2.6: Calupera - A Large Coenocytic Green Algae

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Caulerpa, another example of green algal diversity

Caulerpa is a large green algae that appears to be multicellular because it is organized into different parts, seemingly leaves, stems and roots. But it is actually just a single large cell. And since an individual organism might be two meters in extent, Caulerpa produces the largest cells on earth, except for maybe some plasmodial slime molds. They are mostly found in shallow waters in warmer oceans but a few occur in fresh water.

Five dark green blades (one which has a discolored white tip) of Caulerpa of varying sizes linked by one thin, dark brown stolon which are fixed to the sandy substrate by rhizoids — *Caulerpa prolifera*

Phylogeny and Taxonomy

Caulerpa is in the green algal phylum, in a group generally considered to be a class labelled the Ulvophyceae, a group not considered to be closely related to land plants although other green algae are.

Structure

Caulerpa is siphonaceous (coenocytic) meaning that the cells are multinucleate and specifically in this case there is only a single cell per organism. They typically have a horizontally running structure off of which come extensions to attach it to the substrate ( 'roots' ) and extensions to increase photosynthetic area. Depending on species, these may be blade-like, feather-like or spherical.

Sex and Reproduction

Most commonly Caulerpa reproduces asexually by fragmentation but it is also capable of sexual reproduction, although the details are not completely known. The algae sometimes concentrate their protoplasm in the tips of blades/leaves where it becomes cellular and is released as uninucleate gametes while the rest of the plant senesces and dies. The released gametes are capable of fusing with each other. At this point the knowledge of the life cycle falters. Meiosis must occur at some point but it is not known if it occurs right after syngamy, right before the production of gametes or perhaps that there are both haploid and diploid plants that look alike (cf. Ulva, see chapter 13).

An underwater image of several growths of the dark green thalus with feather-like fronds arising from one stolon, they are upright and branched though some are damaged and bend slightly. — *Caulerpa sertularioides*

Matter and Energy

Caulerpa is a typical photosynthetic autotroph, acquiring carbon as bicarbonate ion (HCO₃^–) which is produced when carbon dioxide reacts with water to form carbonic acid which then ionizes. This is used to produce hexose sugars that are used both for material needs (to make cell walls, cell membranes, amino acids, etc) and also for energetic needs when the hexose are oxidized in oxidative phosphorylation. Caulerpa acquires 14 other elements obtained by absorbing small ions from the water.

Interactions

Caulerpa oxygenates the environment it is in and serves as a food source for a number of animals. However, it does produce toxins. Like the toxins associated with red tides and produced both by dinoflagellates and cyanobacteria, some toxins affect certain herbivores while others herbivores are unaffected but may accumulate the toxin, thereby allowing it to affect consumers of the herbivore. Caulerpa is native to warm waters around New Zealand and Australia but is invasive in parts of the Mediterranean and off southern California, where it is sometimes causes a variety of problems. It's spread to new areas may be partially due to its use in aquaria and their disposal in nearby waters.