Culturing Phytoplankton or green water (Nannochloropsis)

I have been culturing Nannochloropsis Oculata Phytoplankton in a glass demijohn for nearly three years. Nannochloropsis is a fast growing species which is easy to maintain. Personally I think culturing phytoplankton is relatively simple requiring not much effort to produce a continuous supply of live phytoplankton to feed your reef tank and copepods – if you want to culture those too! So far …. touch wood … my cultures has not failed me. Foaming (think protein skimmer) is said to indicate a depleted nutrient or failed (crashed) culture, it can also be a sign that your culture is old, damaged or over-aerated. Not good.


To culture your own phytoplankton, you need RO water, sea salt, light, nutrient and a few basic pieces of equipment: two culture vessels  – two makes it easier when need to swap & clean them roughly once a month (I use demijohns), rubber bung, rigid air tube and flexible airline tubing, a small air pump (mine has two individually adjustable air outlets so I can also culturing copepods using one air pump).

Water Salinity 26 ppt  / 1.020 SG
Nutrient Guillard F/2 based
Illumination (Lux) 5,000 – 10000
Temperature Range 60-86 F / 15.6 -30C

I usually make about 20 litres of seawater which lasts about 6weeks – 2 litres / week for phytoplankton culture  and  1 litre / week to culture artemia

The most difficult part is slowly doubling up the phytoplankton over a few weeks until have the right amount to half it – 50% for feeding to your reef tank over period of a week and 50% to continue culture, for me that’s 4 litres (hence the demijohn). To get you started you can buy live Nannochloropsis Oculata phytoplankton and nutrient online. The nutrient I use is based on Guillard F/2. I started with enough phytoplankton to add to 1/2 litre of seawater. Four litres of culture takes about a month and then simply a matter of splitting the culture every 8 days.

Day One Start with 1/2 litre seawater & nutrient
Day Eight Add 1/2 litre seawater  plus nutrient
Day Sixteen Add 1 litre seawater  plus nutrient
Day Twenty-Four Add 2 litre seawater  plus nutrient
Day Thirty-two Time to split culture

I don’t use any additional heating, so during the UK winter if cold enough the phytoplankton does sink a little (much as they do when stored in fridge prior to use). A quick stir and all is well.

I aim for 16 hours ‘day’ light with 8hours of darkness. Living in UK this means using artificial day light during the late Autumn, Winter and early Spring by supplying a light source which gives an intensity of 5000 – 10,000 Lux on the cultures. In the summer month I have found the natural light works well – but not direct sunlight. Overtime, the sides of your culture vessel will green up, reducing light penetration and culture growth so you will need to clean it. Using two demijohns I have one cleaned and ready to go when this is required. The only thing I need to do is rinse out the vinegar (my natural disinfectant) I leave in it and it’s good to go.

Both my demijohns have incremental measurement marks on outside. This aided in initial doubling up of phytoplankton, is very useful when you split your culture for measuring new seawater and cultured phytoplankton and it allows you to check for any evaporation when you will need to top up with RO water to maintain salinity. I also have stick on thermometers, so I can keep an eye on the temperature of the culture – just as when it is too cold the phytoplankton all go dormant, too hot (over 30C) and it will start to die off. The bung has a small piece cut out to allow air to escape and the rigid tubing is cut to length that reaches from rubber bung to the bottom of your culture vessel (demijohn) at the top of this rigid airline you attach the soft flexible airline, which comes from your pump valve.

Every eighth day, I divide the phytoplankton. Detach airline and pour phytoplankton into a clean large jug. Add salt water half the original volume of the phytoplankton into your now empty culture vessel . Add nutrient  in accordance with instructions

Sea water (my salinity 26 ppt / 1.020 G)  and nutrient (Guillard F/2 based)

Add half of the phytoplankton back into your culture vessel, using a funnel makes this easier. I also filter the phytoplankton through a 200 micron mesh sieve.

Gunk left after filtering phytoplankton into culture vessel & bottle

The other half of the phytoplankton is decanted into a clean plastic bottle

Phytoplankton (green water) split, fed and ready for return to culture station

Re-attach air hose to culture vessel and store the bottle of phytoplankton in the fridge to dose your tank.

Your fresh bottle of phytoplankton need to be stored in the fridge to keep it fresh. Phytoplankton go dormant in the colder temperature, so it  will settle. This is quite handy as the following day  I pour off half of the water so I have a litre of more concentrated phytoplankton. As it settles during the rest of the week, just give it a shake every day to prevent the microscopic organisms dying off.

I add phytoplankton daily at 20:30 (8.30pm) with the skimmer off for two hours to let the reef tank inhabitants feed. If you don’t turn the skimmer off all your efforts will be wasted, just remember to turn it back on again after a couple of hours.

If you’ve never added phytoplankton to your tank you do need to allow for adjustment so start small, add it incrementally (every few days) and increase gradually to your desired amount.

My culture station – phytoplankton & copepods




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