Effects of Elevated CO2 on a Natural Diatom Community in the Subtropical NE Atlantic

Abstract

Diatoms are silicifying phytoplankton contributing about one quarter to primary production on Earth. Ocean acidification (OA) could alter the competitiveness of diatoms relative to other taxa and/or lead to shifts among diatom species. In spring 2016, we set up a plankton community experiment at the coast of Gran Canaria (Canary Islands, Spain) to investigate the response of subtropical diatom assemblages to elevated seawater pCO 2. Therefore, natural plankton communities were enclosed for 32 days in in situ mesocosms (∼8 m 3 volume) with a pCO 2 gradient ranging from 380 to 1140 µatm. Halfway through the study we added nutrients to all mesocosms (N, P, Si) to simulate injections through eddy-induced upwelling which frequently occurs in the region. We found that the total diatom biomass remained unaffected during oligotrophic conditions but was significantly positively affected by high CO 2 after nutrient enrichment. The average cell volume and carbon content of the diatom community increased with CO 2. CO 2 effects on diatom biomass and species composition were weak during oligotrophic conditions but became quite strong above ∼620 µatm after the nutrient enrichment. We hypothesize that the proliferation of diatoms under high CO 2 may have been caused by a fertilization effect on photosynthesis in combination with reduced grazing pressure. Our results suggest that OA in the subtropics may strengthen the competitiveness of (large) diatoms and cause changes in diatom community composition, mostly under conditions when nutrients are injected into oligotrophic systems.

Acidification diminishes diatom silica production in the Southern Ocean

“…Given the depth-dependent fluctuations in pCO2 183 in Antarctic waters 41 and accelerated pH decline 48 , it will not require much anthropogenic enhancement 184 of CO2 before levels will rise above those shown to affect diatom silica deposition. In contrast with the 185 positive effects generally reported by previous studies 11,27,32 , we saw no overall effect on diatom 186 photosynthesis, and no effect on growth until the highest [H + ], suggesting that Antarctic diatom growth 187 and photosynthesis at the community level are not significantly influenced by lowered pH. Instead, if 188 these data accurately represent future diatom physiologies, the changes to frustule density, measured 189 here as reduced silicification rate, have the potential to strongly diminish the grazing resistance 21 and 190 sinking capacity 43,49 of cells, undoubtedly altering the efficacy of the ocean biological carbon pump.…”

“…Changes to any of these traits can therefore have consequences for the efficiency of the 53 biological carbon pump, the process by which CO2 is converted to organic carbon via photosynthesis 54 and sequestered to ocean depth through sinking particles. Current understanding of the impacts of OA 55 on diatoms is limited to its effect on growth 8,11,12,27 , community composition 8,12,[28][29][30][31][32] and productivity 8, 56 13,[32][33][34] , in many cases showing positive responses with increased pCO2 11,32 . While some studies have 57 looked at the mechanistic relationship between pH and silica biomineralisation 35,36 , few have 58 investigated environmental pH shifts on silicification rates in diatoms 37,38 , the unique process that 59 underpins their sinking capacity.…”

CO<sub>2</sub> effects on diatoms: a synthesis of more than a decade of ocean acidification experiments with natural communities

“…Such interspecific differences in pCO 2 sensitivity are an important feature as this could alter the composition of diatom assemblages in a changing ocean. In this regard, it is interesting to note that paleolimnologists have long been using diatom species composition as palaeo-proxy to reconstruct lake pH (Battarbee et al, 2010). Hence, there is ample evidence that high CO 2 conditions have the potential to change the diatom species composition.…”

Diatoms Dominate and Alter Marine Food-Webs When CO2 Rises

“…A shift to larger diatoms may increase the trophic transfer of energy to marine animals by shortening food chains [13], and promote production in higher trophic levels. Carbon dioxide-driven shifts towards larger diatoms has been observed in ocean acidification experiments using natural communities [7,11,12,[14][15][16][17], suggesting that larger diatoms may become favoured in the future [5].…”

“…Research into the response of diatoms to ocean acidification has mostly focussed on their growth, productivity, and community composition by using mesocosms or flask culture experiments [5], with predominantly positive effects observed. Projections of the effects of ocean acidification on diatoms suggest that increased availability of CO 2 as a substrate for photosynthesis will benefit these algae where sufficient nutrients are available [6], and that these algae may indirectly benefit through reduced grazing pressure [7]. Changes in the ecological balance of life in our world ocean due to ocean acidification is a key focus of current research as it underpins the 'blue economy' [8].…”