The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)


c. 2.5 Billion BCE

BASE OF THE FOOD CHAIN. Algae range in complexity from a single simple cell to millions of cells. In size, they span over seven orders of magnitude—from tiny Micromonas (1 µmeter in diameter) to giant kelp (200 feet or 60 meters). By the process of photosynthesis, algae form organic food molecules from carbon dioxide and water; these represent the base of the food chain upon which the existence of all marine life is dependent. Oxygen is a by-product of photosynthesis, and algae produce 30–50 percent of the global oxygen required by land animals for respiration. Crude oil and natural gas are photosynthetic products of ancient algae.

The heterogeneous nature of algae defies a universally agreed-upon biological classification. Some share features with protozoa and fungi, which diverged from algae over one billion years ago. As a group, algae are not closely related, nor do they form a single evolutionary linkage. A massive spike in atmospheric oxygen levels around 2.3 Billion BCE, believed to result from photosynthesis by cyanobacteria (blue-green algae), suggests that their evolutionary history began 2.5 billion years ago. Red algae and green algae evolved from a common ancient ancestor more than one billion years ago, with the oldest red fossil dating back some 1.5 billion years. The lineage that produced green algae were the forebears of land plants, and some biologists have proposed including green algae in the plant kingdom.

Some classifications of algae are based on whether they have a cell nucleus (eukaryotic) or lack one (prokaryotic) or are ecologically grouped by their habitat. Since the 1830s, algae have been classified into major groups predicated upon their color (red, green, brown), a photosynthetic accessory pigment that masks the green of chlorophyll. There are some 6,000 known species of red algae, with their different shades dependent on the depth of the sea, and they are most abundant in the warm coastal waters of tropical oceans. Most red algae are multicellular, the largest of which is referred to as “seaweed.” There are over 7,000 species of green algae of the chlorophyte group, most found in fresh water.

SEE ALSO: Prokaryotes (c. 3.9 Billion BCE), Eukaryotes (c. 2 Billion BCE), Fungi (c. 1.4 Billion BCE), Land Plants (c. 450 Million BCE), Cell Nucleus (1831), Photosynthesis (1845), Food Webs (1927).

All life depends upon photosynthesis in algae. By this process, organic molecules are formed, upon which marine life is dependent. As a by-product of photosynthesis, oxygen is produced, which is essential for the survival of land life.