White sea foam is a natural element of the sea, especially where there is wind or wave turbulence. Normal sea water foams more than fresh water mainly because it contains more organic matter and is more dense due to its salt content. One obvious feature of the algal bloom is excessive thick, discoloured sea foam found on affected beaches, especially after storms or large swells, as seen in the video above.
Foam is a complex structure in which bubbles of air are trapped in the water. For a long lasting foam to form, it needs a combination of suspended solids and some kind of surfactant. Surfactants are chemicals like detergents and soaps that reduce the surface tension of water, thereby facilitating the formation of bubbles with very thin surfaces.
Some of the most obvious algal foams are thick and brownish in colour. These foams have high levels of suspended organic matter, mostly derived from dead microalgal cells together with decaying animals and plants killed by the bloom. The thicker foam is maintained not only by lipid surfactants but also mucopolysaccharides (mucus) from decaying algae1 together with proteins from decaying animal matter which act as a binding agent to trap the bubbles (like whipped egg white). They are also contain a range of bacteria that feed on the organic matter and contribute to its decomposition. These foams tend to have small bubbles and can support other materials such as sand, pieces of seaweed and so on. They can persist for quite a while.
The following video shows this process in action.
Some of the foams show the iridescence of soap bubbles. These foams are relatively free of suspended organic matter but they must contain some kind of surfactant. In this case, it would most likely be derived from lipids (fats) in the cell membranes of the microalgae and perhaps the cells of creatures that have been affected by the cytolytic action of the Karenia mikimotoi toxin. Like soap bubbles, these foams are much more delicate and do not last long.
Iridescent colour is generated by the interference of light reflecting off the inner and outer surfaces of the bubble, which typically need to be about 1/4 of the wavelength of light thick (or thin!). Wavelengths of visible light range from about 380-750nm (nm, nanometres, where 1 nanometre is 1 millionth of a millimetre), where the shortest wavelengths are violet and blue while the longest are red.
The organic matter in thick discoloured foam probably only contains Karenia as broken up cells, since they are very fragile. Broken up Karenia mikimotoi cells have reduced cytotoxicity compared with intact cells but high levels can still cause cytolytic damage on contact. Consequently, contact with discoloured foams may lead to irritation of sensitive tissues such as nasal linings and eyelids. These foams also contain large numbers of diverse types of bacteria, some of which may contribute to irritation of the eyes and throat via their own cytotoxic actions2.
Conditions that lead to the formation of large accumulations of foam also lead to the formation of aerosols capable of containing brevetoxin from any Karenia cristata or Karenia papilionacea that might be in the area. Brevetoxin can persist in the foams themselves3. These same conditions would also promote the distribution of bacteria carried by the foam and aerosols.
Selected references
- Danovaro R et al (2009) Climate change and the potential spreading of marine mucilage and microbial pathogens in the Mediterranean Sea. PLoS ONE 4: e7006, https://doi.org/10.1371/journal.pone.0007006
Mühlenbruch M et al (2018) Mini-review: Phytoplankton-derived polysaccharides in the marine environment and their interactions with heterotrophic bacteria. Environmental Microbiology 20: 2671–2685 https://doi.org/10.1111/1462-2920.14302
Wang FQ et al (2024) Particle‑attached bacteria act as gatekeepers in the decomposition of complex phytoplankton polysaccharides. Microbiome 12: 23, https://doi.org/10.1186/s40168-024-01757-5
↩︎ - Landsberg JH (2002) The effects of harmful algal blooms on aquatic organisms. Reviews in Fisheries Science 10: 113-390, https://doi.org/10.1080/20026491051695
↩︎ - Pierce RH et al (2003) Brevetoxin concentrations in marine aerosol: human exposure levels during a Karenia brevis harmful algal bloom. Bulletin for Environmental and Contamination Toxicology 70: 161-165, https://doi.org/10.1007/s00128-002-0170-y ↩︎
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