Blog

Dr John S. Douglas (PhD in Marine Microbiology)

INTRODUCTION

Algal blooms are caused by a number of genera of single celled Protists, mainly marine, and a few in freshwater. Most blooms are caused by Dinoflagellates, some by Diatoms and Coccolithophores. They produce the “Smell of the sea” that we all sense as we near the coast. This is Dimethyl sulfide (DMS) and is produced by dinoflagellates. DMS in the atmosphere results in cloud condensation nuclei. DMS also has antiviral properties and may protect the remaining dinoflagellates in a bloom. Large blooms can be imaged from space, in this image from MODIS red represents high chlorophyll levels caused by an algal bloom( fig 7).

Dinoflagellates belong in the Kingdom Protista; Phylum Dinoflagellata, however their taxonomy is complex and still in a state of confusion (Hoppenrath, M. Dinoflagellate taxonomy — a review and proposal of a revised classification. Mar Biodiv 47, 381–403 (2017).

Dinoflagellates live in a wide variety of habitats, and all have complex life cycles involving distinct stages, morphologies and resting-cysts (fig 4). Their size ranges from small, armoured dinoflagellates, which are 8 to 15 µm, to Noctiluca which are 200 to 2000 µm (figs 5 and 6). Around 50% are autotrophs and use photosynthesis to provide nutrients, the other 50% are heterotrophs and feed on bacteria and other protists including other dinoflagellates. There are around 2500 extant species, 150 are parasitic including on lobsters and prawns. A few are pathogenic.

The genus Symbiodinium engage in mutual symbioses with a wide range of marine invertebrates especially corals and protists. When the temperature rises in coral reefs the symbiodidia leave and the coral loses their colour. This is called bleaching.

Fig 1: The author at his Olympus BX53 research microscope.

A NEW SPECIES OF DINOFLAGELLATE:

During his PhD research John discovered a unique endosymbiosis between a Folliculinid ciliate and a Symbiodidium (a dinoflagellate) at Edithburgh. This is Edithofolliculina symbiodidia, n. gen. n. sp. Douglas, J.S. 2016, along with its endosymbiotic dinoflagellates, also described as a new species – Symbiodidia folliculinii, n. sp. Douglas, J.S. 2016. Details are on this site.

WHAT ARE ALGAL BLOOMS?

Algal Blooms are an explosion of dinoflagellates cells in coastal and marine waters. This is a consequence of their complex life cycles and increased nutrient levels along with temperature changes. Importantly less than 60 of the 2500 extant species are toxic and form Harmful Algal Blooms or HAB – so let us not condemn all because of a few. Blooms are controlled by the supply of nutrients and that is why they are often patchy.

ORIGINS, MORPHOLOGY, LIFECYCLES, HAB AND POSSIBLE ACTIONS:

The first fossil evidence of Dinoflagellates dates back to the Silurian around 430 million years before present. The Silurian was a stable marine time when Coral Reefs first appeared along with the symbiosis between Dinoflagellates and coral. Thus, Dinoflagellates have been around for an exceedingly long time, and without them there would be no coral reefs, including the GREAT BARRIER REEF.

There are two morphological types of dinoflagellates:

Desmokonts: with two dissimilar flagella emerging from the anterior end (fig 2A) and Dinokonts: With two flagella one transverse and housed in a cingulum and the other in a longitudinal sulcus. The transverse provides propulsion and the longitudinal provides direction (fig 2B).

Fig 2: The two Morphological Types of Dinoflagellates.

Taxonomy:

Dinoflagellate Taxonomy is complex and often requires DNA and RNA analysis which was the case of Karenia mikimotoi. If you are looking in texts before 2000 you will find the original name was Gymnodinium mikimotoi (Miyake & Kominami ex Oda 1935).
Traditionally dinoflagellate taxonomy was (and still is) based on light microscopy using the assignment of plates on the theca, spines, sutures, and cingulum shapes. Genomics are an additional diagnostic when clades are hard to separate.

Morphology, Physiology and Ecology:

The cell wall or Theca can be smooth such as Gymnodinium or armoured with plates made up of polysaccharides often with spines and flanges, as in Pyrodinium. Inside the theca is the protoplasm and the large nucleus. The theca has two grooves which house the two flagella (hence “Dinoflagellate”) that provide locomotion.

Fig 3: Karenia mikimotoi – Differential Interference Contrast Image 14093 processed using Z-stack to achieve full depth focus. NOTE: Chloroplasts present. Transverse flagellum lying in cingulum and posterior longitudinal flagellum caught in motion. (XAPO x40 0.95 na objective).

Dinoflagellates have a complex lifecycle involving many stages:

• An asexual stage which is the motile form that produces blooms,
• A sexual stage when nutrients are low,
• A planozygote stage when their food of bacteria is plummeting,(they form a Planozygote which forms a resting cyst in the bottom sediment). This cyst can stay quiescent for months even years until conditions are favourable,
• Then they begin the cycle again (fig 4)

Fig 4: Life Cycle of a Marine Dinoflagellate.

HARMFUL ALGAL BLOOMS (HAB):

Out of around 2,500 species of Dinoflagellates only about 150 produce toxins and only a few of those cause problems to humans, notably Karenia mikimotoi and Karenia brevis. The main causes are from eating fish and shellfish which have accumulated the toxins. These two dinoflagellates can be distinguished using light microscopy by a marine microbiologist with taxonomic experience in this field. HAB,s produce a number of toxins most are neurotoxins. Algicidal bacteria offer a promising biological approach for mitigating HABs such as the bacterium Alteromonas abrolhosensis JY-JZ1 against Karenia mikimotoi (Yang Jia et al. Jrn Hazadous Materials Vol 490, 15 June 2025).

Algicidal Viruses and Bacteria usually end a HAB Bloom:

Viruses: Viruses are the smallest and most numerous biological entities in the ocean, and are critical to the structuring marine ecosystems and maintaining a balance of bacteria and protists. Viruses are critical in controlling HAB in coastal waters. (Lawrence, J.E. and Suttle, C.A. 2004.)

Seagrass beds along with being nurseries for numerous marine species, also protect against HAB blooms by acting as habitat for Algicidal bacteria. (Imai. I et al 2012a, Bull Fish Sci Hokkaido Univ, 62:21 – 28). The global loss of seagrass beds is matched with increasing algal blooms, the restoration of seagrass beds is an important strategy to prevent HAB blooms.

There are two types of Algicidal bacteria:

Direct – Those that directly attack the host algal cells.
Indirect – That produce algicidal toxins.
Algicidal bacteria play a significant role in bloom termination. Exceptionally large numbers of algicidal bacteria form biofilms on seaweeds; including the green Ulva pertusa, the red Gelidium spp and the brown algae Sargassum muticum and S. thunbergii.

What physical actions can initiate algal blooms:

• Sewage plants, desalinzation plants, agricultural and urban runoff all introduce nutrients which can initiate and feed blooms.
• Dredging in estuaries and coastal zones during a bloom. Dredging will do two things: bring up nutrients into the water column and release any dinoflagellate resting cysts into the water column.

Noctiluca is a large dinoflagellate that is the cause of the “Sea-Sparkle” we all love on warm summer nights. Running our hands through the sea to marvel at the blue fire trail left behind as these tiny cells turn luciferin into blue light. Noctiluca can eat dinoflagellates and reduce blooms. (Images by John Douglas)

Fig 5: Noctiluca scintillans. This large (499 µm) dinoflagellate is Bioluminescent, has no chloroplasts and is phagotrophic.

Fig 6: Noctiluca scintillans showing the large tentacle.

Fig 7: A satellite image showing chlorophyll levels (in red) around the Fleurieu Peninsula (right) on Tuesday 23 July. (Supplied: NASA)

References:

1. Hoppenrath, M. Dinoflagellate taxonomy — a review and proposal of a revised classification. Mar Biodiv 47, 381–403 (2017).
2. Miyake & Kominami ex Oda 1935.
3. Lawrence JE, Suttle CA. 40.  2004. Effect of viral infection on sinking rates of Heterosigma akashiwo and its implications for bloom termination. Aquat. Microb. Ecol. 37:1–7