Heteromita globosa: The Antarctic Sewer Scrubber

Heteromita globosa and its flagella

by AB

Antarctica is home to icebergs, penguins, and…soil flagellates? With average temperatures as low as -80°F and no trees, flowering plants or nutrients, it is difficult to understand how any microorganism could survive in such a cold environment.  Studies show, however, that Antarctica is home to a range of terrestrial microbiota called psychrophiles that can operate at extremely cold temperatures.  These microorganisms not only survive the cold temperature fluctuations, but they fulfill important functions such as maintaining other microbial populations and preventing biofilm build-up. According to H.G. Smith, the microbiota that exists in this environment consists of those that can adapt physiologically in order to survive, grow, and reproduce in unfavorable climates (1).

The “small (5-20 Ixm), nonpigmented bacteriophagous heterotroph” Heteromita globosa is the most prevalent protozoan soil flagellate in the fell-fields of Antarctica (See Figure 1) (2). H. globosa implements a unique form of feeding and locomotion in which its flagella produces currents to bring in essential nutrients and swim from place to place. While these psychrophiles perform optimally at temperatures as low as 1.5°C, cold temperature-induced encystment, or enclosure in a cyst form, occurs below 1.5°C and excystment, or breaking out of the cyst, occurs slightly above 1.5°C (See Figure 2). Due to the extreme temperature fluctuations and thermal periods in Antarctic terrestrial environments, H. globosa can readily adapt, allowing them to survive the freeze and thaw cycles (2).

Heteromita globosa in a cyst

The combined effort of this organism’s psychrophilic adaptations for excystment and encystment, ability to feed off nutrients brought in by the flagella, and mobility make it successful.  Compared to other microorganisms of the same environment, H. globosa confers “greater mobility, growth rates and synchrony of temperature-induced excystement at low sub-optimal temperatures”, or temperatures below 1.5°C (2). From a practical perspective, H. globosa is a huge consumer of bacteria in biofilm and microbial communities.  This trait makes it an important component of biogeochemical cycling in the ground waters of Antarctica as well as a source of carbon transfer and nutrient regeneration (3).

In order to conserve this cycle of encystment and excystment, H. globosa implements specific mechanisms to maintain the fastest growth while competing with similar microorganisms (3). In a study, H.G. Smith compared the population growth rates of H. globosa with the ciliated Protozoa Colpoda cucullus.  While C. cucullus has not been isolated in the Antarctic zone, like H. globosa, it can survive in extremely dry and cold temperature-fluctuating conditions.  When growing H. globosa at sub-optimal temperatures, it was found that the growth rate was slower in conditions favorable to other soil flagellates. Comparing these low temperatures to C. cucullus showed that this organism was not able to withstand the same temperature fluctuations as H. globosa. In fact, C. cucullus could not grow in temperatures below 5°C while H. globosa grew in temperatures slightly lower than 1.5°C. The fact that H. globosa was able to withstand these cold temperature fluctuations demonstrates its versatility as a microorganism. Compared to other microorganisms that are found in similar environments, H. globosa has the mechanisms to best adapt to the Antarctic environments, making it a microbial force to be reckoned with.

In addition to housing the mechanisms needed to survive the frigid seasons in Antarctica, H. globosa is a significant biofilm reducer in aquifers at cold temperatures.  As previously stated, protozoan soil flagellates are the primary consumers of bacteria not only in Antarctica but in several environments. Bacteria clog the tiny pores spaces in aquifers inhibiting sewage disposal, “microbe-enhanced oil recovery, groundwater recharge, and in situ bioremediation” (4). They form biofilms in these pores by expelling exopolymer slime, creating insoluble biogas, and gathering into a biomass.  H. globosa grows in contaminated aquifers along with bacteria, producing nutrients that “stimulate bacterial metabolism” in order to cause biodegradation (4). H. globosa reverses the effects of bacterial biofilms by grazing on these biofilms and increasing porosity and permeability in these aquifers. In doing this, they induce bioremediation, or removal of pollutants, inhibiting the biofilm-forming bacteria from clogging the pores.

These findings have prompted environmental researchers to consider H. globosa as an important mediator of biofilm reduction.  By studying the relationship between bacteria and protozoan soil flagellates like H. globosa, new trategies that increase the effectiveness of these aquifers in cold regions like Antarctica can be improved.  It has been found that the bacterivorous protozoa like H. globosa “coexist with bacteria [by] thriving on organism contaminants on the subsurface” (4). This method of feeding allows H. globosa to remineralize components that bacteria generate, causing biodegradation and destruction of biofilms produced.  This is a huge advantage to bioremediation projects including maintaining hydrolytic conductivity in aquifers that are normally clogged with bacterial biofilms.  By inducing aquifers with H. globosa as well as other protozoan soil flagellates, bacterial biofilms will be reduced, maintaining hydrolytic power and proper sewage disposal.

While it is difficult to understand why anything would prefer to inhabit extremely cold, temperature-fluctuating regions and live in sewage systems, evidence shows that H. globosa contains the microbial machinery to thrive in this type of environment. Its unique flagellar feeding mechanisms, psychrophilic behavior, and ability to degrade bacterial biofilms enable it to be the most prevalent protozoan soil flagellate in Antarctica. So while most Antarcticans will spend the winter season in front of the fireplace, H. globosa will be scrubbing the sewers.

References:

1. H.G. SMITH . 1996. Diversity of Antarctic terrestrial protozoa. Biodiversity and Conservation (5). [cited 2012 Nov 15] 1379-1394. Available from: http://www.springerlink.com/content/t2xr52876rl78j67/fulltext.pdf?MUD=MP
2. H. G. Smith a1, J. Hughes a1 and S. J. Moore a1 . 2004. Growth of Antarctic and temperate terrestrial Protozoa under fluctuating temperature regimes. Antarctic Science. [cited 2012 November 15] Volume 2 / Issue 04 /pp 313-320. Available from: http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=222941
3. Denis V. Tikhonenkov 1 and Yuri A. Mazei 2 . 2006. Heterotrophic flagellates from freshwater biotopes ofMatveev and Dolgii Islands (the Pechora Sea). Protistology (4) [Internet]. [**Last Updated**, cited 2012 November 15] (4), 327337. Available from: http://www.egf-penza.ru/kaf/zoo/mazei/pdf/32.pdf
4. Richard G. Mattison, Hironori Taki, and Shigeaki Harayama. 2002. The Bacterivorous Soil Flagellate Heteromita globosa Reduces Bacterial Clogging under Denitrifying Conditions in Sand-Filled Aquifer Columns. Applied and Environmental Microbiology. [cited 2012 November 15] 68(9): 4539–4545. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC124123/