Spotlight on macronutrients : Carbon (mainly)

Whilst members of the Plant Kingdom might dominate terrestrial biomes, non-Plant Kingdom members are the major photosynthetic organisms in aquatic habitats
Image: Archie Portis/Wikimedia Commons.
Image: Archie Portis/Wikimedia Commons.

The ability of plants – and other plant-like organisms that aren’t in the Plant Kingdom (such as the Protists, algae and seaweeds, and the prokaryotic Moneran cyanobacteria) – to manufacture their own organic food from the simple inorganic materials carbon dioxide (C and O – two nutrients in one!) and water (H and O; ditto) using light energy in the process known as photosynthesis never ceases to amaze. But there are those who need to be reminded of how amazing photosynthesis is, and how fundamental green things are as the conduit by which energy is converted from a physical, electromagnetic form to a chemical form that is then available to the plants, and all those organisms who consume them (whether directly or indirectly).

One way we’ve often done this is to impress upon our students the importance of Rubisco, Ribulose-1,5-bisphosphate carboxylase-oxygenase, the enzyme that ‘catalyzes the primary chemical reaction by which inorganic carbon enters the biosphere’ in photosynthesis, with statements such as Rubisco is the ‘most abundant protein on Earth’. (And which represents a major sink for another essential nutrient – nitrogen (N), an essential component of amino acids from which such proteins are made…) Impressive, certainly, but is such a statement accurate? Well, examining that enzyme in single-celled marine algae, Jenna Losh et al. conclude that ‘unlike in plants, Rubisco does not account for a major fraction of cellular N in phytoplankton’ but constitutes less than 6% of total protein in those microalgae (cf. up to 50% in ‘plants’).

What might one conclude from this? We must try to avoid terrestrial bias in our plant biology! Whilst members of the Plant Kingdom might dominate terrestrial biomes, non-Plant Kingdom members are the major photosynthetic organisms in aquatic habitats, which occupy more than two-thirds of the Earth’s surface. Oh, and never ignore the small things!