Marine phytoplankton consist of the microscopic algae that live in suspension in the surface waters of oceans. Most of them are single-celled organisms, but some form chains of several or many cells. Like terrestrial plants, phytoplankton organisms have the green pigment chlorophyll a, which is essential to photosynthesis. Phytoplankton can only be observed by means of a microscope; most of them are just a few micrometers (thousands of millimeters) in size.
Question 1: What are phytoplankton organisms?
Phytoplankton organisms are not all identical. They include numerous groups that are characterized by a variety of shapes and sizes, and play different roles in marine ecosystems. For example:
Diatoms
These organisms are protected by a very thin, transparent glass cell wall. Thousands of species can be identified based on the shape and ornamentation of their glass cell wall. Diatoms need much nutrients to grow. When the conditions are favorable for growth, they respond quickly! One may then observe what is called a diatom bloom.
Dinoflagellates
These organisms possess two flagella that enable them to move like animals. Several species are characterized by both plant-like traits (they carry out photosynthesis) and animal-like traits (they also feed on organic matter). Dinoflagellates often possess collar-like structures ("cingular lists"), wing-like structures ("sulcal lists"), or horns. Researchers use these characteristics to identify species. Thanks to their flagella, dinoflagellates are capable of vertical migrations to make the most of their environment and utilize both sunlight (near surface) and nutrients (at depth).
Coccolithophores
These organisms possess flagella and, more importantly, they are covered with microscopic plates made of limestone (calcite). When coccolithophores die, they shed their small calcite plates, which sink into the deep ocean when incorporated into heavier particles. They accumulate on the ocean floor for millions of years and form limestone, which is actually chalk!
