Noctiluca scintillans – Jekyll & Hyde of Plankton

This week red tides have been reported all across the western half of Hong Kong including Discovery Bay, Peng Chau, Mui Wo, East and West Lamma Channel. The culprit was once again the plankton species Noctiluca scintillans – neither fully plant not fully animal. It’s a single-called organism from a group called dinoflagellates. They consist of a bubble-shaped cell with two whips called flagella – that propel them through the water.

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Though Noctiluca eats other plankton it doesn’t always kill what it eats: sometimes it leaves algae intact and stores it in little bubbles in its body (cell) where the algae make sugars that leak out and feed Noctiluca while the waste produced by Noctiluca feeds the photosynthesis of the preyed on algae – a process known to most as symbiosis and also found in tropical corals. However Noctiluca can also just eat the algae. Why and how it decides to eat or farm the algae is not really clear.
Noctiluca is a well known and non-toxic local red tide species and its occurrence is not necessarily a sign of pollution, but entirely natural. What is perhaps not natural is the size of the bloom, though. This could well point to agricultural fertiliser run-off and sewage effluent particularly from the fast growing population of the Pearl River Delta (PRD).

Images of the recent red tide at Discovery Bay North:
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Noctiluca has however a redeeming feature – bioluminescence! That beautiful sea sparkle of iridescent blue that night divers in the tropics often sea or beach goers see in the breaking waves at night. So blood-red tides on the one side and beautiful sea sparkle on the other, Noctiluca is the Jekyll and Hyde of HK’s marine environment.

Long exposure image of a Noctiluca scintillans patch (cm via WikiCommons):

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If you would like to know more about Actual toxic red tides in Hong Kong here is a little TV news documentary from 2013 I found on YouTube:

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