Most of us want—and should want—our aquarium systems to more or less function ecologically in a way that resembles natural marine environments. Probably the most critical aspect of this is establishing a naturalistic food web. Unicellular, free-living, perpetually suspended algae (i.e. phytoplankton) account for a great deal of primary productivity in areas over or close to coral reefs. These tiny plants constitute a major food source for numerous filter-feeding invertebrates as well as the pelagic larvae of yet more reef species. As such, phytoplankton is an important constituent of coral reefs ecosystems. It is therefore difficult to overstate the importance of the presence of phytoplankton (of the right kind and in the appropriate density) in reef aquarium systems.
Many products have been developed to suit the needs of marine aquarists and mariculturists. These products may include either dead or live phytoplankton. There are many compelling reasons to opt for live products, particularly when attempting to create an overall more balanced and natural captive ecosystem. Therefore, in addition to having to be easy to grow, be generally nutritious and be accepted as a feed by a wide variety of phytoplanktivorous organisms, the contender for all-time most popular phytoplankter would additionally need to have an extended shelf life in its live form.
With all of these considerations at the forefront, certain species of the genus Nannochloropsis have emerged as the most widely used in the aquarium and aquaculture industries to date. There are six species in the genus, most (but not all) of which are marine. Nannochloropsis gaditana is a small, ball-shaped, unflagellated yellow/green algae that is used most frequently by marine aquarists. Typically referred to as “Nanno” in both industries, it has a broad range of uses and is frequently included in blended-species products (live and otherwise).
It is definitely fair to say that Nanno is easy to grow. This acknowledgment, however, comes with some caveats. Nanno is sort of a micro-weed. It can grow aggressively enough that if it contaminates cultures of other microalgae species, it can quickly dominate and outgrow them to the point of exclusion. This creates challenges for aquaculturists when attempting to grow multiple species. This is not such a bad thing for marine aquarists, on the other hand, as they often observe a reduction in the densities of less desirable algae when regularly adding live Nanno to their tanks. This positive effect can be seen even with benthic, film-forming algae types. The Nanno, though, cannot ever become a nuisance as it lives only in the water column and is very eagerly grazed upon (that is, removed) by the aquarium’s resident phytoplanktivores.
This takes the discussion of this important live feed to another subject: its overall suitability as food for diverse aquarium species. As a food, Nanno stands out for a couple of reasons. For one, at only a few microns in diameter, it is of unusually small size, even among the phytoplankton. In addition to this, its basic, round shape and lack of motility make it easy to capture and ingest for all phytoplanktivorous animals. Thus, on account of its tininess and high palatability, it is a part of most mixed-species feeds as to accommodate the needs of small or fine-filtering animals (such as many sponges, tube worms, tunicates, bryozoans, etc.). It is very widely used in the culture of bivalve mollusks. It is also commonly used to enrich live foods (e.g. brine shrimp, rotifers) by way of gut-loading. It contains no phytotoxins; it is so innocuous that it is often the preferred algal species for “greening the water” of larval fish, which enhances their ability to spot their zooplankton food in the water column. Oh, and its shelf life? Properly stored and refrigerated, Nanno has (particularly for a live product) an exceptionally long shelf life. Some report that it can be stored in the fridge at 30-37 degrees F for as long as three months.
And aside from just being harmless and palatable, Nannochloropsis is extremely nutritious. Most notably, it is very rich in eicosapentaenoic acid (EPA). This is a pretty big deal since EPA is one of the most important polyunsaturated fatty acids in the diets of all animals. Animals are incapable of synthesizing many of the fatty acids (including EPA) that are required for normal bodily function and so must obtain them through their diets. Most of the EPA in the natural marine environment is, in fact, ultimately derived from phytoplankton and passed along through zooplankton such as copepods. This is so whether the animal obtains them directly by consuming the algae or indirectly by consuming herbivores (e.g. copepods). Thus, supplementing this important lipid can positively influence the general health of all aquarium animals. It can even help to enhance the coloration of numerous aquarium species, as it is rich in color-enhancing dietary pigments such as astaxanthin, canthaxanthin, and zeaxanthin.
There are a lot of reasons that Nannochloropsis is the most widely used and recommended phytoplankton by marine aquarists. Of course, it certainly helps that it is easy to keep alive in culture. And that it has high concentrations of essential fatty acids. Another distinct benefit is that it produces no toxins nor is it likely to cause water fouling when used correctly. In fact, it can significantly improve water quality by rapidly uptaking excess nutrients. In so doing, it can improve aquarium appearance by competing with ugly benthic microalgae. Yet, it cannot grow out of control in aquaria. To get the desired density of Nanno, the aquarist needs only to adjust the amount administered in the feeding regimen. Most individual animals, and certainly entire communities of aquarium animals, fare best when offered a mix of phytoplankton as to present a well-balanced nutritional profile as well as a wide range of cell sizes. Nannochloropsis compliments other phytoplankters well and hence is included in high-quality live phytoplankton feeds such as OceanMagik. Used regularly, these prolific microalgae can do a lot to balance water chemistry, sequester nitrate and phosphate, provide excellent nutrition to certain herbivorous filter-feeders, bolster pod populations and intensify animal coloration. Not a small list for such a minuscule plant!