My introductory photosynthesis lessons (check for links on the ‘Basics’ page) were quite plant-focused. While plants themselves are a diverse group, they only represent a subset of the photosynthetic organisms on Earth. Welcome to the wider world of photosynthetic organisms. If you weren’t thinking much about plants until you found this blog, I guarantee the organisms highlighted in today’s post have been completely off your radar.
Algae: Like plants, these organisms are eukaryotic and their cells contain organelles (nucleus, mitochondria, chloroplasts, endoplasmic reiticulum, golgi bodies etc.). Photosynthesis in these organisms takes place in their chloroplasts just as in plants. Many are unicellular, but sometimes they can live in large colonies that may even look like plants.
Cyanobacteria: Prokaryotic organisms that can perform the same kind* of photosynthesis that plants do. Think of them as free-living chloroplasts. These cells do not contain the various organelles that plant and algal cells have. They contain a double outer membrane system and a separate internal thylakoid membrane system for photosynthesis. The cell sizes of these organisms can vary significantly, but some can be as small as 1 µm in diameter. (Eukaryotic cells, like those of plants and algae, can be 10 – 100 µm.) Cyanobacteria also use the Calvin cycle to fix carbon dioxide in the same way as plants and algae.
The organisms mentioned above, along with all plants, perform oxygenic photosynthesis, which has already been described in some detail on this blog. These organisms produce oxygen from their photosynthetic reactions because they use water as their electron donor. The rest of the bacterial photosynthetic organisms perform a different type of photosynthesis called anoxygenic photosynthesis. As the name implies, these organisms do not produce oxygen in their version of photosynthesis. They still use light to fix the carbon they use, but they use something other than water as their electron donor. There are four classes of anoxygenic photosynthetic bacteria: purple bacteria, green sulfur bacteria, green nonsulfur bacteria and heliobacteria.
Purple bacteria: These are metabolically versatile organisms that can grow photosynthetically or scavenge for their carbon. They can use a variety of compounds as their electron donor, just not water. Even though they don’t produce oxygen from their photosynthetic reactions, life isn’t a problem for them if oxygen is available or not. These organisms do not contain thylakoids. When environmental conditions favor a metabolic switch to a photosynthetic lifestyle, these guys start adding to their cytoplasmic membrane system which then folds in on itself to become intracytoplasmic membranes. Membranes are necessary for photosynthesis of all varieties because you need a barrier to create that proton gradient.
Green sulfur bacteria: These organisms have fewer metabolic options compared to the purple bacteria. They are strictly anaerobic (can’t survive when oxygen is around) and use hydrogen sulfide as their electron donor. They can fix carbon dioxide, but they don’t use the Calvin cycle that plants, algae and cyanobacteria use. Green sulfur bacteria perform their photosynthetic reactions at their cell (plasma) membrane, without any extra membrane alterations. The only flair these organisms allow for is a highly specialized light-harvesting antenna system called chlorosomes because these cells are often found growing at very low light intensities. This lifestyle seems overly limiting, but they are able to inhabit an environment (like the very bottom of a lake) that most other organisms cannot. It turns out that you don’t need every metabolic trick in the book if you are living in a niche without much competition.
Green nonsulfur bacteria: This group is not nearly as specialized as their sulfurous relatives. They can grow in the presence of oxygen and have a unique biochemical pathway for carbon fixation separate from the Calvin cycle and that of the green sulfur bacteria.
Heliobacteria: These are the only photosynthetic organisms that are Gram positive bacteria. This means they have a single cell membrane surrounded by a thick layer of peptidoglycan (kind of like a candy shell). This is opposed to all of the other prokaryotes mentioned here, which are Gram negative– meaning they are enclosed by a double membrane, with only a thin peptidoglycan layer and a separate out S-layer. The heliobacteria are the least characterized of the photosynthetic bacteria. These strict anaerobes are capable of photosynthetic activity, but it has not been shown that they can survive solely by their photosynthetic ability.
Today was just an overview of the wide world of photosynthesis. Each of these groups has a more elaborate back-story: specialized biochemical options, unique physical characteristics, strange black-sheep family members etc. There will be more to come later. The majority of these organisms are small in size, but collectively they still have large effects on global nutrient cycles. For example, when it comes to fixing carbon on a global scale, the contribution split between terrestrial plants and aquatic photosynthetic organisms is 55%/45%. Note that plants make up only about 1% of the aquatic photosynthetic biomass. The rest of that productivity comes from algae and the other photosynthetic prokaryotes.
Take home messages for the day:
Plants don’t have the monopoly on photosynthesis.
There are many ways to be photosynthetic.
Being small and metabolically weird doesn’t mean you’re insignificant in the grander scheme of things.
*Remember when I told you there were other forms of photosynthesis? I was talking about anoxygenic photosynthesis. Also, there are multiple ways of being anoyxgenic when it comes to photosynthesis, but there is only one way for photosynthesis to be oxygenic.
Blankenship, Molecular Mechanisms of Photosynthesis. 2002 Blackwell Publishing.
Falkowski and Raven. Aquatic Photosynthesis.2007 Princeton University Press.