2022 started with enforced isolation due to a positive Covid test, so my first post of the year has to be desk- rather than field-based, picking up my tour of the major algal groups. So far, I have covered Cyanobacteria (“Shuffling the pack …”), green algae (“The big pictures …”) and red algae (“Rhapsody in red …”). This post will deal with the last of the major groups of algae, the Chromista, the kingdom which includes the diatoms, chrysophytes, brown algae and yellow-green algae. Microscopic silica-encased diatoms and giant kelp may seem like unlikely relations, so read on …
Be prepared for a veritable porridge of terminology, reflecting the gradual evolution of ideas. From the point of view of someone who wants to understand how different groups of algae are related, all are more-or-less synonyms. However, each also has a precise usage, representing their place in a hierarchy that also includes non-algae. The Chromista (also known as SAR (“Stramenopile, Alveolates and Rhizaria”) is a Kingdom (akin to “plants”, “animals” or “fungi”), within which the algae are spread across four Divisions. Most of the algae that are found in freshwater benthic habitats (the main focus of this blog) are found in the Heterokontophyta (a subkingdom, also known as the Stramenopiles) and, more specifically, in the Ochrophyta (a superphylum). However, some of the other groups are very abundant in freshwater and marine plankton. All are thought to share a common ancestor, a red algal-type cell, although many have subsequently lost their chloroplasts.
A guide to the terminology of the Chromista.
|Chromista||Algae with chlorophyll c but not b, evolved from a secondary endoysmbiosis with a eukaryotic (red) alga, along with protists descended from these which have lost photosynthetic capability and plastids|
|Heterokonta||Organisms possessing two flagellae of different lengths for at least part of the life cycle (hetero = different; kontos = punting pole)|
|Stramenopiles||Organisms with a flagellum with short hair-like extensions. The term is derived from Latin words stramen (= straw) and pilus (= hair). However, I’ve also seen suggestions that stramen refers to the yellow-brown colour of the chloroplasts.|
|Ochrophyta||Refers to the colour of the cells, from a Greek rather than a Latin root (okhra = yellow).|
One further surprise before we go on is that the non-algal representatives of this group include Cryptosporidium, responsible for gastrointestinal illness, Plasmodium, the parasite that causes malaria, and Phytophthora infestans, which causes potato blight.
None of the groups in the subkingdom Hacrobia have been covered in this blog, mostly because they are not abundant in the habitats that I spend most of my time studying. I do see cryptophytes (“Cryptista”) but never in large numbers, and when I do they are invariably moving too fast for me to photograph. The Haptophyta include the Coccolithophores, which are very abundant in marine plankton, but not in freshwaters. The only time one has appeared in this blog is in a feature on the Hilda Canter-Lund competition in 2017 (see below).
Harosa, the other subkingdom, is divided into the Rhizaria, comprising non-photosynthetic amoebae-like organisms and the Halvaria. The Halvaria contains many important groups of algae. The first of these is the Alveolata, which includes the dinoflagellates, again mostly found in plankton, both marine and freshwater. The only mention in this blog (see table below) is not really typical of the group as a whole. Another major group is the Pseudofungi (which includes the organism responsible for potato blight) and the final group is the Heterokontophyta, of whose algal interest lies in the superphylum Ochrophyta.
There are four major groups of algae in the Ochrophyta of which the diatoms, Bacillariophyceae, are widely-covered in this blog, needing no extra explanation here. The Chrysophyceae have been encountered a few times but, as for some of the other Chromistan algae, much of their diversity is planktonic rather than benthic.
The best known representatives of the Phaeophyceae are the kelps and other brown seaweeds of marine littoral zones. I have written about these, but not much. Michiel Vos’ blog An Bollenssor is a good place to start exploring this group. There are freshwater representatives, but their only appearance in this blog was to record their likely extinction from the Bodensee. I was planning to write a post last summer but inadvertently deleted all my photos. They are probably more common in rivers than most people realise but are easily overlooked and, as a result, rarely recorded.
Finally, the Xanthophyceae, the yellow-green algae, include important freshwater representatives such as Vaucheria and Tribonema. They differ from many of the other algae in the Chromista in that they are typically green in colour, lacking the extra pigments that give many other representatives yellow, orange and brownish hues. The technical definition of the Chromista is that they contain chlorophyll c as well as chlorophyll a, and Xanthophyceae fulfil this criterion. However, many of the other Chromista also contain extra pigments (carotenoids and xanthophylls) which are responsible for these other colours. Xanthophyceae do have carotenoids and xanthophylls, but not the types or quantities that alter appearance. For a long time, as a result, they were classified with the green algae. That had a certain superficial logic until people started focussing on aspects of the organisms that were less obvious. The outcome is a more natural classification but problems when faced with a small greenish cell that doesn’t fit any descriptions in the “green algae” section of an identification guide. If you’ve managed to reach the end of this post, you’ll have realised by now that taxonomists and phylogeneticists don’t like to make life too easy for the rest of us.
Adl., S., Bass, D., Lane, C.E., Lukeš, J. et al. (2018). Revisions to the classification, nomenclature, and diversity of eukaryotes. Journal of Eukaryotic Microbiology 66: 4-119.
Derelle, R., López-García, P, Timpano, H. & Moreira, D. (2016). A phylogenetic framework to study the diversity and evolution of the Stramenopoiles (= Heterokonts). Molecular Biology and Evolution 33: 2890-2898. https://doi.org/10.1093/molbev/msw168
Martin, W.F., Garg, S. & Verena, Z. (2015). Endosymbiotic theories for eukaryotic origin. Philosophical Transactions of the Royal Society of London Series B 3702014033020140330 http://doi.org/10.1098/rstb.2014.0330
Ruggiero, M.A., Gordon, D.P., Orrell, T.M., Bailly, N., Bourgoin, T., Brusca, R.T., Cavalier-Smith, T., Guiry, M.G., Kirk, P.M. (2015). A higher level classification of all living organisms. PLOSone DOI:10.1371/journal.pone.0119248
Links to posts describing the major groups of Chromista found in freshwaters (and, in one case, marine habitats). Only the most recent posts are included, but these should contain links to older posts (you can also use the WordPress search engine to find older posts).
|Cryptista||The underwater world of Ennerdale Water …|
|Haptophyta||How to win the Hilda Canter-Lund Prize (4)|
|Alveolata||Invisible worlds at Malham Tarn|
|Bacillariophyceae||Too numerous to mention. Most recently:|
Every cloud has a silver lining …
Eyes wide open in Cassop’s muddy fringe …
|Chrysophyceae||The little tarn of horrors …|
Fade to grey …
A brief excursion to Norway
|Phaeophyceae||Depths of imagination …|
Swimming in a sea of ignorance …
|Xanthophyceae||When a green alga is not necessarily a green alga …|
The littoral ecology of Lough Down …