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Biodiversity

Around the world on a quest for diatoms

How do scientists explore the diversity of tiny cells in the vast ocean? Does diversity change in relation to environmental factors? This study used a series of models to explore diatom diversity around the world on one of the Tara Oceans expeditions. Read on to learn about the wonderful world of diatoms in the global ocean.

Citation: Busseni, G, Caputi, L, Piredda, R, et al. Large scale patterns of marine diatom richness: Drivers and trends in a changing ocean. Global Ecol Biogeogr. 2020; 00: 1– 14. https://doi.org/10.1111/geb.13161

Tara Oceans: A global ocean study

The Tara Ocean Foundation is one of the largest efforts of oceanographic exploration in modern times. The foundation supports revolutionary ocean science research with help from its research vessel, Tara, a 118 foot schooner. The ship has sailed on 12 expeditions around the world, covering 400,000 km and stopping in 60 countries. The 150 scientists and 250 crew members involved in the various expeditions have collected over 80,000 different types of samples. The goals of the project are to explore the biodiversity of our oceans and to understand how a changing climate will impact the organisms that live beneath the waves. To date, 100,000 new species and 150 million genes have been discovered by Tara Oceans scientists. So much discovery, yet so much still to learn!

Map of Tara Oceans expedition from 2009 – 2013. No need to focus on the details of the legend – just examine the incredible route around the world! Image credit: Tara Ocean Foundation

All about diatoms

The current study used data from a Tara Oceans expedition that traveled from the Mediterranean to the Atlantic, Pacific, Indian, Arctic and Antarctic Oceans. So, all over the world, really! This particular expedition took place from 2009 – 2013 and its goal was to examine planktonic and coral ecosystems’ response to climate change. This study focused on diatoms, a particular type of phytoplankton that comes in all shapes and sizes (see below for an example of their incredible diversity). There two commonly used methods to identify diatoms from seawater samples: 1) morphological identification–which means one spends hours and hours looking through a microscope to identify a diatom species and 2) metabarcoding–which involves extracting DNA from diatom cells collected from the water and channeling your inner forensic scientist to use DNA to identify different species. Both methods are frequently used, yet the results from each method can be different.

A clipart version of metabarcoding: the cells are taken from the natural environment via water filtration, the DNA is extracted and sequenced and various programs are used to process the DNA sequences for species identification. Image credit: Wikipedia Commons, Larissa Fruehe

While the metabarcoding approach is more easily able to identify rare species that may get missed during microscope analysis,  there are pros and cons to both methods (eg. cost and time). It can be difficult to compare diatom diversity across studies that use different methodologies because of the separate outcomes. Thus, the goal of this study was to combine these two different approaches in order to get one single measurement of diatom diversity in different regions of the world’s oceans.

An example of just how diverse diatoms (and all plankton for that matter) are. So many different shapes and sizes! Image credit: Carolina Biological Supply Company

What did they find and why does it matter?

The researchers examined diatom richness, or the number of different species present at any given moment in time and space, around the world. They used complex models to examine patterns of diatom richness in relation to environmental factors and found that richness was influenced by nutrient availability and variables related to hydrodynamic structure of the water, such as temperature and salinity. In fact, temperature was the most important variable in the model output, especially at its extremes such as in the Arctic and in tropical regions. The scientists also used models to explore what would happen to diatom richness under a specific climate change scenario. They found that with a changing ocean ecosystem, there will likely be a general decrease of richness. In addition, changes in nitrate, a specific nutrient required for diatom growth, concentrations and temperature distributions may dramatically affect diatom sensitivity to climate change. Understanding diatom diversity is important for a healthy ocean because diatoms are one of the major energy producers at the bottom of the marine food web. A changing ocean ecosystem will mean different diatom species may thrive under different environmental conditions and being able to predict possible changes is important for overall ecosystem function.

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