Ocean acidification changes the structure of an Antarctic coastal protistan community

Abstract

Abstract. Antarctic near-shore waters are amongst the most sensitive in the world to ocean acidification. Microbes occupying these waters are critical drivers of ecosystem productivity, elemental cycling and ocean biogeochemistry, yet little is known about their sensitivity to ocean acidification. A six-level, dose–response experiment was conducted using 650 L incubation tanks (minicosms) adjusted to a gradient in fugacity of carbon dioxide (fCO2) from 343 to 1641 µatm. The six minicosms were filled with near-shore water from Prydz Bay, East Antarctica, and the protistan composition and abundance was determined by microscopy during 18 days of incubation. No CO2-related change in the protistan community composition was observed during the initial 8 day acclimation period under low light. Thereafter, the response of both autotrophic and heterotrophic protists to fCO2 was species-specific. The response of diatoms was mainly cell size related; microplanktonic diatoms (> 20 µm) increased in abundance with low to moderate fCO2 (343–634 µatm) but decreased at fCO2 ≥ 953 µatm. Similarly, the abundance of Phaeocystis antarctica increased with increasing fCO2 peaking at 634 µatm. Above this threshold the abundance of micro-sized diatoms and P. antarctica fell dramatically, and nanoplanktonic diatoms (≤ 20 µm) dominated, therefore culminating in a significant change in the protistan community composition. Comparisons of these results with previous experiments conducted at this site show that the fCO2 thresholds are similar, despite seasonal and interannual differences in the physical and biotic environment. This suggests that near-shore microbial communities are likely to change significantly near the end of this century if anthropogenic CO2 release continues unabated, with profound ramifications for near-shore Antarctic ecosystem food webs and biogeochemical cycling.

Short-term responses to ocean acidification: effects on relative abundance of eukaryotic plankton from the tropical Timor Sea

“…On the other hand, the "bottle enclosure effect" can also be systematically used to allow rare and underrepresented populations to flourish. OA studies often apply acclimation periods with gradually increasing CO2 to let organisms slowly adjust to new pH conditions (Hancock et al 2018), which is probably a more realistic representation of OA than sudden pH manipulations. However, here we used acute elevations of seawater pCO2 to obtain a better understanding of the short-term sensitivity of Ocean acidification effects on planktonic eukaryotes planktonic organisms and because rapid (within a few days) shifts in picoeukaryote communities due to OA were previously detected (Meakin & Wyman 2011).…”

Ocean acidification reduces growth and grazing of Antarctic heterotrophic nanoflagellates

“…Thus, it appears that an acclimation period had no discernible effect on the response of the community to enhanced CO 2 . Hancock et al (2018) did observe a significant change in microbial community composition in all treatments between days 1 and 3 but no further change in community composition was found between any of the treatments during the acclimation. Therefore, they attributed this initial change to acclimation of the community to the minicosm tanks and not a response to increasing CO 2 .…”

“…CC BY 4.0 License. few metazooplankton and nauplii passed through the pre-filter and they were seldom observed throughout the experiment (see Hancock et al, 2018). Thus, it is unlikely that their grazing effected the CO 2 -induced trends in community composition in our study.…”

“…As HNF cells are difficult to identify by microscopy in fixed samples , we were unable to determine whether the reduction in HNF abundance and differences in growth rates among treatments were due to CO 2 -induced effects on the entire HNF community or if species-specific sensitivities changed the community composition. Hancock et al (2018) reported a CO 2 -related change in the relative abundances of two choanoflagellate species at CO 2 levels ≥634 µatm (see 4.4 below) and thus, it is possible that other CO 2 -induced changes to HNF community composition may have occurred. Previous experiments in Prydz Bay, Antarctica also reported a reduction in HNF abundance when CO 2 was ≥750 µatm in both high and low nutrient conditions (Thomson et al, 2016).…”

Delivering Sustained, Coordinated, and Integrated Observations of the Southern Ocean for Global Impact

“…This suggests important localized forcings. Similar comparative studies are not available for other areas of the Southern Ocean, which are differentially affected by sea ice, glacial meltwater (Hernando et al, 2015), iron availability, and other stressors such as CO 2 increase and the consequent ocean acidification (Hoppe et al, 2013;Trimborn et al, 2013Trimborn et al, , 2017Hancock et al, 2018). Other than phytoplankton, bulk community composition and a deeper understanding of the microbial realm, including bacteria, archaea, and viruses, and the role of recalcitrant dissolved organic matter (microbial carbon pump, Jiao et al, 2010) are essential to the food-web carbon flux as well as to the ocean-carbon pump and CO 2 sequestration.…”