Ehux: The Lit­tle Eu­kary­ote with a Big His­tory

by Jaime E. Zla­mal

There is an or­gan­ism out there that is smaller than you, older than you, and that has a house that's prob­a­bly fancier than yours. Meet Emil­ia­nia hux­leyi, or Ehux. Ehux is a minis­cule coc­col­ithophore, a beau­ti­ful sin­gle-celled alga that doesn't get enough at­ten­tion in the world of mi­cro­bi­ol­ogy. The ten­dency of bi­ol­o­gists to ig­nore this phy­to­plank­ton species be­lies its sig­nif­i­cance. In fact, Ehux has a pen­chant for be­ing flashy—it has phe­nom­e­nally beau­ti­ful plates cov­er­ing it, known as coc­col­iths, in­tri­cate com­pos­ites of cal­cium car­bon­ate that when viewed un­der the mi­cro­scope ri­val the most won­drous sculp­tures made by man. And they ex­ist in huge quan­ti­ties. The White Cliffs of Dover wouldn't be so large, or for that mat­ter, white, if it weren't for this tiny ar­chi­tect. Ehux makes blooms near the sur­face of the oceans that are so big they can be seen from space! And it has its own web page.

Coc­col­ithovirus, a gi­ant dou­ble-stranded DNA virus, in­fects Ehux. The virus (pink) was first ob­served in 1999 by W.H. Wil­son and was found to be a "gi­ant-virus" hav­ing 472 pro­tein-cod­ing genes. Source

These blooms can have quite an eco­log­i­cal im­pact. With the sud­den ex­plo­sion of Ehux pop­u­la­tions, the sul­fur cy­cle can be af­fected dra­mat­i­cally, as these coc­col­ithophores pro­duce di­methyl­sul­fo­nio­pro­pri­on­ate (DMSP), which turns into the volatile, cloud-in­duc­ing, di­methyl sul­fide (DMS). Like­wise, the sheer den­sity of this phy­to­plank­ton tends to phys­i­cally block sun­light and cause dark­en­ing be­low. Both of these con­di­tions, along with an up­surge in cal­cium car­bon­ate falling to the sed­i­ment when Ehux blooms col­lapse, can sig­nif­i­cantly af­fect the ma­rine ecosys­tem. The cause of such col­lapses is in­fec­tion by viruses.

The in­fec­tious cy­cle of Ehux viruses. Source

A re­cent pa­per by Mar­tinez et al. probed fur­ther into how viruses cause Ehux blooms to crash. Bloom com­mu­ni­ties of both the phy­to­plank­ton and the viruses are dy­namic; the re­searchers found that as part of the ac­tive in­ter­ac­tions be­tween the host and viruses, the pres­ence of dif­fer­ent viruses and tim­ing of their in­flux can heav­ily in­flu­ence the bloom of Ehux. Mul­ti­ple ge­netic sig­na­tures of both Ehux and the viruses were found, re­veal­ing that both or­gan­isms change across the land­scape over time, rac­ing against each other for sur­vival. Frada and col­leagues looked into the meth­ods used by Ehux to avoid vi­ral an­ni­hi­la­tion, find­ing that the or­gan­ism can switch from a diploid to a virus-re­sis­tant hap­loid stage, be­com­ing in­vis­i­ble to viruses in the process. They called this a "Cheshire Cat" strat­egy. (See also a pre­vi­ous post on this sub­ject.)

Two Ehux cells: (left) a healthy cell; (right) a cell full of viruses. Source

Viruses are not new to Ehux. Rather, it ap­pears that they have played a sub­stan­tial role in the coccolithophore's his­tory for at least the past 7000 years. Coolen used ge­netic sig­na­tures to re­con­struct the his­tory of Ehux and the DNA viruses that in­fect it, specif­i­cally Coc­col­ithovirus.

Coc­col­ithophore skele­tons in sea floor sed­i­ments con­tain resid­ual DNA from both the host and the viruses that can per­sist for thou­sands of years. The ge­netic tech­niques used in this study added to ex­ist­ing mor­pho­log­i­cal and chem­i­cal in­for­ma­tion on Ehux com­mu­ni­ties and pro­vided di­rect ev­i­dence on fluc­tu­a­tions in coc­col­ithophore pop­u­la­tion struc­ture. Geno­typic vari­a­tion cor­re­lated with large en­vi­ron­men­tal changes, in­clud­ing those caused by changes in wa­ter salin­ity. The size of the DNA am­pli­fied by PCR was lim­ited by the degra­da­tion of DNA frag­ments over time; how­ever, vi­ral DNA re­cov­ered from sam­ples dat­ing from 7300 years ago showed re­mark­able sim­i­lar­ity to that of mod­ern Coc­col­ithovirus. This virus is a mem­ber of the Phy­cod­naviri­dae, which are part of theNu­cleo-Cy­to­plas­mic Large DNA Viruses (NCLDV), a group that in­cludes the gi­ant Mimivirus and its rel­a­tives. The Coc­col­ithovirus has a com­plex evo­lu­tion­ary his­tory. Gene se­quenc­ing has shown that in in­fec­tion, parts of its genome are tran­scribed in the host cy­to­plasm, un­like other viruses in the group that are tran­scribed in the nu­cleus. It is prob­a­ble that some of these un­usual genes were do­nated by an un­known or­gan­ism fol­low­ing the evo­lu­tion­ary di­ver­gence of this vi­ral species from the rest of the group.

Ehux bloom south of Corn­wall (UK) from space. Source

Po­ten­tially im­pact­ing the fields of vi­rol­ogy, en­vi­ron­men­tal sci­ence, and many in be­tween, E. hux­leyi is a wor­thy sub­ject of study for mi­cro- and mac­ro­bi­ol­o­gists alike. With re­cent pub­li­ca­tions link­ing it to vi­ral ecol­ogy, phar­ma­col­ogy, and even op­to­elec­tron­ics, it seems that any­one with cu­rios­ity to­wards strange, sin­gle-celled eu­kary­otes would en­joy tak­ing a poke or two at Ehux. The en­vi­ron­men­tal im­pact of Ehux prac­ti­cally begs for a global warm­ing study, and I'd bet a re­searcher cur­rently study­ing bio­fuel-pro­duc­ing al­gae could find in­spi­ra­tion in the long-chain lipids pro­duced by this coc­col­ithophore.

Jaime is a stu­dent in the Uni­ver­sity of Cal­i­for­nia at San Diego/San Diego State Uni­ver­sity In­te­gra­tive Mi­cro­bi­ol­ogy grad­u­ate course.