Introduction

Acropora corals, known for their intricate branching structures and vibrant colors, are cornerstone species in reef ecosystems. Their health is paramount to maintaining a balanced marine environment, particularly in reef tanks maintained by hobbyists. This literature review examines the nutritional dynamics and health of Acropora corals, focusing on the roles of zooplankton, phytoplankton, beneficial marine diatoms, and rotifers.

The health and growth of Acropora corals are closely linked to their unique morphological and genetic characteristics. These corals exhibit a variety of colony morphologies, ranging from simple forms to complex branching structures. This diversity in form is driven by the genetic regulation of colony morphology, which involves several key developmental signaling pathways such as Wnt, Notch, and BMP. Acropora corals show a division of labor between different parts of their colonies, with branch tips specializing in growth and branch bases focusing on maintenance and stability. This specialization is reflected in the differential gene expression between these regions, highlighting the intricate genetic control over coral growth and form​.

Nutritional Dynamics of Acropora Corals

Heterotrophy and Autotrophy

Acropora corals, like other reef-building species, engage in both heterotrophic and autotrophic modes of nutrition. They rely heavily on their symbiotic relationship with zooxanthellae, photosynthetic algae that live within their tissues. These symbionts provide the corals with essential nutrients derived from photosynthesis, which is critical for their energy needs and calcification processes. In addition to autotrophy, Acropora corals also capture plankton and particulate organic matter from the water column. This heterotrophic feeding supplements their diet, especially during periods of low light or environmental stress when photosynthesis might be compromised. This dual feeding strategy is vital for their resilience and overall health​.

Zooplankton and Phytoplankton

Zooplankton and phytoplankton are critical to the health and growth of Acropora corals. Phytoplankton, such as diatoms and dinoflagellates, contribute significantly to the coral's diet by providing a steady supply of photosynthetic products. Zooplankton, including copepods and rotifers, offer rich sources of protein and fatty acids necessary for coral growth and reproduction​.

Phytoplankton are primary producers in marine ecosystems, converting sunlight into biochemical energy through photosynthesis. This process not only generates oxygen but also produces organic compounds that serve as food for higher trophic levels, including zooplankton and corals. Diatoms, a major group of phytoplankton, are particularly beneficial due to their high content of essential fatty acids, which are vital for coral metabolic functions​.

Zooplankton are an essential component of the coral diet, providing proteins, lipids, and other nutrients that are not adequately supplied by photosynthesis alone. Research indicates that the ingestion of zooplankton can significantly enhance coral growth rates, fecundity, and resilience to environmental stressors​.

Beneficial Marine Diatoms

Marine diatoms, a subset of phytoplankton, play a beneficial role in coral health. They are a significant source of essential fatty acids, which are crucial for maintaining cell membrane integrity and overall metabolic functions in corals. Diatoms contribute to the coral's energy requirements and enhance their overall health and resilience by promoting a diverse and balanced diet​.

Diatoms are characterized by their silica-based cell walls and their ability to photosynthesize, which makes them a fundamental part of the marine food web. Their production of long-chain polyunsaturated fatty acids (PUFAs) is particularly important for coral health. PUFAs are essential for coral physiological processes, including reproduction and stress response mechanisms​.

The Role of Lipids in Coral Health

Lipids are essential for energy storage, structural functions, and cellular processes in corals. The lipid content in corals varies significantly among species and environmental conditions. Acropora species can exhibit lipid contents ranging from 14% to 37% of their dry weight. These lipids serve as critical energy reserves during periods of low food availability​.

The primary lipid classes in corals include triacylglycerols, wax esters, phospholipids, and glycolipids. These lipids are integral to the coral's structural integrity and play a role in symbiotic relationships with zooxanthellae, the photosynthetic algae living within coral tissues. Zooxanthellae provide corals with photosynthetically derived nutrients, particularly during daylight hours, which are crucial for sustaining coral energy demands and growth​.

Benefits of Feeding Rotifers

Rotifers, microscopic zooplankton, are highly nutritious and beneficial for Acropora corals. They are rich in protein and essential fatty acids, making them an excellent food source. Feeding rotifers to Acropora corals can enhance their growth rates, improve their coloration, and increase their resilience to stress and disease. The small size of rotifers ensures efficient nutrient uptake by coral polyps, facilitating optimal nutritional benefits.

Rotifers are particularly advantageous due to their ease of culture and high reproductive rates, which allow for a consistent and reliable food source for corals in captivity. Studies have shown that rotifers can significantly improve coral health and growth, highlighting their potential as a staple food source in reef aquariums​.

Endolithic Microbiomes

Recent studies highlight the importance of endolithic microbiomes residing within the coral skeleton. These communities, composed of bacteria, fungi, and algae, contribute to nutrient cycling and metabolite transfer, essential for coral health, especially during thermal stress. Endolithic microalgae, such as Ostreobium spp., penetrate the coral skeleton and engage in photosynthesis, supplying additional nutrients to the coral host.

Diversity and Function

The diversity of endolithic microorganisms within coral skeletons includes cyanobacteria, fungi, and various bacteria, each contributing uniquely to coral health. Cyanobacteria, for instance, engage in nitrogen fixation, supplying bioavailable nitrogen to corals. Fungi and other bacteria are involved in organic matter decomposition and nutrient remineralization, ensuring a continuous supply of essential nutrients.

Impact on Coral Health

Endolithic microorganisms play a dual role in coral health. While they contribute positively by enhancing nutrient availability, they can also weaken the coral skeleton through bioerosion. The balance between these opposing effects is crucial for maintaining coral integrity and resilience.

Metabolite Transfer and Nutrient Cycling

The interaction between endolithic microorganisms and coral tissues involves the transfer of metabolites such as lipids and amino acids. These metabolites support coral energy requirements, particularly under conditions of environmental stress, such as increased seawater temperatures. The nutrient cycling facilitated by endolithic communities ensures a steady supply of essential elements, enhancing coral growth and resilience.

Conclusion

This review underscores the importance of a diverse and balanced diet in maintaining the health of Acropora corals. Zooplankton, phytoplankton, beneficial marine diatoms, and rotifers each play critical roles in providing essential nutrients that support coral growth, reproduction, and resilience. By understanding and implementing these nutritional strategies, reef tank hobbyists can ensure the vitality and sustainability of their Acropora corals, contributing to vibrant and healthy reef ecosystems. Future research should focus on elucidating the specific mechanisms of nutrient assimilation and the role of microbial communities in coral health under varying environmental conditions.