Time to Think Small

Anjani Ganase, marine scientist, proposes a different way of looking at the world. “To think big, we first need to see the small things,” she says, as she looks into the ocean.

What is the most abundant group of organisms in the oceans, and why do they matter?

When I first started my degree in Marine Biology, I - like many others – was excited to begin exploring the world of fish, crabs, critters, whales and even sharks. To my surprise, we had to begin with the smallest of things, the foundation of all ecosystems, understanding the microbial universe. Microscopic organisms, although we can’t see them, are crucial for life and are constantly providing us with all essentials nutrients and removing our waste, they are the cogs in the wheel of life. Microorganisms are invisible to the naked eye and typically constituted of single-celled entities. Furthermore microorganisms are everywhere, yes everywhere! They can be found in deep scalding undersea volcanic vents and even on the surface of your skin. In the open ocean, in one drop of water (1 ml) there are up to one million microbes and this number increases closer to the coastlines, where resources become more available (Glockner et al. 2012). Remember the last time you stared at the crystal clear waters at the beach? Next time try to imagine the bustling microbial universe that is swimming in that water. Although we generally think of microorganisms as disease causing bacteria and viruses, since it is the only time they draw attention to themselves, the majority of microbes are not harmful but in fact helpful. Microbes are composed of organisms from all the kingdoms of life - plants, animals, fungi, bacteria, protists and archaea, and are crucial for the proper functioning of our ecosystems and the stability of our climate.

Cycling of essential elements
It was once thought that the terrestrial plants were the sole providers of oxygen of the world through the process of photosynthesis (the conversion of carbon dioxide and water to sugar using light energy), and replacing the oxygen consumed by animals in respiration. However, we now know that the plants of the ocean, in particular phytoplankton, microscopic single celled plants that live in in the water column near the sunlit surface of the oceans, contribute to half of the supply of the world’s oxygen through the same process (Roach 2004). The combined productivity of both phytoplankton and plants is what maintains our atmosphere at 20 % oxygen, but this is only one of three ways we benefit from photosynthesis. The end product of photosynthesis is sugar, which is incorporated in the biomass of plants and thus forms the foundation of all marine food webs, which is vital for our fisheries. The final benefit of photosynthesis is the removal of carbon dioxide from the atmosphere and converting and storing it as usable organic matter. This is merely one group of microbes.

What are some other processes of marine microbes that are useful to us? Death and decay is another process. Microbes break down and recycle dead organic matter, from dead phytoplankton to dead whales, making it available for other communities. Other microbes make other elements available to be utilised by marine organisms. The element nitrogen is an essential element in DNA and proteins. However, nitrogen on its own is not useful to plants. Nitrogen-fixing bacteria, typically blue-green algae also known as cyanobacteria, converts nitrogen into a useable form by binding it to other elements, which can then be taken in by plants and other bacteria. An example of cyanobacteria in the marine environment is the Trichodesmium species, which play a major role in the marine environment. Trichodesmium species cluster to form large colonial strands that extend hundreds of metres across the surface of tropical waters and can even be seen from space.

Close up of coral colours made up of the symbiotic microscopic zooxanthellae. Photo by Anjani Ganase

Symbiosis
Microorganisms can be free-living, drifting along in the water column or can also be found on the surface of things on the benthos. This is where microbes form the most fascinating symbiotic relationships. The most famous of all is the symbiosis between the microscopic algae, zooxanthellae and corals. Corals are animals similar to the jellyfish; instead of drifting in the water column, they settle in the shallow benthos and build a calcium carbonate skeleton. It is the zooxanthellae that live within the tissues of the coral that gives it, its colour. The blue, pink and brown colours that we see are all because of the microscopic plants residing in the coral. Zooxanthellae feed the coral by supplying it with food from photosynthesis, and in return gets a safe environment to grow and colonise. Unfortunately, when this relationship breaks down - say when the environment around the zooxanthellae becomes too toxic in the coral tissues because of warm temperature - the algae leaves the tissue of the coral, the corals turn white and the coral starves; this is known as coral bleaching. There are arrays of symbioses that occur between invertebrates and microbes. Filter feeders, marine sponges are one other example of invertebrates that host a range of symbiotic bacteria that take advantage of the filter feeding process; many of the relationships between sponges and their symbionts are still being assessed.


Red tide bioluminescence on the New Jersey shore. Photo by catalano82 (https://www.flickr.com/people/95165469@N00/) under the creative commons license (Attribution 2.0)

Bioluminescence
One way we can see the beauty in the little creatures that live in our coastal water is through their stunning light shows on a dark night.  The flashes of electric light we see on the surface of the water as it is disturbed by wave breaks or the movement of a paddle is bioluminescence. This is an oxidative process (a reaction with oxygen) carried out by marine bacteria. Some marine animals have learned to use this trait of the bacteria for their own purposes. They incorporate the bacteria in the bodies, and stimulate the reaction when needed. Predators use the mesmerizing light to lure prey, while prey can use the flashing lights to confuse a predator while making an escape. The light can even be used to attract a mate if the light show is impressive enough. Scientists have only scratched the surface of the diverse, functional roles carried out by the microbes of the ocean. With technology, we may actually see into their world and perhaps we can use what we find to advance our own technology.

Climate Change
The variations in the microbial communities in the ocean can also be used as an indicator of a changing climate. For several years, scientists have been tracking changes in the composition and productivity of surface microbial communities as a result of warming sea surface temperatures through satellites. As the surface of the ocean warms this results in a layer of less dense warm water in the tropics that floats over the cooler water creating a stratification or partitioning. We have often experienced this on a small scale when we go swimming along beaches and our lower body hits a pocket of cold below. This stratification acts as a barrier to the tiny organisms in the water column. Particularly, the phytoplankton that floats near the surface to photosynthesise cannot access the essential nutrients from the deeper colder water to maintain productivity (Ducklow et al. 2010). More persistent and larger warm water bodies occurring in the tropics may result in an overall further decline in phytoplankton productivity. This is bad news for a system that is significant in removing carbon dioxide from the atmosphere. Furthermore reduced productivity of the phytoplankton means limited resources for the marine communities that rely on this for food. Without these microbial communities, the climate and food systems suffer. This is one of many ways in which climate change can affect marine microbial communities.

To maintain life in the ocean, and on the earth – as we know it – we must think of an interconnected web of living organisms. The smallest are as important as the biggest. To put it another way, the big depends on the survival of the small.

References:
Ducklow, H. W., Morán, X. A. G., & Murray, A. E. (2010). Bacteria in the greenhouse: marine microbes and climate change. Environmental microbiology. Wiley-Blackwell [doi: 10.1002/9780470495117. ch1], 1-31.

Glockner F.O., Stal L.J., Sandaa R.-A., Gasol J.M., O’Gara F., Hernandez F., Labrenz M., Stoica E., Varela M.M., Bordalo A., Pitta P. (2012). Marine Microbial Diversity and its role in Ecosystem Functioning and Environmental Change. Marine Board Position Paper 17. Calewaert, J.B. and McDonough N. (Eds.). Marine Board-ESF, Ostend, Belgium.

Roach 2004, Source of Half Earth's Oxygen Gets Little Credit, https://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html

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