Ocean Acidification Accelerates the Growth of Two Bloom-Forming Macroalgae

Coastal ecosystems often face acidification due to natural and human-induced factors like upwelling and nutrient runoff. While calcifying organisms are known to be affected by these changes, the impact on non-calcifying macroalgae remains unclear. Previous studies have shown that acidification can hinder calcifiers, but few have explored how macroalgae might respond. This gap motivated the need to assess how elevated CO2 levels influence the growth of macroalgae. Understanding these responses is essential for predicting future ecosystem shifts. Current knowledge lacks detailed data on macroalgal growth under acidified conditions. This study aims to address that uncertainty by examining two common bloom-forming species. The findings could help clarify the role of acidification in shaping coastal algal communities. This research fills a critical knowledge gap in marine ecology.

The goal of this research was to assess how two macroalgal species, Gracilaria and Ulva, respond to elevated CO2 levels and nutrient enrichment in a coastal estuary. Researchers wanted to determine if acidification could promote the growth of these species. They conducted experiments in situ and under controlled conditions to simulate future ocean conditions. The study aimed to explore the physiological mechanisms behind any observed growth changes. Understanding these responses is key to predicting future bloom dynamics. The researchers focused on growth rates and carbon use patterns. They also examined the interaction between CO2 and nutrient availability. This approach allowed them to isolate the effects of each variable on macroalgal growth.

The study involved field and controlled experiments with Gracilaria and Ulva in Shinnecock Bay, NY, USA. Algae were grown in situ or under manipulated pCO2 and nutrient conditions. Researchers used mesotrophic estuary conditions to simulate realistic environmental scenarios. Growth rates were measured and compared across treatments. Nutrient levels were adjusted to reflect eutrophic conditions. Stable carbon isotopes (δ13C) were analyzed to assess carbon use. Mixing models were applied to determine shifts in carbon uptake. The study included both seasonal and controlled experimental designs. These methods allowed for a comprehensive assessment of macroalgal responses.

Gracilaria growth increased by 70% under elevated pCO2 (p<0.05), but not with nutrient enrichment. Ulva showed a more complex response, with growth boosted by both pCO2 and nutrients. In some cases, these factors interacted to enhance Ulva growth synergistically. Elevated pCO2 increased Ulva growth by 30% (p<0.05), while nutrient effects were less significant. The δ13C content of both species decreased under elevated pCO2 (p<0.001). This shift suggested a change in carbon use from HCO3- to CO2. Growth responses were linked to inorganic carbon limitation. These findings highlight the potential for acidification to promote macroalgal overgrowth.

The study suggests that elevated pCO2 can significantly boost the growth of Gracilaria and Ulva. Acidification may promote macroalgal overgrowth in eutrophic estuaries. The shift in carbon use under high CO2 supports this conclusion. These findings align with the observed δ13C changes in both species. The interaction between CO2 and nutrients further complicates growth patterns. The results imply that acidification could intensify in the coming decades. This process may reshape coastal algal communities. The authors emphasize the need to consider acidification in future ecological models.