Microscopic algae in armoured plates helped build the world's chalk
Most haptophytes are coccolithophores, single-celled algae that wrap themselves in tiny plates of calcium carbonate. They rank among the most abundant plankton in the open ocean, and after death their plates pile up on the seabed in such numbers that they form chalk deposits.
Haptophytes are a clade of algae able to make minerals, and coccolithophores account for 673 of the 762 described species. Their name refers to a structure found nowhere else, the haptonema, from Greek words for touch and thread. It looks a little like a flagellum but has a different internal arrangement of microtubules and does a different job. Most cells also carry two smooth, slightly unequal flagella for swimming.
Their colour comes from an unusual pigment set. They use chlorophylls a, c1 and c2 but no chlorophyll b, and like diatoms and brown seaweeds they contain fucoxanthin, which is probably the main source of their brownish-yellow tint. Their chloroplasts resemble those of heterokont algae while the rest of the cell does not, hinting that both groups may have acquired chloroplasts from similar red algal partners. Instead of starch, most haptophytes store sugar as chrysolaminarin, in short chains of 20 to 50 units kept in vacuoles.
Some are troublemakers. Chrysochromulina and Prymnesium periodically cause toxic blooms at sea, and Phaeocystis blooms whip up foul foam that washes onto beaches. Others are valuable: Pavlova lutheri and Isochrysis are grown to feed young oysters and shrimp because they are rich in omega-3 fats such as DHA.
Classifying them has taken time. They were first lumped with the golden algae until electron microscopy showed they were distinct, and evidence now splits them into five orders. They appear closely related to the SAR clade. Uncultured cells only 2 to 3 micrometres across turn out to matter ecologically.
Source: Haptophyte
