As Americans we drive in cars.  We use electricity for our houses and cell phones.  And we burn fossil fuels to power these processes. The average American releases more carbon into the atmosphere per capita than any other country in the world.  Carbon dioxide gas in the atmosphere reacts with ocean water to form an acid called ‘carbonic acid’.  This breaks up into bicarbonate and hydrogen ions and raises the acidity of the oceans.  When we think of climate change, most people think about the sky and atmosphere.  Yet the U.S. National Oceanic and Atmospheric Administration tells us the oceans stored ninety percent of global heat increases from 1970 to 2010. That’s ninety percent of warming stored in the oceans!  Ocean acidity today is up 26% since the beginning of the Industrial Revolution in 1800.  This is due to carbon dioxide reacting with seawater.

Ocean Acidification Threatens Phytoplankton

Ocean acidification threatens tiny marine organisms called phytoplankton.  These small photosynthetic organisms are an important food source for marine ecosystems.  Without phytoplankton, marine ecosystems such as fisheries could not survive.  Phytoplankton also produce over half of the earth’s oxygen.  That means the oxygen in every other breath you take comes from them!  By performing photosynthesis, they also trap carbon in the oceans.  This is important to slow climate change because carbon dioxide traps heat in the atmosphere.  In this way phytoplankton reduce harmful levels of carbon dioxide in the air and slow climate change.

The Diatom Family of Phytoplankton

One family of phytoplankton is called diatoms.  This family uses silicate minerals to build its cell walls.  Scientists know that ocean acidification interferes with this process in large diatoms.  Some scientists also believe small diatoms may not be affected by acidification as much as large diatoms due to their lesser reliance on this silica based process.  In order to investigate this idea, I chose a small diatom called T. weiss and tested whether more acidic ocean conditions affected its growth.  I hypothesized that more acidic conditions would hinder T. weiss growth compared to normal ocean acidity (about pH 8.1).  I predicted this since it is known that high acidity interferes with the silica needed for diatom cell walls formation.

The Experiment

I grew phytoplankton in moderate sunlight and temperature conditions.  They grew in plastic test tubes loosely covered to allow ventilation.  Each tube contained an “f/2 medium” beneficial to growth.  This contained distilled water, “Instant Ocean” Sea Salt, a vitamin solution, trace metals, and other compounds to mimic ocean conditions.

Acidic samples were made using Tris Hydrochloride and its conjugate base Tris.  In the first week I grew samples at pH 7.7, pH 7.9, and the control ocean pH of 8.1.  The second week tested the acidic condition at pH 7.5 compared to control ocean conditions. Every few days I took a visual reading of the phytoplankton’s color using a color-scale, or gradient of colors.  This allowed me to record the number of phytoplankton cells.  This was because our lab used an automated cell counter to measure what number of phytoplankton each color on the scale matched to.

Figure 1: The bottom color gradient was used to quantify the number of phytoplankton present throughout the growing period.  Color scale from: Abualhaija, Rana et al. “The fifth shade of green: A novel approach to phytoplankton color index assessment in an oligotrophic system.” Association for the Sciences of Limnology and Oceanography.  18 March, 2020. p. 498.

Results

The first batch was grown for 16 days and the second batch for 10 days.  The results were then graphed as number of cells versus time.  Error in the color-scale measurement technique was estimated at plus or minus 5 points on the scale or 30,000 cells.  The error in measurement time of day was estimated as plus or minus four hours.  When graphed, no statistically significant difference was observed between growth at more acidic pH of 7.5, 7.7, or 7.9 when compared to the control growth.

Batch 1 Graph: pH 7.5 and pH 7.9 vs. time

Batch 2 Graph: pH 7.5 vs. time

Figure 4 above shows how no difference  between pH 7.7, pH 7.9, and the control pH 8.1  No difference was observed between pH 7.5 and the control in batch 2 after 10 days (fig. 5).  This suggests T. weiss growth is not hindered by ocean acidification. Acidity increases that could arise in the oceans over the next century do not appear to inhibit T. weiss’ growth.  A possible explanation is that these smaller diatoms rely less on the silica process than large diatoms.  This supports the idea that smaller diatoms which are less reliant upon silica may be less harmed by acidification compared to larger diatoms.  High future carbon dioxide levels in the atmosphere therefore may not harm T. weiss as much as large diatoms and other phytoplankton species.