CHAPTER 26 INTRODUCTION

CORE CONCEPTS

26.1 The tree of life has three main branches, called domains: Eukarya, Bacteria, and Archaea.

26.2 Bacteria and Archaea are notable for their metabolic diversity.

26.3 In addition to their key roles in the carbon cycle, Bacteria and Archaea are critical to the biological cycling of sulfur and nitrogen.

26.4 The extent of bacterial diversity was recognized only when sequencing technologies could be applied to non-culturable bacteria.

26.5 The diversity of Archaea has only recently been recognized.

26.6 The earliest forms of life on Earth were Bacteria and Archaea.

In Chapter 25, we outlined the carbon cycle as it operates in the oxygen-rich environments of our daily existence. But can carbon cycle through the deep waters of the Black Sea, within black muds beneath swamps and marshlands, or in other habitats where oxygen is limited or absent? Can biological communities survive without oxygen? The answer, emphatically, is yes. Indeed, life existed on Earth for more than a billion years before oxygen-rich habitats first appeared on our planet.

Exploration of seemingly alien oxygen-poor environments on today’s Earth reveals further aspects of the carbon cycle. These habitats are populated by organisms that neither produce nor consume oxygen, yet are able to cycle carbon. The organisms responsible for this expanded cycle share one fundamental feature: They have prokaryotic cell organization. Prokaryotes, which include bacteria and archaeons, lack a nucleus (Chapter 5). We’ll look at the carbon cycle again later in this chapter, but first we discuss Bacteria and Archaea, the prokaryotic domains of microscopic organisms so critical to the cycling of carbon and other elements essential to life. We examine their diversity and how they have evolved through time.