Calcification in Three Common Calcified Algae from Phuket, Thailand: Potential Relevance on Seawater Carbonate Chemistry and Link to Photosynthetic Process.

Pimchanok Buapet,Sutinee Sinutok

doi:10.3390/plants10112537

Pimchanok Buapet, Sutinee Sinutok

Open Access

https://doi.org/10.3390/plants10112537

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Journal: Plants (Basel, Switzerland)	Publication Date: Nov 21, 2021
Citations: 3	License type: CC BY 4.0

Affiliation: Prince of Songkla University

Abstract

Calcifying macroalgae contribute significantly to the structure and function of tropical marine ecosystems. Their calcification and photosynthetic processes are not well understood despite their critical role in marine carbon cycles and high vulnerability to environmental changes. This study aims to provide a better understanding of the macroalgal calcification process, focusing on its relevance concerning seawater carbonate chemistry and its relationship to photosynthesis in three dominant calcified macroalgae in Thailand, Padina boryana, Halimeda macroloba and Halimeda opuntia. Morphological and microstructural attributes of the three macroalgae were analyzed and subsequently linked to their calcification rates and responses to inhibition of photosynthesis. In the first experiment, seawater pH, total alkalinity and total dissolved inorganic carbon were measured after incubation of the macroalgae in the light and after equilibration of the seawater with air. Estimations of carbon uptake into photosynthesis and calcification and carbon release into air were obtained thereafter. Our results provide evidence that calcification of the three calcified macroalgae is a potential source of CO2, where calcification by H. opuntia and H. macroloba leads to a greater release of CO2 per biomass weight than P. boryana. Nevertheless, this capacity is expected to vary on a diurnal basis, as the second experiment indicates that calcification is highly coupled to photosynthetic activity. Lower pH as a result of inhibited photosynthesis under darkness imposes more negative effects on H. opuntia and H. macroloba than on P. boryana, implying that they are more sensitive to acidification. These effects were worsened when photosynthesis was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, highlighting the significance of photosynthetic electron transport-dependent processes. Our findings suggest that estimations of the amount of carbon stored in the vegetated marine ecosystems should account for macroalgal calcification as a potential carbon source while considering diurnal variations in photosynthesis and seawater pH in a natural setting.

Highlights

Since the mid-18th century, atmospheric CO2 has risen from 280 to 400 ppm, and is predicted to increase to 730–1200 ppm by 2100 as a result of human activities, such as the burning of fossil fuel, deforestation, agriculture and industrialization [1,2]
This study aims to provide a better understanding of the calcification process, focusing on its role in carbon economy and its relationship to photosynthesis in three dominant calcified macroalgae in upper sublittoral areas in Phuket, Thailand, P. boryana, H. opuntia and H. macroloba [44,45,46,47]
CaCO3 crystals of P. boryana were precipitated at the cell surface, whereas H. opuntia and H. macroloba precipitate CaCO3 in intercellular spaces

Summary

Introduction

Since the mid-18th century, atmospheric CO2 has risen from 280 to 400 ppm, and is predicted to increase to 730–1200 ppm by 2100 as a result of human activities, such as the burning of fossil fuel, deforestation, agriculture and industrialization [1,2]. Calcifying macroalgae are intriguing organisms and have become a subject of interest for recent marine frontiers research [12,13,14,15] as they are a source of primary production via photosynthesis as well as CaCO3 production via calcification with relatively fast growth and turnover rates compared to corals [16,17]. Their role in the carbon economy, whether they act as a net sink or source of carbon, remains a knowledge gap and debatable [18,19,20] mainly due to uncertainties about the fate of carbon associated with calcification. A recent study by Kalokora et al [20] found that calcification in a calcifying alga, Corallina officinalis L., can release a significant amount of CO2 to the atmosphere and become a source of CO2 if not refixed via photosynthesis in the system, highlighting the complex role of algal calcification on carbon capture potential in coastal areas [20]

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