Haptophytes – algae little known, but very important

In terms of algae, databases list tens of thousands of confirmed species (the number can vary depending on the definition of these organisms), which is several times more than vascular plants. However, our knowledge of this group of organisms is still far from complete. In recent years, the public has become more familiar with one species representing haptophytes – Prymnesium parvum, and the scientific community with another – Braarudosphaera bigelowii. The first is a so-called. golden alga, whose toxic bloom caused the ecological disaster of the Oder River in the summer of 2022. The second is the first eukaryotic organism in which an endosymbiotic bacterium that fixes atmospheric nitrogen is so integrated into the cell that it has been recognized as a new type of cell organellum – a nitroplast.

Little-known haptophytes

Both species belong to haptophytes – microscopic algae. This group is poorly known even to hydrobiologists and botanists. Polish textbooks devote very little space to them or omit them altogether. They are more popular among oceanologists and paleontologists. The main group of haptophytes, coccolithophores, are algae that can dominate the oceanic phytoplankton during blooms, and their shells – coccoliths – are not only important fossils, but often the main component of chalk deposits.

Oceanic coccolithophores produce calcium carbonate deposits

Blooms of a particular coccolithophore haptophyte – Gephyrocapsa huxleyi – are so abundant that it is listed as one of the most abundant species found on Earth. This is supported by its wide ecological tolerance, making it found from tropical to polar zones in the Atlantic, Pacific and their seas. In the literature, it is more commonly encountered under the name Emiliania huxleyi (while it was first described as Pontosphaera huxleyi in 1902. from the sea near Syracuse).

Coccoliths of this species are transparent, but at higher densities, reflecting sunlight, they turn white. They are visible on satellite images, at first glance resembling clouds covering the ocean. The blooms near the British Isles are particularly impressive, matching their size. Chrysochromulina, on the other hand, is known for its golden-colored blooms, from which it takes its name.

Blooms of various species of coccolithophores are estimated to account for half of the modern production of oceanic calcium carbonate deposits, competing mainly with other microscopic organisms – borers. Compared to them, haptophytes seem to tolerate water acidification better, so their role is growing. The production of calcareous shells absorbs carbon dioxide, but the decomposition of bicarbonate ions can mix in the balance. The bloom shades the water’s bottom, and its milky whiteness reflects sunlight, which collectively cools the ocean, but also reduces photosynthesis of the remaining algae – lower because of shading, higher because of photoinhibition (counterintuitively, more light in the water does not always favor photosynthesis).

Because of their abundance, haptophytes make an important contribution to global photosynthesis, in which oceanic phytoplankton play a major role. They are also an important source of sulfur aerosols, key to cloud formation over the oceans. A recently discovered nitroplastic indicates their hitherto unknown role in nitrogen cycling and ocean fertilization.

How can tiny haptophytes cause an environmental disaster?

Example Prymnesium parvum indicates that under specific conditions haptophytes can cause an ecological disaster. True, most of their species do not produce toxins, but a bloom of one is enough to cause a plague of thousands of animals. In addition to Prymnesium parvum, such abilities are attributed to at least some other species of it and its sister genus Chrysochromulina. Initially, mass fish die-offs were observed in the limited environment of farm ponds, but recurring disasters have been recorded for several decades in rivers in the United States, England and elsewhere around the world.

One such catastrophe in Greece has also been linked to bird deaths, although given other observations, this is an unlikely factor ( Prymnesium toxins, or prymnesin, must somehow overcome body barriers, and the process occurs most easily in the gills). Haptophyte toxins are highly complex organic compounds, and the enzymes discovered this year for their synthesis are the largest proteins currently known to be produced in nature.

These catastrophes rarely occur in the natural habitat of Prymnesium, which, like almost all haptophytes, prefers marine waters (though not necessarily with high salinity), possibly brackish coastal lakes. The first observations of Prymnesium parvum were made in a pond on the English Isle of Wight and in the Gulf of Gdansk. In some such sites, no problems appear for several decades. Toxin release usually occurs in inland reservoirs where fresh water mixes with saline water, such as due to pollution.

A series of similar disasters have occurred since 1989 in the fjords of the Norwegian Sea, where meltwater enters the salty seawater. Conjecture has been put forward that a similar cause was a fish plague in the Danish Straits not long before the discovery of the species. It is estimated that such phenomena may have been repeated for two hundred million years.

Unclear taxonomy of haptophytes

Growing awareness of the environmental and economic role of haptophytes is turning them from a biological curiosity to an object of increasing research. Known since the 19th century. For many years they were included in the group of goldilocks. Today, it is known that the assessment of relatedness based on the similarity of chloroplasts and the related physiology of photosynthesis of sugars is illusory, since these by way of successive symbioses may have a similar history in different hosts. Both the goldworms and haptophytes have chloroplasts derived from ancient kelp.

In taxonomic systems that still distinguish kingdoms, they are included in chromists. However, they form a separate group – the type Haptophyta or Haptista. Most taxonomists combine this type with the cryptomonads (Cryptophyta/Cryptista) and possibly the sunfishes (Heliozoa) into the suborder Hacrobia. The goldilocks lie in the parallel subtribe Harosa (Stramenopile-Alveoloata-Rhizaria), and thus are more closely related to diatoms or brown algae, and even to periwinkles or fungus-like larvae, than to haptophytes.

Haptophytes are divided into two main groups (classes): Pavlovophyceae and Coccolitophyceae (Prymnesiophyceae). Sometimes they also include a third, Centrohelea, which includes only one genus: Meringosphaera (usually included in the sunflowers). There is also a group of species that are known only from coccolith fossils and it is impossible to determine their closer relationship to modern species. An interesting case is another class of Rappephyceae. In 2011. As part of analyses of DNA extracted from the ocean, genes belonging to previously unknown algae were discovered. It wasn’t until a decade later that the organism that was the source of these genes – Pavlomulina ranunculiformis – was pinpointed, and considered a specific haptophyte.

Haptonema – a diagnostic feature of haptophytes

All representatives of haptophytes have cells covered with lamellae, but in each class they have a different structure and origin. The most massive are the plates in the Coccolithophyceae. The name of the class suggests that all its representatives have shells made of calcareous coccoliths, but this is not true. Examples are Prymnesium and Chrysochromulina, whose shells are organic. The name haptophytes comes from the appendage of haptonema. At first it seemed to be a third thread, but the development of microscopy in the mid-20th century. showed that it is a different structure.

It is usually shorter than the tendrils and is not used for movement, but for attachment to various structures, including potential prey, as many haptophytes can not only perform photosynthesis, but also feed heterotrophically. It may be noted that the name haptophytes is – although rarely – used in hydrobotany to designate aquatic plants attached to the substrate. Sometimes the haptophyte is barely visible, and in some, including the best studied species, Gephyrocapsa huxleyi, it is not noticeable at all.

Currently, almost two thousand species of haptophytes are distinguished. By comparison – there are slightly more brown algae, furrowed algae twice as many, diatoms ten times as many, but the classic goldilocks currently include less than one and a half thousand. These numbers are changing dynamically. One reason for this is the discovery that organisms classified into different categories are actually different generations of the same species.

A common situation is that one generation has coccolithophores and the next generation is devoid of them. This was the case with Braarudosphaera bigelowii, which is a crystal-shaped coccolithophore, and the next generation is a pear-shaped flagellate with delicate organic plates and was formerly described as Chrysochromulina parkeae. Also Prymnesium parvum until recently had its twin, Prymnesium patelliferum. As you can see, haptophytes are a group of algae with no shortage of exceptions to the rule and deserve more space in textbooks than they have had so far.

Photo. main: Agnieszka Kolada