Phytoplankton: Use in Marine Aquariums

What is phytoplankton?

"Phytoplankton" is a term that covers a broad group of autotrophic micro-organisms. Though not usually included in the aquarist's concept of phytoplankton, diatoms and dinoflagellates are also types of phytoplankton. The types of phytoplankton commonly used in aquarium feeding products are Chaetoceros, Nannochloropsis, Tetraselmis and Isocrysis species (among others).

These are commonly used in aquarium feeding likely because they are easy to culture and thought to be of the approximate size best for the organisms we want to feed with them.

3 Sources of Phytoplankton (for aquarists):

1) Phytoplankton in Your Tank:

Some phytoplankton is actually already in your system. In fact, when you wipe your glass clean with a sponge or Magfloat, and you see green clouds of green or brown released from the glass--that's phytoplankton (along with bacteria films, pods and other coral/invert yummies).

2) Culturing Phytoplankton at Home:

Phytoplankton can also be cultured at home. In my opinion, unless you need a very "pure" (containing only one particular species of phytoplankton) culture, culturing at home is not quite as difficult or cumbersome as it's rumored to be. I've done so on a windowsill in a simple 2.5 gallon tank with an airstone. I simply used DT's to start (which is a mix of phytoplankton
species). The resulting culture was likely only one of these species (when a mix is used to start a culture, one will almost always out compete the others).  My culture was also likely populated with bacterias and other microbes. But for my purposes, this was just fine (if not a good thing). All you really need to culture your own phytoplankton is a dedicated tank (or other water holding container), airstone, light source, and "food"/fertilizer for the algae. (Note: For more step-by-step instructions: Melev's article on phytoplankton culture)

3) Store Bought Phytoplankton:

There are many different types and brands of phytoplankton products sold online and aquarium stores. They differ in concentration, mix of species and the form in which they are stored/distributed (i.e. frozen, refrigerated, spray-dried, etc).    Phytoplankton won't survive in the freezer, nor will it survive at room temperature.  Most will not survive under refrigeration.  However, there are a few species that will. The freezer is too cold.  At room temperature, the algae would need light and air to survive.  However, in the refrigerator, a few species of phytoplankton can go into a kind of "dormant" state, which allows them to survive with little air or light.

Frozen and spray-dried phytoplankton products are certainly dead phytoplankton cells.  Again, that doesn't mean they are useless.  For spray-dried phytoplankton, you'll want to soak the dry phytoplankton first.  If you don't, it will float (and likely get sucked down an overflow). While live phytoplankton is generally better and will feed more organisms more effectively, frozen and spray-dried phytoplankton still has its uses.  Chances are, at least some critters in the aquarium will eat it.  It can also be used to feed/raise brine shrimp, rotifers, etc.

Reasons to feed phytoplankton:

There are a few aquarium corals which feed on phytoplankton. The most common ones are Dendronephtya and ahermatypic (lacking zooxanthelle) Gorgonians. It's quite possible/likely that other ahermatypic, azooxanthelle corals also feed on phytoplankton, but the Dendronephtya and ahermatypic (lacking zooxanthelle) Gorgonians absolutely need phytoplankton.

Most the corals we keep in aquariums are Hermatypic (both autotrophic and heterotrophic). This is a fancy way of saying that they both eat food and use light for photosynthesis. For the most part, these corals don't feed on phytoplankton (or, if they do, not primarily). However, phytoplankton can feed these corals indirectly by feeding the "critters" in our systems. These critters, when well fed, can continually reproduce, providing a great source of "natural" food for hermatypic corals. Feeding corals in this way is beneficial also because it provides a more constant (vs. intermittent) food supply (which is closer to how corals feed in the wild). [Note: In most cases, it's best not to try to target feed your corals phytoplankton... especially if the phytoplankton is cold, concentrated store-bought product. The best way to feed phytoplankton is to either drip it in continuously or pour it into a high current area of your tank.]

Another argument for phytoplankton feeding is that phytoplankton feeds benthic and refugium/sump critters that help process waste. Keeping healthy populations of these organisms can help keep your tank "clean" and healthy all around.

Phytoplankton is also used in the culturing of zooplankton used to feed, among other things, fish larvae. [Note: For some further reading on this use of phytoplankton: The Breeder's Net by Frank Marini]

Arguments against phytoplankton feeding:

If unconsumed, the phytoplankton will degrade and just become another source of nitrogenous waste. This might be a concern in small systems without refugiums.

References/Further Reading:

http://www.dtplankton.com/articles/refugiums.html

  http://www.personal.psu.edu/sbj4/aquarium/reeffood/feeding.htm