Influence of Exposure to Sandy Sediments on Bottom-Dwelling Diatoms

Review of Microphytobenthic Primary Production on Exposed Coastal Sandy Sediments of the Southern Baltic Sea Using Ex-Situ Sediment Cores and Oxygen Opcodes

Written by Sofia Perez 

In a world deeply entwined with our own but more alien than the landscape of Mars, there lives an impressive diversity of organisms that hold together Earth’s ecological balance. With constantly-changing climate and shifts in weather events, it is now more important than ever to be able to understand them. 

As with every seemingly barren place, upon closer inspection the seafloor is home to  a complex range of organisms all with their own jobs. Since such diversity is common in the ocean, a recent study in Germany investigated the microscopic residents of the Baltic sea, including chlorophytes, euglenids, cyanobacteria, dinoflagellates, and diatoms. 

Microscopic Residents of the Baltic Sea

Why They Matter
ChlorophytesImportant food source to marine animals (e.g. plankton)
CyanobacteriaCarry out photosynthesis, which removes carbon from atmosphere & produces oxygen
DinoflagellatesImportant food source to marine animals
Mutually beneficial relationships with coral
DiatomsImportant food source to marine animals
Use carbon to build their cell wall, removing it from the atmosphere
EuglenidsCarry out photosynthesis, which removes carbon from atmosphere & produces oxygen
Mutually beneficial relationships with various organisms

The research group decided to study diatoms because they are the largest demographic of this microscopic aquatic community.  This helped to fill an important gap in our understanding of primary production in exposed sandy sediments, with the word ‘exposed’ referring to waves and currents. 

Like humans, diatoms have different preferences, each species preferring different levels of exposure to waves and currents, different sediment types, different amounts of light, and different preferences in regard to living attached to sediment grains or between them. 

However, with a recent increase in global temperatures, weather events are becoming more sporadic and extreme, meaning more wind and waves on the German coast. With these sediments facing constant disturbances and the distribution of microorganisms shifting in response, the researchers sought to better understand the current levels of production occurring in the changing sand. In order to do this, they used state-of-the-art approaches such as oxygen opcodes to take sediment cores from four sampling stations along the Hütelmoor coast of Germany. The following parameters were used to determine production:

ParameterWhat it shows
chlorophyll a per m2It shows the quantity of microorganisms of that sample which carry out photosynthesis, as chlorophyll a is found in the chloroplast of all cells which carry out this process. (There is also chlorophyll b, which absorbs a different frequency of light.)
Grain sizeThe mean grain size determines what type of diatoms are present in the sampling station, as different diatoms have different preferences when it comes to the size of sediment. 
POC:PON (particulate organic carbon to particulate organic nitrogen ratio)Particulate organic carbon= the mass of small particles of carbon from living organisms in the sample
Particulate organic nitrogen= the mass of small particles of nitrogen from organic compounds in the sample
The ratio of carbon to nitrogen in a sample can be used to determine the source of organic matter (i.e. The type of animal it comes from).  
OxygenBecause it is produced during photosynthesis and used for respiration, it can be used to show the rate at which these cellular processes take place. This indicates the presence of organisms which carry out photosynthesis. Oxygen optodes were used to detect dissolved oxygen and oxygen saturation levels by emitting a specific wavelength of light and measuring the glow given off by the oxygen in the water.
Fig. 1 The experimental setup: Sediment cores were placed inside a plastic tray. 
Experimental setup of sediment cores.” 2021. Oceanologia. Microphytobenthic primary production on exposed coastal sandy sediments of the Southern Baltic Sea using ex-situ sediment cores and oxygen optodes

After measuring these parameters, the measurements showed that total primary production strongly varied across the different stations and depths, while respiration didn’t fluctuate drastically. Additionally, sediments from the sampling locations were smooth, round, and inhabited by only a few diatoms, most of which preferred to be wedged in cracks and crevices rather than on the surface of the grain. Of these shy creatures, Planothidium delicatulum was the most abundant, making up 25% of the community. Araphid diatoms, which are tube-like in shape, were also found stuck to the surface of sediments by mucus coming from special pores.

Fig. 2 Results after measuring chlorophyll a concentrations in the sediment cores,
showing an increase in photosynthetic activity at greater depths 
Area Chlorophyll a Concentration as Proxy for Phototrophic Biomass (Chlorophyll a Mg M−2) in All Sediment Samples, as Measured in Different Water Depths (M) and at Three Sampling Dates in April, June and July 2017.” 2021. Oceanologia. Microphytobenthic primary production on exposed coastal sandy sediments of the Southern Baltic Sea using ex-situ sediment cores and oxygen optodes

In conclusion, the hypothesis could be confirmed that at greater depths, more photosynthesising microorganisms could be found due to the reduced disturbance of the upper layer of sediment. At this depth, these organisms carry out more primary production and are therefore vital for understanding various ecological phenomena. Moreover, because they make up one third of all the production in the Baltic Sea, they will be important for understanding its future fate. Overall, this topic warrants further investigation so that in the face of a constantly evolving planet, these tiny creatures are not left behind.

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