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  • Culturing Phytoplankton For Reef Aquarium Food



    Culturing Phytoplankton

    Live microalgae is a natural food source used for feeding clams, sponges, soft coral, and other filter feeders.  It's rich in Omega-3 fatty acids, carbohydrates, lipids, and plant sterols.  As the foundation of the aquatic food chain, phyto provides food for zooplankton, which are then eaten by: stony coral, planktivores, and other invertebrates.  In addition, pods that feed on microalgae are more nutritious prey than pods which feed on detritus.


    When we talk about phytoplankton (phyto), we are usually referring to one or more of the thousands of species microalgae.  However, phytoplankton also includes other protists, including cyanobacteria and dinoflagellates.  For this article, phyto will mean microalgae (including diatoms, which are a type of brown microalgae that is often included in live phyto blends).


    Live phyto blends, containing various species of microalgae, are available from local and online retailers.  Some of these products are filtered and concentrated.  However, the nutritional value of these cultures may still decrease when refrigerated, and cost (especially considering next day shipping) can often become prohibitive.  In this article, I will discuss how to culture your own phytoplankton.

    Nannochloropsis oculata

    072318b.jpg.gifI am currently culturing the microalgae species, Nannochloropsis oculata.  It's often used to culture rotifers due to its high levels of vitamin B12 and Omega-3.  And while it's also beneficial to copepods, larvae, and filter feeders, its relatively thick cell wall can make it harder for certain animals to digest.[1]  Another species might be more suitable if you are specifically culturing copepods.


    I started my culture using AlgaGen PhycoPure Greenwater (Nannochloropsis) that I purchased from Live Aquaria.  AlgaGen claims that customers have reported good results feeding Nannochloropsis to rotifers, copepods, amphipods, corals, shrimp, feather dusters, clams and other filter feeders.[2]  Plus, it's pretty easy to culture.


    Florida Aqua Farms is another good source for a starter culture.  They also sell f/2 fertilizer, which as been used to culture microalgae for over 30 years.[3]  Live phyto blends purchased from your LFS can be used to start a culture.  However, this will likely result in a monoculture of a single species (often Nannochloropsis).  Finally, a starter culture can come from another reefer who is culturing phyto (check with your local reef club).

    Phyto Culture Containers

    072318e.jpg.gifMost commonly, hobbyists use one or more clear plastic 2 liter bottles to culture microalgae.  I'm currently using two 1 gallon Hawaiian Punch jugs.  Simply drill a hole in the cap for the airline tubing, with a little room for air to escape (to avoid a build up of air pressure which could affect air flow).  I used a 1/4” bit, which seems to work fine.  I have read where people have used floss to cover open gaps; but with a 1/4" hole, this really isn't necessary.


    I didn't actually drink the Hawaiian Punch, so I just rinsed out the bottles after pouring it down the drain.  I didn't bother to sterilize them, but have read where others have recommended it.  Actually, I haven't sterilized any of my containers or other equipment.  I do, however, keep everything clean, and prevent contamination from tank water.  Also, I make sure that any of the equipment that I use for rotifers is never also used to culture phyto.

    Specific Gravity

    072718a.jpgAlthough most phyto species are pretty tolerant to various specific gravities, it's commonly recommended to culture it at 1.020 sg.  Likewise, rotifer cultures can tolerate a relatively wide range of salinities; however, they tend to be most productive between 1.014 and 1.017 sg.[1]  So if you are culturing rotifers, you might wish to culture your phyto using a lower specific gravity.  I'm currently culturing both my rotifers and phyto at 1.019 sg.


    Always use new saltwater for your phyto cultures (never use water from your tank).  Contamination from tank water, or from a rotifer culture can compromise your phyto culture.  A 3/8 cup scoop of salt mix should make a gallon of saltwater with a suitable specific gravity.  I'm using a scoop that was included with some protein powder.  Those scoops come in various sizes, so test it first (mine mixes to 1.019 sg).  But just like in a reef tank, a specific salinity isn't as critical as consistency.

    Dosing Nutrients and Trace Elements

    Nitrate and phosphate are needed to grow microalgae; but trace elements (like iron, copper, zinc and manganese) are needed too.  In addition, brown/tan microalgae (diatoms) would need silicate.  In order to supply our culture these elements and nutrients, we dose fertilizer with trace elements.


    When starting a new culture, or splitting an existing culture (for each 2 liter bottle), I'll add 1ml of Micro Algae Grow (Guillard f/2 formulation) from Florida Aqua Farms, and around 0.5ml of Kent Essential Elements.  I recommend using Micro Algae Grow, but if just culturing phyto to raise rotifers, Miracle-Gro Liquid All Purpose Plant Food can be substituted.  I confesss that I have occasionally dosed my tanks with phyto fed with Miracle-Gro before.


    Note: 20 drops is roughly 1 ml


    Aeration is necessary to supply carbon dioxide for photosynthesis and to help maintain pH.  However, excessive aeration can potentially fracture the cells and cause foaming.[1]  Air stones aren't necessary, but I have used them in the past without any problems.  Instead of air stones, most people recommend rigid air line tubing.  In addition, you'll need an air pump, flexible air line tubing, a gang valve, and a check valve.


    Aeration circulates the non-motile algae, which exposes the individual cells to the light and helps prevent them from settling to the bottom.  Remember to shake the culture(s) at least twice a week. I have an extra cap for my bottles, in order to shake them up without spilling; but you could just put your finger over the hole instead.


    Room temperature is typically fine.  Plus, you don't want to use incandescent light bulbs for lighting, as they may heat the culture too much.


    I originally used compact florescent work lights, but I had one melt down and nearly cause a fire; so I switched to a plastic clamp on light with a standard 75W equivalent LED light bulb (daylight spectrum).  I leave the light on 24/7, but 16 hours a day would be adequate.  Try to light the side of the bottle(s), versus the smaller top (which is also partially blocked by the cap).

    Harvesting Phyto


    You must regularly harvest your culture to keep it going. I harvest half of it weekly by doing the following:


    • Gently shake the culture.
    • Strain the entire culture using a 53 micron plankton sieve (available at Amazon), to remove the larger particles.  Nannochloropsis is only about 4 to 6 microns in diameter.[1]  This step is optional, but I feel it's helpful.  Also, in a pinch, a new brine shrimp net can be used instead.
    • Remove half the culture (1 gallon in my case).
    • Make replacement saltwater with added Micro Algae Grow and Essential Elements, then add this water to the remaining culture.
    • Clean the empty culture bottle(s) with a bottle brush.
    • Pour the diluted culture back into the cleaned bottles with the help of a funnel.


    Foaming on top of the culture usually indicates that harvesting is overdue.  Most of my harvested phyto culture goes to keep my rotifer culture alive, but I also use it to dose both of my tanks when I harvest it.  If using more than one culture bottle, you could potentially harvest them on different days.


    When just starting a culture, it will be pretty pale in color.  Let it darken up before you start diluting it.  However, you should still continue to add f/2 and Essential Elements weekly.  You might be surprised just how small of a sample is required to start a culture.


    Use (clean) empty water bottles to store harvested phyto in a refrigerator.  Label them with a date so that you know how old they are, and that nobody mistakes them for something else.  At least once a week, gently shake or invert the cultures several times to prevent settling.  You can keep phyto in the refrigerator for up to a month; although fresh phyto provides the best nutrition.

    Dosing Phytoplankton

    Gently shake or invert the culture before dosing.  You can broadcast feed your tank, or target feed specific specimens with the help of a clean syringe, pipette, or eye dropper.  I suggest target feeding any livestock that requires phytoplankton.  This can be done by releasing the phyto a couple of inches upstream from the target.  Avoid contaminating the culture with aquarium water (pour some phyto into another container if you are target feeding multiple specimens).


    To broadcast feed your tank, start slowly with 1 drop per gallon of tank water, once a week.  Dose it into a high flow area of your tank.  Eventually you can increase the dosage and/or frequency.  However, excessive dosing could negatively affect water quality.

    Water Quality

    I tested my phyto culture for phosphorus just prior to harvesting it by diluting a sample with 9 parts of clean saltwater. Through that, I determined that the undiluted phosphorus concentration was 310 ppb (or roughly 0.95 ppm of phosphate), which was actually lower than I had expected.


    There are about 3,785 ml in a gallon, so dosing 1 ml per gallon would cause an immediate increase in phosphorus of just about 0.08 ppb (or about a 0.00025 ppm increase in phosphate).  That's really not that much phosphate.

    Nutrient Consumption

    AlgaGen states that Nannochloropsis, “is also known to be a great water conditioner” consuming and binding nitrate, phosphate, and heavy metals.[4]  This is contrary to phyto's reputation for adding phosphate to your tank (which, as I indicated above, it initially does).  However, I assume that the live phyto continues to consume nutrients within our reef tanks after dosing, potentially lowering nutrient levels (versus raising them).


    1. Wilkerson, Joyce D.. Clownfishes. Microcosm Ltd.. Kindle Edition.
    2. https://www.liveaquaria.com/product/3249/phycopure-green-water
    3. http://floridaaquafarms.com/
    4. https://www.algagen.com/phycopure1.html
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    Excellent write up.  Easy to read and extremely practical.

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    The inclusion of Dino, cyno and diatoms as phytoplankton shows how interconnected the algae kingdom is.

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    How often do you dose the Micro algae grow and Kent essentials?  Would the initial salt batch not include all of the essential elements?

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    I dose both when starting the culture, and each week when I harvest it.  Depending on how large of a culture you are starting with, you might have to dose weekly without harvesting any (until it darkens up).  Also, when starting a culture, you might choose to double up on the Micro Algae Grow.

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    So this is what happens when you don't shake your culture and let it go for too long.  If this happens, don't worry, your culture is still probably fine.  Shake the culture, add some more nutrients, and filter out the larger chunks.



    Here's the culture after shaking and straining it.



    Here's the culture afterwards.



    Don't be afraid to add more microalgae fertilizer if you feel that the culture isn't darkening properly.


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    I'm also going to include this link to some experiments that I performed in starting a phytoplankton culture just from sitting out a bowl of water on my screened in porch:


    It demonstrates how easy it is to start a culture.  It also documents how much f/2 I used to get the culture going.


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    Nice!! I also started my own phytoplankton cultures, I use for my tiger pods. 


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    My university grows phytoplankton sometimes for experiments. Some of it is done in containers that are on top of machines that shake them constantly, which is a bit noisy but probably very helpful in keeping it from settling it out. Some of it just gets shaken manually, though. 

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