Amelioration of ocean acidification and warming effects through physiological buffering of a macroalgae

Abstract

Concurrent anthropogenic global climate change and ocean acidification are expected to have a negative impact on calcifying marine organisms. While knowledge of biological responses of organisms to oceanic stress has emerged from single‐species experiments, these do not capture ecologically relevant scenarios where the potential for multi‐organism physiological interactions is assessed. Marine algae provide an interesting case study, as their photosynthetic activity elevates pH in the surrounding microenvironment, potentially buffering more acidic conditions for associated epiphytes. We present findings that indicate increased tolerance of an important epiphytic foraminifera, Marginopora vertebralis , to the effects of increased temperature (±3°C) and p CO 2 (~1,000 µatm) when associated with its common algal host, Laurencia intricata . Specimens of M. vertebralis were incubated for 15 days in flow‐through aquaria simulating current and end‐of‐century temperature and pH conditions. Physiological measures of growth (change in wet weight), calcification (measured change in total alkalinity in closed bottles), photochemical efficiency ( Fv/Fm ), total chlorophyll, photosynthesis (oxygen flux), and respiration were determined. When incubated in isolation, M. vertebralis exhibited reduced growth in end‐of‐century projections of ocean acidification conditions, while calcification rates were lowest in the high‐temperature, low‐pH treatment. Interestingly, association with L. intricata ameliorated these stress effects with the growth and calcification rates of M. vertebralis being similar to those observed in ambient conditions. Total chlorophyll levels in M. vertebralis decreased when in association with L. intricata , while maximum photochemical efficiency increased in ambient conditions. Net production estimates remained similar between M. vertebralis in isolation and in association with L. intricata , although both production and respiration rates of M. vertebralis were significantly higher when associated with L. intricata . These results indicate that the association with L. intricata increases the resilience of M. vertebralis to climate change stress, providing one of the first examples of physiological buffering by a marine alga that can ameliorate the negative effects of changing ocean conditions.

Decrease in volume and density of foraminiferal shells with progressing ocean acidification

“…Numerous studies have discussed the potential of planktic shell information (e.g., shell weight, size, size-/area-normalized shell weight) as a proxy for environmental parameters; ambient seawater [CO 3 2− ] 18 , 28 – 31 , p CO 2 18 , 32 , 33 , temperature 33 , 34 etc. In addition, several other environmental parameters, such as nutrients, salinity, and marine algae have also been reported to have affected shell calcification 35 – 39 . Osborne et al 19 examined the shell area density (area-normalized shell weight) from sediment trap samples and proposed that the intensity of calcification is primarily controlled by [CO 3 2− ], whereas temperature influences shell size.…”

Differential Sensitivity of a Symbiont‐Bearing Foraminifer to Seawater Carbonate Chemistry in a Decoupled DIC‐pH Experiment

“…In addition, local environmental variations in the microhabitat of the organism may be highly significant. For example, a protective effect of a macroalgae microhabitat, acting as physiological buffer for increased temperature and p CO 2 , was recently described in a laboratory experiment with the epiphytic LBF M. vertebralis (Doo et al, 2020).…”

Coral reef productivity and diversity—Contributions from enhanced photosynthesis via demand for carbohydrate from the host

“…This response is consistent with downregulation of algal photosynthesis as a result of reduced demand for photosynthetically produced organic carbon from the host when the host uses DOC as an alternative energy source. In addition, when an intact symbiotic system (algal symbiont and foraminifera) was exposed to acidified water, increased rates of photosynthetic CO 2 fixation and associated amelioration of ocean-water acidification were observed; similar effects were not seen in isolated algal symbionts exposed to the same conditions (Doo et al, 2020).…”

“…This response is consistent with downregulation of algal photosynthesis as a result of reduced demand for photosynthetically produced organic carbon from the host when the host uses DOC as an alternative energy source. In addition, when an intact symbiotic system (algal symbiont and foraminifera) was exposed to acidified water, increased rates of photosynthetic CO 2 fixation and associated amelioration of ocean‐water acidification were observed; similar effects were not seen in isolated algal symbionts exposed to the same conditions (Doo et al., 2020). This finding is consistent with the assumption that a high demand for organic carbon from the host is a prerequisite to allow the algal symbiont to take advantage of greater CO 2 availability for photosynthesis in more acidic water and counteract acidification in the process.…”