How Did We Handle the Purplish Biofloc Problem?

RAS Aquaculture recently encountered a purplish biofloc problem from our small biofloc system in the crab farm. Because of the molasses, the normally brownish biofloc turned purplish. We were fortunate that it did not result in a high rate of shrimp mortality. Otherwise, it would be a huge setback for us. Now that the problem has been resolved, we'd like to share our methods for restoring the biofloc to normalcy. To learn more, keep reading this post until the end.

It is actually quite hard and typical to see photosynthetic bacteria (PSB) on biofloc. However before this, we used a microscope to identify the source of the purplish biofloc, which is most likely photosynthetic bacteria that appear to have grown on its own. As the name implies, these microorganisms are special types of bacteria with light-absorbing pigments and reaction centres that allow them to convert light energy into chemical energy. These bacteria contain bacteriochlorophyll, a compound that functions similarly to chlorophyll in plants and allows them to perform photosynthesis.

In the previous video, we planned to dose photosynthetic bacteria into the system to see if we could replicate the results we saw in the microscope. We did not, however, do so after confirming that the bacteria was, in fact, a photosynthetic bacteria which is PSB, as we had expected. Below is a video on the PSB under the microscope:

Because we don't yet have expertise in PSB, we should prioritise the problems. The "purplish biofloc" has a very fast doubling time, which means it can multiply quickly and consume oxygen. Normally, it can range from 5 to 40 in a single day. That was our issue. However, perhaps due to our system's good aeration, the excessive growth did not result in high mortality.

We then increased another unit of aeration to account for this explosive growth. In the event of uncontrolled growth, we have plenty of oxygen to spare. We continued to run the system and monitor the water parameters on a regular basis. Despite the bacteria's rapid growth, the ammonia removal efficiency is very low. We still have an ammonia level of 1-2 ppm every day. The ammonia level of the biofloc can be seen in the table for two weeks before the purplish biofloc is completely gone.

Ammonia level from when the purplish biofloc started

The first thing we tried was to reduce the settling tank flow velocity; these bacteria take longer to settle, as evidenced by the Imhoff cone settling test. (Floc picture) So we reduce the flow rate from 500l/hr to 200l/hr to give the floc plenty of time to separate. Then we cut the shrimp’s feeding, forcing them to eat the purple biofloc. We also reduced the molasses while maintaining ammonia and feeding requirements.

The floc is now stable after about two weeks, and the shrimps are growing quickly as well. The purplish biofloc has vanished and the colour has returned to a brownish hue again. Comment down below if you have experienced this issue, or if you are an expert in PSB and share your thoughts on our solutions.


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