Tag Archives: bacteria

Photosynthesis: The Breath of Life

large-leafPhotosynthesis is a chemical process, by which plants, algae and some bacteria convert solar energy into chemical energy. Basically, the organisms take in carbon dioxide and water, and use sunlight to make glucose, thus releasing oxygen as a by-product. The organisms are able to make their own food (glucose) by capturing sunlight. Other elements, such as nitrogen, phosphorous and magnesium are also needed to complete the process.

Sunlight provides the energy needed to transfer electrons from water molecules, an essential part of the process. The electrons are extracted from water molecules and passed along a chain, and finally forced onto carbon dioxide to make sugars. Photosynthetic organisms are called autotrophs. People and animals cannot photosynthesize; they are called heterotrophs. Today, plants are the most familiar form of autotrophs, but they were not the inventors of photosynthesis.

It Started Way Back

The evolution of photosynthesis is believed to have stared about 3.4 billion years ago. It was developed by primitive bacteria, first using elements such as hydrogen, sulfur and organic acids. These early bacteria manufactured food without using water, and did not produce oxygen (a process called anoxygenic photosynthesis). This was, however, an important development in the evolution of life.

algaeAround 2 billion years ago, a variant form of photosynthesis emerged. These bacteria lived in the ocean, used water as electron donors, and the release of oxygen into the atmosphere was the result (oxygenic photosynthesis). Photosynthesis was one of the most important developments in earth’s history. Learning to use sunlight to produce food took advantage of an endless supply of energy.

The introduction of free oxygen into the atmosphere was a game changer for all life. For some microbial life that existed then, oxygen was a toxin; this led to a significant extinction event. However, it opened up new opportunities for complex life to evolve, which it did. Oxygenating the atmosphere was an extremely slow process, as it took in the range of 1 billion years before complex life emerged.

How Did Plants Acquire the Ability to Photosynthesize?

Inside the cells of plants there is a separate compartment called the chloroplast. The chloroplast contains the green-colored pigment chlorophyll, which absorbs blue and red light. These are the wavelengths used in photosynthesis. The green wavelengths are deflected out, and that is why plants look green to us. In the chloroplast a number of complex interactions occur to produce food for the plant cell.

The chloroplast originated from primitive bacteria. In fact, in an earlier period chloroplast were bacteria, which eventually formed a symbiotic relationship with more complex cells. This relationship was the beginning of plant life on earth. It seems that the complex cells took advantage of the bacteria in order to get a free lunch. However, there was probably a benefit for the bacteria as well, perhaps the protection of an outer membrane or some other survival advantage.

The Ultimate Recycling Project

Photosynthetic organisms are the original source of oxygen and food for other life forms. If not for plants, there wouldn’t be any food for animals to eat. So the food chain in any ecosystem begins with photosynthesis. The food chain starts with plants, which are consumed by herbivores; the chain continues with carnivores and omnivores.

athmosphereThe earth’s atmosphere contains 20.95% oxygen and .039% carbon dioxide. The remainder is mostly nitrogen (78.09%). What concerns us here is the oxygen/carbon dioxide relationship. Most of the oxygen is provided by terrestrial green plants and microscopic phytoplankton in the ocean (they consume carbon dioxide and release oxygen). Non-photosynthetic organisms, like humans, animals and fish do the opposite (they breathe in oxygen and breathe out carbon dioxide).

Oxygen and carbon dioxide are continually recycled into the air. Life as a whole has evolved to function with the present mix of oxygen and carbon dioxide. What we don’t know is how delicate the balance is and if human carbon emissions will drastically change the balance. Scientists know that the atmosphere has changed significantly over evolutionary time; the result being the extinction of some species and the evolution of other species.

Given that life has created and maintained the atmosphere for billions of years, it will no doubt continue to do so. Life as a whole is safe. The questions for humans are: What are the long-term implications of releasing more carbon dioxide in the atmosphere? Which species will adapt successfully? And whether the atmosphere is changing in a direction that is less favorable for us?

 

References: In Our Time: Science, Photosynthesis, May 14, 2014.

livescience, What is Photosyhthesis? by Aparna Vidyasagar, July 31, 2015. http://www.livescience.com/51720-photosynthesis.html

Earth and Sky, How much do oceans add to world’s oxygen? June 8, 2015. http://earthsky.org/earth/how-much-do-oceans-add-to-worlds-oxygen


 

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The Evolutionary Arms Race

Evolution is guided by an intense competition for survival. When one individual or species gains an advantage, natural selection will cause competitors to catch up. Because there is always competition, over time species are pushed to improve, and a stable balance is generally established. It would be more economical for all to keep things as is, but that’s not how it works. Evolution requires change, and change is continual. An evolutionary arms race is an unavoidable consequence of evolution. To see how this works let’s look at a few examples:

The Tree Canopy

tree canopyHave you ever wondered why mature trees in a forest are roughly the same height. Given the fact that there is tremendous diversity in nature, why not have trees of various heights. Although different species can naturally grow to different heights, they are not found in the same forest environment. The reason is due to a race upward for sunlight. All the trees in a forest are competing for solar energy.

Forests could easily have been populated by low growing trees, at an energy cost savings for all. But nature has selected the trees that gained a competitive advantage (those that grew a little taller). The other trees were forced to keep pace or be left behind. Trees have evolved to grow higher because competing trees were also reaching for sunlight. Individual trees compete with their own species and also with other species. It does not matter whether it’s an individual or an entire species, those that cannot keep pace will not be successful at passing on their genes. In The Greatest Show on Earth, Richard Dawkins writes:

“In fact, what we actually see is a forest in which each tree species evolved through natural selection favouring individual trees that out-competed rival individual trees, whether of their own or another species.”

Of course, this was a slow process that was played out over evolutionary time and at the genetic level. Genes favorable for growing tall trees were passed on, because the trees that contained them were more likely to survive and seed the next generation. This competition continued for millions of years until an optimal height was achieved. There is a limit to the amount of energy a tree can divert towards growth, and a limit to the height a tree’s structure can support. Eventually the forest settled at a maximum height, when it was no longer an evolutionary advantage to grown higher.

Running Speed of Predator and Prey

predator and preyThe relationship between predator and prey is a complicated one, with each trying to outwit the other. Both predator and prey will evolve their own skill set. Some traits are specific for catching prey, while others are specific for avoiding predators. But there is an overlap as some traits are shared. For example, let’s consider the running speed of animals, which is just one of many skills needed by both sides. Running speed is valuable for both predator and prey.

Predators like cheetahs have evolved to run faster and faster, while gazelles (their prey), have also evolved to run faster. The end result being that neither gains ground. Richard Dawkins explains:

“Natural selection drives predator species to become ever better at catching prey, and it simultaneously drives prey species to become ever better at escaping them. Predators and prey are engaged in an evolutionary arms race, run in evolutionary time.”

Running speed is important, but it is part of a delicate balance with other important traits: such as endurance, strength and eyesight. The evolutionary winners will be those that get the balance right, yet running speed will be in the mix. Although less obvious, it is just as important for an individual to outrun individuals from the same species. For instance, a gazelle which runs slightly faster than the average gazelle will escape the predator at the expense of the slower gazelles. The fastest gazelle in the herd will be favored just as much as the overall speed of the herd.

Bacteria, Viruses and Human Defenses

In the two examples listed above (the tree canopy and the running speed of predator and prey), evolution acted at the subconscious level. No conscious agent designed any particular trait. The arms race was fought by natural selection. Because of our knowledge of viruses and bacteria, another level of the arms race is added. That is, the production of vaccines and antibiotics.

The Influenza Virus and Vaccines: In 1918 the Spanish Flu was responsible for the death of about 50 million people (the worst pandemic in world history). The pandemic struck in the last year of World War 1. The world war was critical in spreading the disease as masses of soldiers moved across the globe. The poor living conditions and ill-health of the solders may also have contributed. Pandemics have reoccurred throughout history, and experts caution that it could happen again. Different strains of the flu still come around every year. Today, much progress has been made in developing vaccines, which along with sanitation is our best defense against viruses.

influenza virusViruses attack the human body by invading cells. The immune system produces antibodies that fight off the virus. Therefore, a specific virus can only infect a host body once. However, viruses evolve very fast and are inaccurate replicators. As a result they evolve into different strains that can evade the human immune system. Viruses are not trying to change; they change because of chance mutations. The ones that are resistant to antibodies populate. It may appear that viruses are attempting to outsmart the immune system, but they simply evolve through the process of natural selection in their environment.

An arms race between viruses and their host is ongoing. Each year experts predict which strains of influenza will be dominant, and they product vaccines in accordance. It is an educated guest, which sometimes they get right and sometimes they don’t. Except for particularly dangerous strains, such as the H1N1 pandemic in 2009, it is debatable whether wide-scale vaccinations for the flu are effective or necessary.

The arms race will surely continue. We have the natural competition between viruses and antibodies, and the additional armament of vaccines. The speed in which viruses replicate and evolve insures that they are here to stay. Humans are faced with every-changing viruses, which we have to keep pace.

Bacteria and Antibiotics: Common bacterial infections for today’s standards were often fatal in the past, but thanks to antibiotics are now easily treatable. Antibiotics can kill bacteria inside a human body, however it kills good bacteria as well as bad bacteria. Good and bad are subjective descriptions based on their influence on humans. Bacteria are single cell organisms that have evolved to live in symbiotic relationships with humans (bacteria cells in the body outnumber human cells). The beneficial bacteria¬†will defend its turf against invading bacteria, and thus can be considered as part of the immune system.

bacteriaAlthough the antibiotics are engineered to target the invaders, they are not perfect and they kill some of the symbiotic bacteria. Because antibiotics have been widely used, this has changed the balance of bacteria that dwell in humans. The effects of these changes is not clear, but there is evidence it can contribute to some diseases.

When an antibiotic is used to treat an infection it does not kill all the invading bacteria. And similar to viruses, the bacteria develop resistance to the antibiotic. The surviving bacteria will multiply and evolve until that specific antibiotic becomes ineffective. New antibiotics need to be developed in order to keep up. Even though a patient is cured, surviving bacteria can still spread to other people. Once again, the battle is similar. Unwanted bacteria verses beneficial bacteria and human ingenuity. The arms race is on with no end in sight.

 

References: Richard Dawkins, The Greatest Show on Earth (New York: Free Press, 2009), 380, 381.

Big Picture Science, Skeptic Check: Evolutionary Arms Race (June 22, 2015).