acropora can exude nearly 5 liters of mucus/meter2 reef area/day. Gobs of released mucus traps suspended POM, increasing its organic carbon and nitrogen content by three orders of magnitude within two hours! Final-phase aggregates may harbor bacterial densities that exceed those of ambient water by four orders of magnitude. This can increase overall nitrogen content of the particle by over 70x.
As it drifts through the water column, it is enriched by microbes which enhance aggregation through the production of sticky biofilm. Some of these aggregates washed back out to sea. Someare swept into nearby lagoons where they are remineralized by benthic microbes. At least as importantly, a considerable mass of these enriched aggregates is consumed by the very same corals. As such, the formation and consumption of marine snow on coral reefs serves to recycle food energy and nutrients.
The perfect simulation
Sure, there is plenty of coral mucus produced in a well-stocked reef tank. There generally is no shortage of organic debris, either. So why do we not observe the formation of marine snow in aquaria?
There are a few reasons for that. For starters, most suspended POM is rapidly removed via mechanical filters. Most EPS and beneficial bacterioplankton are rapidly removed by protein skimming. Even when flocculation processes do occur, aggregates are quickly shredded or blasted apart by water pumps. Perhaps even more significantly, the residence time in the water column is not long enough for the right microbes (if they are even present) to fully colonize and enrich the particles.
PNS YelloSno was developed to fill this gap. Photofermented with live purple non-sulfur bacteria (Rhodopseudomonas palustris), it is the first and only simulated marine snow coral diet that is made under naturalistic conditions. That is important, because corals don’t just eat anything that falls on their lips. They indeed show a high level selectivity for what they ingest. They certainly won’t eat common detritus (e.g. fish crap). They might even refuse marine snow that is not enriched with the “right” types of bacteria. For example, they exhibit a strong preference for probiotic rhizobial bacteria such as R. palustris.
Purple non-sulfur bacteria (PNSB) occur widely in nature, including marine habitats. In the ocean, these photosynthetic heterotrophs reside from the surface down to around 150 meters and are most abundant at around 35 meters--right at and just above the export flux zone. PNSB are known to colonize marine snow. And they’re found in the coral gut. Coincidence? We think not.
Like other PNSB, R. palustris is by itself extremely nutritious and has been proposed as a mass produced diet for aquaculture. For example, it boasts a crude protein content of around 72–74%. It is rich in stearic acid and oleic acid. And, it is highly digestible.
Problem is, at their very tiny size of 1.2-2.0 µm, not every filter-feeder can enjoy these microbes. However, when they’re grown on larger particles such as marine snowflakes, creatures such as Christmas tree worms, barnacles, crinoid stars, etc. can capture and consume them with relative ease. That’s exactly what PNS YelloSno is for! The core of the aggregate in this product is a natural (i.e. crustacean and fungal) source of chitin. The medium is rich in minerals, trace elements, polysaccharides and vitamins. But perhaps most importantly, it is rich in B vitamins. It is particularly rich in B12 (cobalamin), which is an essential vitamin across the food chain from certain microbes (including some nitrifying bacteria such as Nitrobacter) to zooxanthellae to corals.
Use, dosage and storage
For general use, we recommend small doses of PNS YelloSno (a little bit goes a very long way!) at 1 mL/gallon system water weekly. However, we have used this dose (and higher) in our test systems daily with favorable results. As with all foods, recommended dosage is somewhat arbitrary since every aquarium is built, stocked and maintained differently! We therefore suggest starting with the minimal dose and then slowly increasing dosage (if desired) until best results are achieved.
For best results, we also recommend the following:
Use just after lights-out (i.e. when filter-feeders are most active).
Remove mechanical filter media.
Cut protein skimmers.
Shake the bottle lightly, but not vigorously, as to introduce both large and small aggregates.
Finally, when using live zooplankton foods, you may try gutloading them with YelloSno. This significantly nutritionally enriches feeds such as adult brine shrimp and amphipods. And yes, it can be added to maintain live food cultures as well. For example, this product has been observed to enhance the growth and quality of Daphnia.
One final note: Unlike PNS ProBio, PNS YelloSno is not a live product. Rather, it is heat inactivated just prior to bottling; this preserves the culture at the optimal stage of fermentation (otherwise, the bacteria would simply consume and completely degrade the medium from inside the bottle!). Because the product is hot-filled into special bottles, it is stable and has a fairly long shelf life (be watchful of expiration date on the back label). Nevertheless, it must be capped and refrigerated when not in use!