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

Abstract

Abstract. Diatoms account for up to 50 % of marine primary production and are considered to be key players in the biological carbon pump. Ocean acidification (OA) is expected to affect diatoms primarily by changing the availability of CO2 as a substrate for photosynthesis or through altered ecological interactions within the marine food web. Yet, there is little consensus how entire diatom communities will respond to increasing CO2. To address this question, we synthesized the literature from over a decade of OA-experiments with natural diatom communities to uncover the following: (1) if and how bulk diatom communities respond to elevated CO2 with respect to abundance or biomass and (2) if shifts within the diatom communities could be expected and how they are expressed with respect to taxonomic affiliation and size structure. We found that bulk diatom communities responded to high CO2 in ∼60 % of the experiments and in this case more often positively (56 %) than negatively (32 %) (12 % did not report the direction of change). Shifts among different diatom species were observed in 65 % of the experiments. Our synthesis supports the hypothesis that high CO2 particularly favours larger species as 12 out of 13 experiments which investigated cell size found a shift towards larger species. Unravelling winners and losers with respect to taxonomic affiliation was difficult due to a limited database. The OA-induced changes in diatom competitiveness and assemblage structure may alter key ecosystem services due to the pivotal role diatoms play in trophic transfer and biogeochemical cycles.

Diatoms Dominate and Alter Marine Food-Webs When CO2 Rises

“…Silicate concentrations were replete at our reference sites (4.62 µM SiO 4 ± 0.61 SD, n = 3) and elevated pCO 2 sites (7.79 µM SiO 4 ± 0.08 SD, n = 3) in the RCP 6.0 conditions), so a lack of silicate likely does not explain the lack of benthic diatom blooms at the reference site and surrounding area. Clearly, diatom community responses will not only be dictated by direct physiological responses to CO 2 and macronutrients [5]; they will be indirectly affected by acidification-driven impacts on grazers. Given the high grazing pressure within our reference pCO 2 site [22,51], predominantly associated with both herbivorous fish and sea urchins, it is possible that B. biddulphiana is being excluded.…”

“…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].…”

“…There is a growing consensus on how ocean acidification will affect marine phytoplankton [3,4]. 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].…”

“…Experiments using natural communities of marine organisms are useful when projecting the impacts of ocean acidification as these help assess cascading effects through trophic levels that can only be assessed when interactions and competition among species are considered [18]. Studies investigating the effects of ocean acidification on natural diatom communities have mostly focussed on plankton (review: [5]), typically performed using closed containers [19]. In recent years, alternative approaches have increasingly been used including in situ and long-term mesocosm experiments (e.g., [16,20]) and the use of natural gradients in pCO 2 .…”

Higher sensitivity towards light stress and ocean acidification in an Arctic sea‐ice‐associated diatom compared to a pelagic diatom

“…Light utilization increased, as evident by higher α and light‐harvesting pigments, which translated into higher ETR HL and slightly higher POC production in cells grown under OA conditions (Figs 2, 3). Increasing seawater p CO 2 levels may increase diffusive carbon uptake and thus lower the energy and resource requirements for CCM operations, which could allow for increased rates of photosynthesis (Raven et al , 2011; Bach & Taucher, 2019). Under HL, however, a significant reduction in growth rates was observed in response to high p CO 2 , while beneficial effects on photophysiology disappeared (Table 2; Fig.…”