Experiments on Mixotrophic Protists and Catastrophic Darkness

Abstract

Catastrophically darkened photic zone conditions in water bodies are postulated to be induced by a diversity of mechanisms that are recorded in the geological record, including asteroid and comet impacts and large-scale volcanic eruptions. Giant wildfires, such as those that followed the great fires in Siberia in 1915, have been directly shown to cause large reductions in sunlight penetrating to the ground. Previous studies on the response of phototrophs to sudden prolonged darkness have focused on the survival of axenic strains. In this paper, we describe laboratory experiments to investigate the survival and growth of isolated and mixed cultures of freshwater and marine mixotrophs after 6 months of darkness and in the low light that would follow these events. Mixotrophs could survive 6 months of darkness. Some species used dissolved organic carbon, which can be released from dead biomass after loss of light and was shown to improve feeding rates. Mixotrophs also improved the survival and subsequent growth of obligate phototrophs at low light levels when grown in mixed cultures. The ability of mixotrophs to switch from photosynthesis to heterotrophy following sudden darkening would not only allow them to survive but to grow and contribute to active food chains. The experiments suggest that, following the return of light, resumption of photosynthesis can be rapid. These experiments improve our understanding of the collapse of photosynthesis following catastrophic darkening and emphasize the important role of mixotrophy in the resilience of the photosynthetic biosphere during such periods. We speculate on the implications for the Cretaceous-Tertiary impact event and periods of global freezing.

Physiological Responses of Three Species of Antarctic Mixotrophic Phytoflagellates to Changes in Light and Dissolved Nutrients

“…Photosynthetic flagellates utilizing phagotrophy for longterm dark survival was previously suggested [24,25]. As noted above, this is consistent with observations of G. cryophila and other Antarctic cryptophytes that engage in autotrophic energy production, but may not require photosynthesis for survival.…”

“…Additionally, micronutrient deficiencies, particularly iron, have been hypothesized as factors controlling phytoplankton growth in the Southern Ocean and could be a major resource targeted by the phagotrophic activity of mixotrophic flagellates [22,23]. Mixotrophy has even been proposed as a strategy that phytoflagellates could use to survive the austral winter to enter spring as actively growing populations [24,25].…”

Experimental Evolution of Heterotrophy in a Green Alga

“…Reboud and Bell (1997) showed that experimental populations of Chlamydomonas subjected to an environment fluctuating between light and dark conditions every few generations tended to evolve into generalists capable of growing well in both. Jones et al (2009) have speculated that mixotrophic protists could survive the "catastrophic darkening" associated with volcanic eruptions and asteroid impacts and be responsible for the rapid resumption of primary production after the event.…”

Phytoplankton growth after a century of dormancy illuminates past resilience to catastrophic darkness

“…Although mixotrophy might have been a survival advantage to some species17, it cannot solely explain the differential extinction. Phagotrophic mixotrophy is unknown in diatoms and some mixotrophic dinoflagellates have been reported to encyst when exposed to sudden darkness18.…”