Cryptomonad

• 1. Introduction

  ›

  Cryptomonads are single-celled organisms so small that dozens could fit across the width of a human hair, yet they carry inside them one of the strangest evolutionary secrets in all of biology. Each cell runs between 10 and 50 micrometres long, flattened like a tiny leaf, with a groove or pocket near its front end and a pair of slightly unequal whip-like flagella beating at its rim. In freshwater ponds, in coastal seas, in brackish estuaries, these organisms are quietly everywhere. But what makes them remarkable is not their abundance. It is what lives inside them. Hidden between the inner membranes of their chloroplasts sits a shrunken cell nucleus called a nucleomorph, the remnant of an ancient red alga that was swallowed whole and never fully digested. How a eukaryote ends up inside another eukaryote, and what happens over deep time when it does, is the question that makes cryptomonads one of the most studied puzzles in the science of cell evolution.

• 2. Anatomy of a Tiny Cell

  ›

  Christian Gottfried Ehrenberg first noted cryptomonads in 1831 while studying Infusoria, though he would not have had the tools to see what was truly going on inside them. The cell body is wrapped not in an ordinary membrane but in a periplast, a cryptophyte-specific outer covering. Beneath it, and associated with the anterior pocket, sit structures called ejectisomes: two connected spiral ribbons held under tension like a coiled spring. When the cell is disturbed by mechanical force, chemical stress, or a sudden change in light, those ribbons fire. The discharge propels the cell away in a zig-zag course, a microscopic escape manoeuvre. Larger ejectisomes sit near the pocket and are visible under a standard light microscope; smaller ones are distributed beneath the periplast across the cell body. The flagella themselves are not bare whips. They are covered in bipartite hairs called mastigonemes, assembled inside the endoplasmic reticulum and carried to the surface, with small scales sometimes present on both the flagella and the cell body.

• 3. The Stolen Alga Inside

  ›

  Most cryptomonads carry one or two chloroplasts that can appear brown, red, or bluish-green depending on their pigment mix. Each chloroplast is wrapped in four membranes rather than the two found in most algae, and between the middle two membranes sits the nucleomorph. Genetic studies confirmed what the structure implied: the plastid originally belonged to a red alga that was engulfed by an ancestral cryptomonad host in a process called secondary endosymbiosis. The plastids, however, are strikingly different from those of living red algae. Phycobiliproteins are present, but only inside the thylakoid lumen, and only in the forms phycoerythrin or phycocyanin. In the genus Rhodomonas, researchers have resolved the crystal structure of a phycobiliprotein to 1.63 angstroms, and found that the alpha subunit bears no relation to any other known phycobiliprotein. Meanwhile, Cryptomonas paramecium, once called Chilomonas paramecium, has leucoplasts rather than photosynthetic chloroplasts, and the class Goniomonadea lacks plastids entirely, an arrangement that supports the view that the cryptomonad lineage was originally heterotrophic and only later acquired the ability to photosynthesize.

• 4. Life Strategies and Survival

  ›

  Some Cryptomonas species can shift into a palmelloid stage, clustering together inside a shared mucus, only to dissolve back into free-living flagellates when conditions improve. Certain species go further still, forming immotile cysts with rigid cell walls to ride out periods of stress. Mitosis in cryptomonads is open, meaning the nuclear envelope breaks down during cell division. Sexual reproduction has also been documented, though it is not universal across the group. A particularly striking strategy is mixotrophy: some cryptomonads can both photosynthesize and consume organic matter, giving them flexibility when light or nutrients are scarce. The flat cristae of their mitochondria link them structurally to other groups in the tree of life, a detail that has been central to debates about how the major algal lineages relate to one another.

• 5. Contested Relatives

  ›

  Botanists historically placed cryptomonads in the class Cryptophyceae or the division Cryptophyta; zoologists treated the same organisms as the flagellate protozoan order Cryptomonadina. For a time, their pigmentation looked similar enough to that of dinoflagellates that some classifications grouped both lineages together as the Pyrrhophyta. Cavalier-Smith later united cryptomonads with the heterokonts and haptophytes as the Chromista, based on the similarity of their chloroplasts. That proposal ran into trouble when ultrastructural analysis revealed deep differences in overall cell organisation among the three groups, suggesting the three major chromist lineages had each acquired plastids independently and that the grouping was polyphyletic. Molecular work by Parfrey and colleagues, and separately by Burki and colleagues, placed Cryptophyceae instead as a sister clade to the green algae, or to green algae combined with glaucophytes. The sister group to the entire cryptomonad superclass is likely the kathablepharids, flagellates that share the ejecting ejectisomes but do not have plastids. Genetic studies dating back to 1994 supported the placement of Goniomonas as sister to Cryptophyceae within the superclass, helping anchor what had long been a structurally defined grouping onto a molecular foundation.