ICP-OES Determination of Metal Bioaccumulation in Caulerpa racemosa ramuli from Guánica, Puerto Rico

Technical Summary: ICP-OES Determination of Metal Bioaccumulation in Caulerpa racemosa Ramuli from Guánica, Puerto Rico

Problem Statement
While marine macroalgae, particularly Caulerpa species (sea grapes), are increasingly recognized as valuable resources for the novel food market due to their nutritional and biochemical properties, there is a significant gap in knowledge regarding the elemental composition of Caribbean populations. Existing literature heavily focuses on cultivated populations in Southeast Asia and tropical Australia. Furthermore, the elemental profile of marine algae is dynamic and governed by local physicochemical variables; therefore, data from the Indo-Pacific cannot be extrapolated to Caribbean ecosystems. There is an urgent need to establish baseline data on the elemental bioaccumulation profiles of native Caulerpa racemosa in Puerto Rico to evaluate their safety as food sources, assess local environmental quality, and explore their potential in marine biotechnology.

Methodology
The study employed Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) to characterize the elemental composition of Caulerpa racemosa var. ramuli collected from the subtidal zone of Tamarindo Beach in Guánica, Puerto Rico, in March 2025.

• Sampling: Fresh algal biomass (~1 kg) and adjacent seawater samples (collected in triplicate) were obtained.
• Preparation: Algal samples were washed with distilled water to remove epiphytes and salts, air-dried, and dissected to isolate upright ramuli from stolons and rhizoids.
• Digestion: 1.0 g of fresh ramuli was subjected to microwave-assisted digestion using 2% nitric acid (HNO₃) at 200°C.
• Analysis: Both digested algal samples and filtered seawater were analyzed for 61 chemical elements (major, trace, heavy metals, and rare earth elements).
• Calculation: The Bioconcentration Factor (BCF) was calculated to determine accumulation efficiency, defined as the ratio of analyte concentration in algal tissue to that in seawater. Accumulation was categorized as low (BCF < 100), moderate (100–1000), or high (> 1000).

Key Results

• Elemental Profile: While 61 elements were detected in the surrounding seawater, only 11 elements were detected in the C. racemosa ramuli biomass, indicating a distinct selective uptake mechanism rather than passive reflection of the environment.
• Selective Accumulation: The algal tissue showed comparatively higher concentrations of Boron (B), Tantalum (Ta), Zirconium (Zr), and Chromium (Cr). Ytterbium (Yb) and Manganese (Mn) were detected at the lowest concentrations.
• Bioconcentration Factors (BCF):
  • High Accumulation (BCF > 1000): Rhenium (Re), Zirconium (Zr), and Gadolinium (Gd) exhibited high accumulation. Re showed the highest BCF, surpassing 10,000.
  • Moderate Accumulation (BCF 100–1000): Ytterbium (Yb) and Tantalum (Ta).
  • Low Accumulation (BCF < 100): Manganese (Mn), Bismuth (Bi), Lithium (Li), and Chromium (Cr).
• Heavy Metal Presence: The seawater contained a diverse heavy metal profile, with Aluminum (Al), Arsenic (As), and Lead (Pb) being most abundant. Nickel (Ni), Cadmium (Cd), and Mercury (Hg) were present in seawater but undetected in the algal tissue. Conversely, Chromium (Cr) was the only heavy metal detected at appreciable levels in the algal tissue (approx. 95 µg/kg), showing enrichment relative to seawater.
• Unique Elements: Boron (B) and Beryllium (Be) were detected exclusively in the algal tissue and not in the adjacent seawater. Boron was the predominant unique element (~5,000 µg/kg), while Beryllium was present at ~450 µg/kg.

Significance and Claims
The paper claims that these findings provide new baseline information on the elemental profile of Caribbean C. racemosa, supporting its potential use in marine biomonitoring.

• Bioindicator Potential: The selective accumulation of Chromium (Cr) suggests that C. racemosa could serve as a bioindicator for local metal contamination. The presence of B and Be in tissues, particularly Be which has no known physiological role in algae, further supports the species' capacity to accumulate non-essential trace elements, warranting its investigation as a biomonitoring agent.
• Anthropogenic Indicators: The high enrichment of Rhenium (Re), Tantalum (Ta), and Zirconium (Zr)—elements typically found at very low concentrations in natural seawater—suggests potential anthropogenic pollution sources in the study area.
• Cautious Interpretation: The authors explicitly state that the unusually high concentration of Rhenium (Re) is not common and should be interpreted cautiously. They recommend confirmatory analysis using independent ICP-MS methods to validate this specific finding.
• Limitations: The study acknowledges limitations, including the analysis of only ramuli (excluding stolons and rhizoids), a single sampling location and time, and the lack of metal speciation or physiological uptake mechanism analysis. Consequently, the paper does not claim definitive applications in metal remediation or resource recovery but highlights the need for further studies to validate these observed patterns and explore such applications.