7.31: Green Algae

Green algae, also referred to as chlorophytes, are different from red algae in having the chloroplasts containing chlorophylls a and b, which give them their distinct green hue. However, they lack phycobiliproteins, preventing them from developing the red or blue-green pigmentation seen in red algae. In terms of photosynthetic pigment composition, green algae closely resemble plants and share a close evolutionary relationship with them. Taxonomically Green algae belong to Phylum Chlorophyta in the Kingdom Archaeplastida.

Green algae are categorized into two primary groups: chlorophytes, such as microscopic Chlamydomonas and Dunaliella, and charophytes, such as Chara, which are macroscopic and often bear a resemblance to terrestrial plants. The latter group is considered the closest relatives of land plants. Most green algae are found in freshwater environments, though some inhabit moist soil or even grow in snow, imparting a pink hue to their surroundings. Others exist as symbionts in lichens. Morphologically, chlorophytes exhibit a wide range of forms, from unicellular organisms to filamentous structures, where cells are arranged in a linear fashion, and colonial arrangements, where cells aggregate together. Some green algae even attain a multicellular form, such as the seaweed Ulva. The life cycle of most green algae is complex, incorporating both sexual and asexual reproductive stages.

Among the smallest known eukaryotes is the green alga Ostreococcus tauri, a common unicellular species found in marine phytoplankton. With a cell diameter of approximately 2 micrometers, O. tauri has the smallest genome of any known phototrophic eukaryote, roughly 12.6 million base pairs. Due to its minimal genomic content, Ostreococcus has served as a model organism for studying genome reduction and specialization in eukaryotes.

At the colonial level of organization, green algae include Volvox, which forms colonies comprising several hundred flagellated cells. Some of these cells primarily engage in photosynthesis, while others specialize in reproduction. The individual cells in a Volvox colony remain connected by thin cytoplasmic strands, facilitating coordinated movement of the entire colony. Volvox has long been studied as a model organism for understanding the genetic mechanisms underlying multicellularity and the division of labor among cells in multicellular organisms.

Certain colonial green algae show promise as potential biofuel sources. The species Botryococcus braunii, for instance, secretes long-chain hydrocarbons (C30–C36) with a consistency similar to crude oil. Approximately 30 percent of B. braunii’s dry cell weight consists of these hydrocarbons, generating significant interest in its potential as a renewable petroleum source. Biomarker research suggests that some existing petroleum reserves may have originated from ancient green algae, such as B. braunii, which settled in prehistoric lakebeds. If the challenges associated with large-scale commercial production of algal petroleum can be addressed, green algae could contribute to the global oil supply through photosynthesis.

Some green algae are capable of living within rocks, a trait that classifies them as endolithic phototrophs. These organisms inhabit porous rocks, particularly quartz-containing formations, and typically form layers near the rock surface. Endolithic phototrophic communities are prevalent in extreme environments, such as deserts or the cold, arid regions of Antarctica. In the McMurdo Dry Valleys of Antarctica, where temperatures and humidity levels are exceptionally low, life inside rocks presents significant advantages. The sun heats the rock, while snowmelt provides a reliable source of moisture. Additionally, when porous rocks absorb water, they become more transparent, allowing greater light penetration to support the algal layers within.

A diverse array of phototrophic organisms can form endolithic communities, including cyanobacteria and various green algae. In addition to existing as free-living phototrophs, green algae and cyanobacteria often form symbiotic relationships with fungi, contributing to endolithic lichen communities. The metabolic activity and growth of these rock-dwelling microbial communities contribute to the gradual weathering of rock surfaces. Over time, small gaps form, allowing water to enter, freeze, and expand, leading to further cracking of the rock. This process creates new habitats for microbial colonization and contributes to the breakdown of rock into primitive soil, which, under favorable conditions, can support the development of plant and animal life.