The ecophysiology of coralline algae in southern New Zealand

Abstract

Coralline algae (Rhodophyta) are an important indicator of global environmental change in coastal ecosystem due to their global distribution and sensitivity to changing ocean climate. In temperate latitudes, coralline algae are a dominant component of macroalgae communities, are autogenic ecosystem engineers. They provide hard substrate for colonization of other marine algae and invertebrates, and food that helps sustain diverse communities of associated organisms. The metabolic processes, growth and reproduction of coralline algae heavily depend on environmental factors including light, temperature and ambient nutrients in seawater. These changes, in turn, directly affect physiological responses of coralline algae. The primary aim of this study was to compare the nutrient and photosynthetic physiology between an important articulate coralline alga species (Arthrocardia sp.) and a group of indistinguishable crustose coralline species and their response to environmental fluctuations over seasonal cycles and depth gradients. Light, temperature and seawater nutrient concentration were sampled at 2 m and 10 m depth strata in a kelp forest where coralline algae were the dominant encrusting group over 21 months. The seasonal patterns in underwater light and temperature showed maximal values in summer and minimal values in winter, while the ambient nutrient concentration in seawater seasonally varied with higher concentration in winter, especially nitrate (7.68 ± 1.69 µM and 5.5 ± 0.77 µM at 2 and 10 m depths in winter, respectively). The highest temperatures at these depths were recorded in December 2017 (approximately 20 ˚C at 2 m depth and 17 ˚C at 10 m depth) when a marine heatwave occurred in New Zealand. Depth, season and the interaction between them clearly influenced to environmental conditions in seawater. These environmental parameters at the 2 m depth stratum were more seasonally variable, whereas these factors at 10 m depth were more stable. The nutrient and photosynthetic status of Arthrocardia sp. at 2 m depth, and crustose coralline algae at 2 m and 10 m depths also were investigated to evaluate effects of depth and season on nutrient and pigment concentration. Season influenced nutrient and photosynthetic pigment concentration of both Arthrocardia sp. and crustose coralline algae. Depth did not influence these concentrations of crustose coralline algae except for soluble tissue ammonium, C%, chlorophyll d and chlorophyll c concentration. Time-course nutrient depletion experiments were conducted in winter and summer to determine the uptake rates and incubation time for nutrient uptake kinetics experiments for Arthrocardia sp. and crustose coralline algae at 2 m depth. An incubation of 120 minutes was suitable for multiple-flask experiments to determine the kinetics of nutrients of these coralline algae. The kinetics parameters (Vmax and Ks) of ammonium, nitrate and phosphate of these coralline algae was determined. The ammonium uptake of both species exhibited a saturable kinetics with Vmax of Arthrocardia sp. (2.07± 0.32 µmol. gDW-1. h-1) was significantly higher than that of crustose coralline algae (0.58 ± 0.17 µmol. gDW-1. h-1). A linear relationship with concentration was observed in nitrate and phosphate uptake by both coralline algae. Higher surface area to volume ratio of Arthrocardia sp. was probably the primary factor of higher nitrogen and phosphorus uptake rates and differences in kinetic parameters than the group of crustose coralline algae. Photosynthesis versus irradiance for Arthrocardia sp. and crustose coralline algae at 2 m and 10 m depths was determined to understand the effects of depth distribution on photosynthesis. The differences between the two coralline groups were the main factor driving the differences in maximal photosynthesis rates normalized to dry mass, wet weight and chlorophyll a between Arthrocardia sp. and crustose coralline algae at the same stratum. The maximum photosynthetic rate Pmax of Arthrocardia sp. (20.38 ± 2.38 µmol O2. gDW-1. h-1) was significantly higher than crustose coralline algae (3.72 ± 0.74 µmol O2. gDW-1. h-1) at the same 2 m stratum. Differences in depth distribution did not affect the photosynthetic parameters of crustose coralline algae measured. The photosynthetic characteristics of these coralline algae showed a shade adapted organism with low saturation irradiance (Ek values are less than 100 µmol photons m-2 s-1). This study provides the first information on nutrient and photosynthetic physiology of coralline algae on the subtidal rocky reef habitats under kelp forest communities in southern New Zealand. The findings from this study are broadly applicable to temperate rocky reef ecosystems and provide fundamental data for further studies related to coralline algae and to environmental conditions in coastal ecosystem.