Diatoms, calcium, and the continuing challenge of ocean acidification

Zhang, Z., Ma, J., Chen, F., Chen, S., Pan, K. and Liu, H. (2024), Effect of increased CO₂ on calcium homeostasis and signaling in a marine diatom. Limnol Oceanogr, 69: 1365-1377. 

Diatoms under a microscope, Andrei Savitsky, Wikimedia Commons

Diatoms are no fans of low pH 

At this point, most people have heard about ocean acidification. As atmospheric CO₂ levels increase, so does the amount of CO₂ in the world’s oceans. After some chemical reactions, this lowers the ocean’s pH, which can affect the organisms that reside in the marine environment.  

One of the affected groups is diatoms. Small critters with silica shells that contribute ~40% of total marine primary productivity and play a major role in carbon and silicon biogeochemical cycles. A variety of studies have been done that look at how rising CO₂ levels can impact diatoms and their use of environmental elements, such as metals. While most of these studies have focused on iron, due to its crucial role in photosynthesis and primary productivity (check out John Martin’s iron hypothesis), another element, calcium, deserves its day in the sun.  

Calcium, not just another supplement 

Iron gets all the headlines when people study the ocean, but calcium, especially in reference to diatoms, is just as important. While relatively understudied, it plays a role in diatom silification (i.e., how diatoms make their houses), signal transduction, and ion regulation. It also aids in surface adhesion and movement in benthic diatoms, allowing them to glide (pretty neat!). How rising CO₂ levels affect calcium’s role in diatoms is a vital question. 

Calcium in glass tubes, Wikimedia Commons

The people, the cells, and the science 

A group working in Shenzhen and Hong Kong sought to answer this question. Led by Z. Zhang, they obtained a culture of Phaeodactylum tricornutum from the Center for Collections of Marine Algae. This diatom was chosen because it is one of the few diatoms whose DNA sequence has recently been made available in its entirety.  

Th team looked at climate change predictions made in 2019 and exposed their diatoms to 3 different pCO₂ levels: 400 µatm (low), 800 µatm (medium), and 1200 µatm (high). Currently, our atmosphere sits at ~425 ppm ( A variety of tests were done to look at how changing CO₂ affects diatom physiology, overall calcium content, calcium-related gene expression, and calcium changes in response to environmental stresses. What they found can be broken down into three main sections: how the outside of the diatoms are affected (physiology), how the inside is affected (calcium homeostasis), and the effects of the environment. 

Outside and in, diatoms need calcium 

First, while the growth of the diatoms did not significantly change, the cells did get smaller as pCO moved from low to high levels. This has been seen before in other studies, and it’s important to note that smaller cells will sink more slowly, which can impact the movement of elements like carbon to the deep ocean. Further, increased pCO₂ also led to a rougher exterior and weaker adhesion of the diatom surface. Both of these can impact the ecology of diatoms, especially how they deal with external forces such as predation from copepods (think Plankton from SpongeBob). 

SEM image of marine diatoms, Wikimedia Commons

Second, an increase in pCO₂ corresponded to an overall decrease in the ability of the diatoms to accumulate calcium. They displayed reduced surface, internal, and total calcium content, all of which can have a further effect on the diatom’s fitness. If the cell cannot obtain and hold onto calcium for its normal processes, it’s more susceptible to environmental changes, which the ocean is known for! 

Finally, the ability of marine creatures to respond to change is crucial for their survival. Using genomic data, Z. Zhang and colleagues examined how calcium-related genes in P. tricornutum respond to different pCO₂ levels. They found that the activity of the TPC1A gene, which regulates calcium movement in and out of the cell, decreased with higher pCO₂. Consequently, if diatoms cannot efficiently manage their calcium stores, their ability to respond to environmental change diminishes, impacting their overall fitness. 

In summary, a continued rise in pCO₂ will surely affect the ability of diatoms to respond to their environment and maintain their calcium levels. Not only does more pCO₂ lead to rougher exteriors, but it also hampers the ability of the cell to respond to cues in the surrounding environment. 

Diatoms will be okay, probably. 

It’s important to note, as the authors of this article do, that this study was conducted on a single strain of diatoms with constant pCO₂ levels. However, the ocean is inherently dynamic, continually changing in ways that captivate our attention. As such, these results cannot be broadly applied. Nonetheless, considering that the ocean is a living community, it’s crucial to study all its members and their interactions, as changes affecting one group can have wide-reaching impacts. 


Cover photo by NASA on Wikimedia Commons.

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