Algae after dark

The Moon (2020)

Reading Kath Kenny's article in the most recent issue of The Monthly about the dire consequences of ice melt in Antarctica, where 90% of the world's ice is currently stored, I was momentarily distracted by an algal conundrum. 

Biogeochemist, Delphine Lannuzel, was quoted as saying her research team were 'surprised' at finding phytoplankton (small algae) growing deep in the ocean, 'in the dark'. Everyone knows algae (and plants) need light to photosynthesise.

The article moved swiftly onto the implications to our planet of ice melting due to climate change, as it should, but there was a later passing mention of the famous 24-hour daylight in summer, and 24-hour night in winter, once you are within the Antarctic Circle.

Which made me think that even phytoplankton nearer the surface would have to survive complete, or almost complete, darkness for weeks or months at a time.

When I was young post-doctoral fellow at The University of Melbourne, I became a little obsessed myself about how algae in streams covered almost totally in detritus and other algae could still survive. Like the Antarctic algae at great depths, they received no sunlight.

I could understand how their thick-walled spores - the equivalent of seeds in flowering plants - could cope but I was less convinced by their vegetative parts. These algae are entirely 'autotrophic', meaning they have no way of consuming other organisms or their exudates to stay alive.

Cladophora glomerata (1989)

While doing other laboratory experiments, I wrapped petri-dishes containing small samples of blanketweed - a green, filamentous alga called Cladophora glomerata (above in situ in Darebin Creek, Ivanhoe) - in aluminum foil, placing them alongside unshielded cultures in a growth cabinet. After six months, they were still green and when unwrapped seemed happy to resume growth. That was all I needed to know at the time, and it helped me explain how seasonal dominance by different species was possible.

Clearly algae (and green land plants, which are fairly closely related) do have ways to cope without sunlight. How long they do that will depend on the species and the conditions: when I froze my blanketweed, it died within days, covered or uncovered.

Off the back of Kenny's story, I took a quick squiz at the latest (or at least since my own cursory experiment in 1989) on 'algae growing in the dark'. You can genetically engineer them to grow without light, but that's cheating. Other algae - not my filamentous green algae - can live in the dark by finding other ways to get food. But there is not much about how your typical photosynthesising alga (or plant) could just hang around for long periods of complete darkness. 

There are studies showing at least some growth even when light levels are extremely low (for example, under many metres of ice) but this may be a mix of limited photosynthesis and other means of obtaining food (which some planktonic algae in places like Antarctica can do). Not a solution for my totally blacked out blanketweed.

I'm sure the answer is out there and the cells in my alga probably just lived off fatty storage products, which did accumulate in my study organisms. 

Incidentally, these sorts of deposits often obscure the internal contents of wild-collected algal cells when collected at the end of the growing season, making it difficult to identify them. You see it's the photosynthetic apparatus - the green plastids (chloroplasts) - that are often most informative. 

And most essential for life, as is Antarctic ice.

Cladophora glomerata (1989)