Mobile diatoms flourish in acid ocean

Thick mats of single celled microalgae, ‘pulsing’ in the seafloor sediments under the Antarctic sea ice, could flourish as ocean acidification intensifies.

The microalgae mats are a mix of more than 60 species of single-celled diatoms, which migrate up and down through sediment in response to daylight. They are expected to be amongst a small number of ‘winners’ as the Southern Ocean continues to absorb carbon dioxide from the atmosphere.

Institute for Marine and Antarctic Studies PhD student, James Black, has been studying the migratory behaviour and photosynthesis of these diatom mats, which can grow about 0.5mm thick, as part of the Antarctic Free Ocean CO2 Enrichment (antFOCE) experiment*, conducted over two months at Casey research station in 2014 (Australian Antarctic Magazine 27: 4–5, 2014).

“The antFOCE experiment focused on longer term ecological changes in the benthic [seafloor] community under ocean acidification conditions, but I was interested in how the short-term physiological changes could influence the longer term changes — what happens to benthic organisms over hours and days,” Mr Black said.

“I was particularly interested in diatoms because they are responsible for between 60 and 90 per cent of the primary production [plant growth] at the study site.”

To assess short-term physiological changes in the diatom-covered sediment, Mr Black deployed two ‘mini-chambers’ alongside larger FOCE chambers under the sea ice in the bay near Casey (see photo). Each mini-chamber contained seawater enriched with carbon dioxide to pH 7.8 (0.4 pH units below natural ocean pH). Every six hours this water was refreshed and the cycle repeated for up to 144 hours.

To monitor the health of the diatoms during the experiment, an instrument sent a pulse of light over the microalgae mats every 30 minutes and recorded how much light they absorbed — a measure of their ‘photosynthetic yield’.

Preliminary analysis of the results suggests there is an interaction between the intensity of the light cycle and ocean acidification that affects diatom behaviour and photosynthetic yield, and that the diatoms require a regular day/night cycle to coordinate their sediment migration.

“When the diatoms experienced regular light/dark cycles under ocean acidification conditions, they coordinated their migration 5–10mm up and down through the sediment over the course of the day. This appeared to regulate their exposure to ocean acidification and resulted in a positive photosynthetic response under most circumstances,” Mr Black said.

“They also migrated more rapidly upward through the sediment under acidic conditions. This may be related to the increased availability of carbon dioxide in the water, which the diatoms use for photosynthesis and growth.”

While the field experiments showed some strong positive trends in microalgae behaviour and response to light under ocean acidification conditions, the natural range of light conditions and experimental replication were limited. To tease out the findings further, Mr Black set up replicate experiments in the laboratory with the help of the Australian Antarctic Division’s Science Technical Support team.

“Performing long-term laboratory experiments on Antarctic organisms is no easy task, and requires a team of people skilled in transport and logistics, software and electrical engineering, and science, to ensure success,” Mr Black said.

The sediment faunal communities and diatoms were collected from Antarctica and placed into 20 chambers set up in a smaller but similar fashion to the original mini-chambers. These natural assemblages of marine organisms were then exposed to 10 weeks of ocean acidification or control conditions.

“The laboratory experiments replicated both the regular and longer day/night light cycle conditions seen in the field and we observed similar results to our field experiments,” Mr Black said.

As diatoms can modify the pH of their immediate environment through photosynthesis, Mr Black will now look at how much the diatoms alter the pH of the sediment and ‘boundary layer’ (water just above the sediment) under high light conditions.

While the experimental results will take some time to fully process, Mr Black is excited at the prospect of having found a photosynthetic species to better understand the effect of ocean acidification on usually sedentary photosynthetic marine organisms.

“You can’t look at the behaviour of seaweeds because they don’t move,” he said.

“So these diatoms are not just important in an ecosystem context, they’re important as a model species to study the effects of ocean acidification on photosynthetic organisms.

“To be able to see the migration of diatoms and link it to ‘choices’ they’re making, provides a greater insight into how ocean acidification affects photosynthesis and microalgae growth.”

Wendy Pyper
Australian Antarctic Division

*Australian Antarctic Science project 4127