Uncovering Seasonal Changes in the Algae Our Oceans Depend On

 

NSF Intern Emily Spiegel hybridizes RNA samples for analysis

Emily Spiegel, Bryn Mawr College

A study conducted in the Hollings Marine Laboratory under Dr. Peter Lee recently discovered evidence on the impact of the seasons on our oceans. Diatoms, highly productive and important photosynthetic algae, form the base of the marine food web. Their interaction with the wide range of environments they live in is incredibly interesting to marine biologists because so much may be garnered from these studies in the way of understanding the marine ecosystems at large.

This study focused on a polar diatom, F. cylindrus, and despite the harsh temperatures of its home habitat, this diatom is awesomely productive. It can form blooms under sea ice so thick, it looks like grass! Marine organisms feed on these blooms which allows the entire ecosystem to survive. The poles are subject to constant change: in temperature, in ice formation, even in light exposure. Since they are situated at the ends of the Earth, light ranges in the poles from continuous light to continuous darkness. How are photosynthetic organisms like F. cylindrus able to adapt to this stressful change? Their ability to produce is dependent on light levels: too much and these cells can be overwhelmed, too little and there may not be enough light to sustain the population.

Researchers found that at the low light exposure of polar autumn (6h light: 18h darkness), F. cylindrus begins to reproduce sexually, instead of asexually. This was found through analysis of RNA expression, which is an indicator for how much a certain gene is being transcribed into proteins to do work within the cell. Sexual reproduction leaves behind a trace in the RNA, based on the particular genes necessary to pull of this mode of reproduction. As opposed to the primary form of diatom reproduction (asexual), sexual reproduction conserves resources and produces fewer cells. So the population does not grow to the same extent as populations reproducing asexually, but it’s also able to survive in stressful conditions better than asexual populations.

Interestingly, scientists also found that stress could reduce the ability of F. cylindrus to remove carbon dioxide from the atmosphere, in a process known as carbon fixation. This could have major implications for our well the polar oceans remove CO2 from the atmosphere throughout the year. Could autumnal months in the poles show dramatically decreased carbon fixation rates? What does this mean for current global carbon models? Further research must be conducted in the poles themselves to determine whether this relationship exists in nature, and how it is affecting the carbon flux within the polar oceans.

This research was conducted in the lab of Dr. Peter Lee at Hollings Marine Laboratory in collaboration with the Medical University of South Carolina. Many thanks to all members of the lab, particularly Nicole Schanke, MSc.

Some Dramatic Microorganisms and Targeted Genetic Analysis

Emily Spiegel, Bryn Mawr College

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Genetic analysis has become the name of the game in many fields of biological research. Genes encode proteins, and in biology, proteins are king. Proteins guide biological pathways throughout the entire organism, so if you can track the genes, you can understand how the animal functions. Advances in technology like CRISPR, RNA sequencing, and PCR have improved the accessibility and accuracy of high-level genetic analysis in laboratories across the world. Some scientists utilize this technology to study the entire genome of an organism, while others attempt to understand the response of specific genes to various environmental factors or other external influences. This summer, I’m conducting an experiment focused on the latter. I’ll be studying how the polar algae species, Fragilariopsis cylindrus (affectionately known as Frag) copes with environmental stress by reproducing sexually. To do so, I’ll use targeted RNA sequencing to track genes related to sexual reproduction.

In order to understand how a Frag, responds to environmental stresses, you need a lot of algae. I reared nearly 100 liters of this algae in different artificial conditions. These conditions varied by two factors: photoperiod (the length of day and night), and nutrient levels. If you missed my previous post, “Stressing Out My Algae,” you should check it out for more details on the background for this experiment. We suspect that in conditions of stressful light energy (24 hours of continuous light), Frag will respond by reproducing sexually as opposed to its normal asexual mode of reproduction. This could possibly be a mechanism to rid itself of excess energy in times of stress, since sexual reproduction is more energetically expensive than asexual reproduction. By reproducing sexually, Frag may improve its chances of survival against this stress. Compounded with this is our hypothesis on nutrient deprivation. Previous experiments have shown that when a major nutrient, nitrogen, is limited, the algae cannot grow at full capacity and sexual reproduction is inhibited. We predict that when the stress of nitrogen limitation is combined with the stress of high light energy, we’ll see a reduction in the algae’s ability to survive in the stressful conditions due to the inhibition of sexual reproduction. So if we stress out the Frag enough and take away their ability to have sex, they’ll probably die. They’re some very dramatic microorganisms.

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24 bottles of algae were grown in six different experimental conditions varied by length of light exposure and nutrient levels. Algae was reared in 4-liter bottles filled with seawater.

So we grew our Frag, four bottles per six experimental conditions. Every day for eight days we extracted biomass from the bottle. From this sample we could test chlorophyll levels and cell counts, both of which give us a good idea of how well the algae in that bottle are growing in their conditions. We also took samples to be used for RNA extraction. Remember how genes encode proteins and proteins are king? Well before you can have your protein product, you need RNA. You’ve probably heard of DNA, which is the double stranded genetic cookbook. RNA is its single stranded offspring, which is then used as a the direct template to make proteins. A lot of genetic analysis therefore looks at RNA instead of DNA in order to understand how genes are being transcribed for protein production. We’re currently working on extracting the RNA from the original biomass sample and then running that pure RNA through a specialized machine called Nanostring. This is extremely targeted analysis, as Nanostring focuses in on the specific RNA we’re most interested in. In this case, we’re interested in RNA which is encoded from genes related to sexual reproduction. Using Nanostring will tell us how active the genes for sexual reproduction are in each bottle, which we can analyze to derive any correlation between our environmental stress factors and sexual reproduction.

If our hypothesis is correct, then we’ll see the greatest expression of sexual reproduction genes in the conditions of high light energy (24 hours of continuous light). We’d expect to also see low growth performance in nitrogen limited populations, indicated by low cell counts and chlorophyll levels. In these populations we predict we’ll see little if any expression of genes related to sexual reproduction. By the end, we’ll hopefully have a clearer picture of how phytoplankton like Frag deal with environmental stress.

Funding for this project is provided by the National Science Foundation in collaboration with the College of Charleston Grice Marine Laboratory and the National Oceanic and Atmospheric Administration. Acknowledgements to the entire lab of Dr. Ditullio and Dr. Lee in the Hollings Marine Laboratory facility.