Defining “algae” as an informal (non-valid) taxonomic term In modern usage, the informal term “algae” generally refers to which group of organisms?

Introduction / Context:
“Algae” is a convenient umbrella term, not a formal clade. It spans diverse eukaryotic lineages that convergently perform oxygenic photosynthesis. Distinguishing this informal category from plants and from prokaryotic phototrophs helps avoid common classification mistakes in microbiology and botany.

Given Data / Assumptions:

• The term is informal and paraphyletic.
• Oxygenic photosynthesis requires chlorophyll a.
• Prokaryotic oxygenic phototrophs (cyanobacteria) are not algae in the eukaryotic sense.
• Vascular plants are excluded because of their complex tissues (xylem/phloem).

Concept / Approach:
In everyday and educational usage, “algae” denotes eukaryotic photoautotrophs with chlorophyll a that release oxygen—ranging from microalgae (diatoms, green microalgae) to macroalgae (seaweeds). This excludes land plants (with true roots, stems, leaves) and excludes prokaryotes. The key is to separate function (oxygenic photosynthesis) and cell type (eukaryotic) from formal taxonomy.

Step-by-Step Solution:
Identify the core functional trait: oxygenic photosynthesis via chlorophyll a.Specify cellular organization: eukaryotic.Exclude vascular plant features (conducting tissues and organs) from the definition.Exclude prokaryotes (cyanobacteria) from the informal eukaryotic term.Choose the option that matches these boundaries.

Verification / Alternative check:
Introductory phycology texts define algae as eukaryotic, chlorophyll a-containing oxygenic phototrophs while explicitly excluding cyanobacteria and land plants, confirming the selected statement.

Why Other Options Are Wrong:

• Conducting tissues and true organs define vascular plants, not algae.
• Both (a) and (b) contradict each other because (b) refers to plants.
• “None of the above” is incorrect because (a) correctly summarizes informal usage.
• Prokaryotes only: excludes many eukaryotic algae and mislabels cyanobacteria as algae.

Common Pitfalls:
Equating cyanobacteria with algae. Cyanobacteria are oxygenic phototrophs but are prokaryotes; in most modern contexts, “algae” is reserved for eukaryotes.

Final Answer:
Eukaryotic organisms that possess chlorophyll a and perform oxygenic photosynthesis (O2-evolving).