Algae, a diverse group of photosynthetic eukaryotic organisms, grow in fresh and saltwater environments. These organisms range from microscopic phytoplankton floating in aquatic environments to large seaweeds adorning rocky shorelines. Algae contribute significantly to atmospheric oxygen, producing more than 50% of its total. Furthermore, algae's importance extends to environmental monitoring, predominantly in assessing water toxicity and pollution impacts. 

In this article, we will explain what algae are, providing a rigorous definition of algae and remarking on their multifaceted contributions to ecosystem health and environmental management.


Defining Algae: Types and Characteristics


What are Algae?

It is not easy to define algae, as they are informally categorized organisms that are not taxonomically unified but encompass various eukaryotic organisms with similar functional characteristics. Algae can range from unicellular microorganisms like phytoplankton and diatoms to large multicellular forms like seaweeds.

A generally accepted definition of algae refers to their photosynthetic functions, describing them as primarily aquatic organisms ranging from unicellular to multicellular forms. These organisms are found across diverse groups, mainly within the Plantae and Chromista kingdoms. Therefore, algae definition remarks their high ability to perform photosynthesis using chlorophyll despite the lack of complex structures such as leaves, roots, and stems. Instead, many algae have a thallus structure corresponding to the organism's entire body.


Types and Characteristics 

Algae are classified into several groups based on their habitat, pigmentation, type of chlorophyll, storage products, and cell wall composition. Major groups include Chlorophyta (green algae), Phaeophyta (brown algae), Rhodophyta (red algae), and Diatoms. Algae possess several unique characteristics that distinguish them from other plant-like organisms. Unlike land plants, algae lack many distinct cellular and tissue structures, such as stomata and vascular tissues. Still, despite their simplicity, they can inhabit various environments, including freshwater, marine, and terrestrial systems. Photosynthesis in algae uses multiple forms of chlorophyll, and some species can also utilize other pigments to capture light and transform it into energy.



The Importance of Algae in Environmental Health


Algae play a key role in global carbon cycling and oxygen production. Algae convert carbon dioxide into organic matter through photosynthesis, releasing oxygen as a by-product. This process contributes significantly to the oxygen content of the Earth’s atmosphere but also acts as a carbon sink, mitigating the impacts of CO2. Phytoplankton, in particular, are responsible for approximately 50% of the planet's oxygen production, highlighting the importance of algae in maintaining the atmospheric balance.

In aquatic ecosystems, algae are fundamental to food webs. They serve as primary producers, forming the base of the food chain and supporting a wide range of aquatic life, from zooplankton to fish and marine mammals. Algae's presence and abundance are vital for the biodiversity and productivity of marine and freshwater habitats. They create habitats and provide food resources for various species, influencing community structure and ecosystem dynamics. 

Due to these characteristics, changes in algal populations can reflect shifts in ecosystem balance. Their inherent sensitivity to changes in water quality, temperature, and chemical composition makes them exceptional bioindicators and environmental toxicity models for evaluating the impacts of pollutants. One prominent example of environmental toxicity assays is the Microplate Alga Growth Inhibition Test, a scaled-down, simplified, time and cost-effective version of the Freshwater Algae Growth Inhibition Test defined by the (OECD) Guideline 201 and ISO standard 8692, both using the Alga specie Pseudokirchneriella subcapitata. They utilize algae to establish the harmful effects of contaminants on aquatic ecosystems, evaluate the long-term environmental consequences of pollutants and facilitate the classification of chemicals based on toxicity levels.

Moving forward to regulatory acceptance, Biobide will offer the OECD TG 201 assay under Good Laboratory Practices, as well as the simplified miniaturized version of the alga growth inhibition test for pre-screening.

Simplified assays provide relevant information for rapid decision-making in the Early Development process. They allow a prescreening assessment, streamlining the tested compounds to select appropriate leads. OECD tests are required to comply with regulatory requirements. Leading compounds can be tested once the simplified assays have narrowed the list of candidates.

Environmental toxicity assessments often employ various model organisms, as the regulatory requirements ask for tests in different organisms as models of different environments. They usually include fish, like Zebrafish, Daphnia, and algae. Algae's characteristics for such testing include their simple structure, rapid reproduction rate, and ease of cultivation, representing a highly cost-effective option. These characteristics allow for indirect measurements of growth rates through cell counting, fluorescence, and optical density. Zebrafish and Daphnia show similar advantages and perfectly complement those algae assays, each showing specific sensitivities. 

These features make them the preferred model in aquatic toxicity tests, particularly those defined by regulatory standards and international guidelines adopted by numerous countries. These regulatory guidelines aim to ensure water safety for human consumption and environmental protection, underlining the need for increasingly effective measures to safeguard the environment.




Bellinger EG, Sigee DC. Freshwater Algae: Identification, Enumeration and Use as Bioindicators. 2nd ed. Hoboken, NJ: Wiley-Blackwell; 2015. 

Falkowski PG, Raven JA. Aquatic Photosynthesis. Princeton (NJ): Princeton University Press; 2007

Field CB, Behrenfeld MJ, Randerson JT, Falkowski P. Primary production of the biosphere: integrating terrestrial and oceanic components. Science. 1998 Jul 10;281(5374):237-40

Hoek C van den, Mann DG, Jahns HM. Algae: An Introduction to Phycology. Cambridge (UK): Cambridge University Press; 1995

Organization for Economic Co-operation and Development. Freshwater Algae Growth Inhibition Test with Unicellular Green Algae. OECD Guideline for the Testing of Chemicals, No. 201. [Internet]. Paris: OECD; 2011 [cited 22 April 2024]. Available from: