Diatom of the Month – September 2017: Rhabdonema adriaticum

by Shelly Wu*

Figure 1. Mount Hope Bay in Bristol, Rhode Island (June 2017).

I am a poor grad student who likes to sample diatoms for fun. During my summer teaching job in Rhode Island, I went the grocery store and assembled a cheap, algae sampling kit with plastic travel bottles, spoons, a notebook, a permanent marker, and a Ziploc bag (Fig. 1A). I also used the iNaturalist app to obtain GPS coordinates. If you are on a budget, you can do it too and sample anywhere! 

How did I find the Diatom of the Month?

My exploration along the Mount Hope Bay in Rhode Island began with lifting up a rock to find an Asian Shore Crab (Fig. 1B). It got crabby with me so I gently put the rock back. From a distance, the surrounding rocks near the shoreline were covered with dried, hairy-looking algae and I joked with my friends that the rocks needed to shave (Fig. 1C). I closely inspected the rocks and realized that the dried algae were Ulva lactuca, also known as sea lettuce (Fig. 1D). I wanted to know what diatoms could be living on the sea lettuce so I grabbed a fresh sample. As I scanned the sample later, Rhabdonema adriaticum caught my attention with its two alien-like ocelli on each end, where a diatom secretes mucilage (Fig. 1E). On the side view, also known as girdle view, several of these diatoms attached to form a beautiful, ribbon-like colony (Fig. 1F). Kützing’s original description shows the drawings in girdle view (Fig. 2).

Figure 2. Original Description for Rhabdonema adriaticum by Friedrich Kützing in 1844.1 
(Image Credit: F. Kützing).

Crash Course on Diatom Sex Education
(Note: The following information is an oversimplification of the process to make it informative)

Rhabdonema is special when it comes to diatom sex. First, I need to fill you in on the ‘dirty’ details of diatom life histories. When new life begins for a diatom, it will divide over time with mitosis, in which one diatom cell will produce two daughter cells, and those two daughter cells will produce four cells and so on (Fig. 3). As time progresses with each division, the diatom cells become smaller and smaller in size, to the point that they will reach a critical size threshold. Then, imagine one diatom ‘having the hots’ for another diatom and saying “I have noticed we have reached our minimum size,” code word for sex  (thanks Iowa Lakeside Lab diatomists for the pickup line). Sexual reproduction is a strategy for diatoms to restore their original size. Depending on the diatom species, there are different ways for sexual reproduction to occur, but the key idea is that gametes will fertilize to form a zygote called an auxospore.2 The auxospore will become a large, happy diatom and divide mitotically (Fig. 3). In fact, without sexual reproduction, diatoms would eventually go to diatom heaven (Fig. 4).

Figure 3. Diatom life history3 (Image Credit: Minnesota Sea Grant).

Figure 4. Rest in peace homies (Image Credit: SimRiver).

Diatoms are broadly grouped into three strategies of sexual reproduction (Fig. 5).
·        The first strategy is Oogamy in Centric diatoms in which a flagellated sperm fertilizes a non-motile egg.
·       The second strategy is Isogamy (= similar gametes) in Araphid and Biraphid diatoms, in which non-flagellated gametes fuse in an amoeboid-like manner.
·       An exception to this is Rhabdonema, an Araphid diatom that does not sexually reproduce this way. What is up with that?! Rhabdonema uses a third strategy called Modified Oogamy in which an amoeboid-like sperm fertilizes a non-motile egg.
If we look at the evolutionary hypothesis for diatom sex, Rhabdonema is thought to be a ‘transitional state’ between Oogamy and Isogamy.4,5 Super cool! 

Figure 5. Phylogeny of Sexual Reproduction4 (Modified from Edlund and Stoermer, 1997; Image Credit: Diatoms of the United States).

So why learn about diatom sex? Because we still don’t know much about it and it has only been documented in about 1% of all known species.4 Future research remains to be done to reveal reproduction mechanisms and the environmental conditions conducive to diatom sex.4 It is a rare phenomenon to observe so you better believe that diatomists get pretty damn excited when we see it happen. This past summer, the diatom class at Iowa Lakeside Lab scraped diatoms off a buoy and we spied on the diatoms in action!

Why should we care about this diatom?
Species of Rhabdonema appears to be present and increase its abundance in response to algal blooms, although I caution that the information on this is sparse!!! In 1977, a dinoflagellate bloom appeared on the coast of South Kerala (India) and the phytoplankton that were present during the bloom included Rhabdonema sp.; however, its abundance was not as high as some other taxa such as Skeletonema costatum.6 In 2013, Rhabdonema adriaticum was found in high abundance in relation to the algal bloom in Biscayne Bay. Even though it is non-toxic, diatom blooms can create ecological problems, nicely illustrated below by Anna Wachnicka (Fig. 6).

Figure 6. Ecological Consequences of non-toxic microalgal blooms. 
(Image Modified from: Diatom Blooms in Biscayne Bay).7

Please don’t hate this diatom, though. It is not intentionally trying to ruin ecosystems and harm us. Algae tend to get a bad reputation and obviously blooms are not an ideal situation. However, I think humans need to reflect upon how we are contributing to these environmental problems, while all algae, including diatoms, are our environmental sentinels telling us how ecosystems are impacted.

*Andrews Institute of Mathematics and Science Education, Texas Christian University, Fort Worth, TX 76129


1. Kützing, F.T. (1844). Die Kieselschaligen Bacillarien oder Diatomeen. pp. [i-vii], [1]-152, pls 1-30. Nordhausen: zu finden bei W. Köhne.
      2. Smol, J. P., & Stoermer, E. F. (Eds.). (2010). The diatoms: applications for the environmental and earth sciences. Cambridge University Press.
      3. Image Retrieved from: http://www.seagrant.umn.edu/newsletter/2013/01/what_good_is_a_diatom.html

4. Edlund, M. B., & Stoermer, E. F. (1997). Ecological, evolutionary, and systematic significance of diatom life histories. Journal of phycology, 33(6), 897-918.
      5. Mann, D. G. (1993). Patterns of sexual reproduction in diatoms. In Twelfth International Diatom Symposium (pp. 11-20). Springer, Dordrecht.
      6. Venugopal, P., Haridas, P., Madhupratap, M., & Rao, T. S. S. (1979). Incidence of red water along South Kerala Coast. Indian Journal of Marine Sciences, 8, 94-97.


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