Ecology of Diseases Affecting Crustose Coralline Algae : characteristics, environmental drivers and effects on coral recruitment

Abstract

Pristine coral reefs are characterized by high densities of large fishes, high cover of reef-building calcifiers and high densities of coral recruits. Such reefs have become rare and have been replaced in many places by macroalgal-dominated reefs. Marine diseases have been recognized as a key driver in coral reef decline worldwide, eradicating keystone species and altering fundamental ecological processes. In the past four decades, investigations aiming at a better understanding of diseases and their consequences on reef communities have multiplied. However, as most of these studies have focused on coral species, other key organisms of reef ecosystems have received little attention. Crustose coralline algae (CCA), along with scleractinian corals, are important primary producers and framework builders that play crucial roles in reef structure and function, including enhancing coral larval settlement. Like other reef organisms, CCA have been affected by striking disease outbreaks, yet, scientific knowledge on CCA disease ecology is lacking. This thesis aims to investigate CCA diseases through different steps, from describing their characteristics and distribution to studying their causes and consequences, which are reflected in the three publications of the thesis. For that purpose, we combined various field and laboratory methods. First, Publication 1 gives a detailed description of the type of diseases affecting CCA species on coral reefs of Curacao, their macroscopic characteristics, their distribution and occurrences, and how they fluctuate under the influence of various environmental factors. Then, Publication 2 investigates the microscopic characteristics and potential causal agents of the diseases. Finally, Publication 3 deals with the subsequent effects of CCA diseases on important ecological processes such as the survivorship and settlement of coral larvae. The thesis includes a general introduction to the papers and concludes with a synoptic discussion. In Publication 1, we conducted a survey along the coast and established a current baseline on the status of CCA diseases in Curacao. Two types of diseases were recorded, Coralline White Band Syndrome (CWBS) and Coralline White Patch Disease (CWPD), the latter being reported here for the first time. Species-specific occurrences revealed that all CCA species encountered, i.e. 10 species among 9 genera, were vulnerable to diseases and that disease occurrences were not influenced by CCA community composition. We found that disease occurrences were significantly higher during the warm/rainy season than during the cold/dry season and thus suggest that sea water temperature and rainfalls could be driving the increase in CCA diseases. The investigation of disease manifestation at the cellular level in Publication 2 revealed the complete depletion of protoplasmic content in diseased cells while cell walls remained intact. Diseased empty cells in CWPD were in the immediate vicinity of healthy cells, while a transition area appeared in CWBS in which diseased cells were only partially deprived of cellular content. We also found that various boring organisms (i.e. cyanobacteria, boring sponges, helminths and other macroborers) were more abundant in diseased thalli than in healthy thalli, which highlights a weakening of the skeleton in diseased CCA. We did not observe any signs of bacterial or fungal infection. Although the role of bioeroders in pathogenesis is not resolved, this study allowed to narrow down the range of potential pathogens for the diseases. Finally, in Publication 3, we investigated the effects of diseases affecting three CCA species on the larval survivorship and settlement success of two common scleractinian corals. Our findings demonstrated a negative effect of disease affecting the well-known settlement inductive CCA species Hydrolithon boergesenii on larval survivorship of Orbicella faveolata. We also showed that the ability of H. boergesenii to activate coral settlement in O. faveolata and Diploria labyrinthiformis was reduced when diseased. The outcomes of this thesis allow to better understand the ecology of CCA diseases by providing novel information on host, causative agents and environment, the three components of a complex disease triangle. This thesis indicates that a large range of CCA species is vulnerable to diseases, that disease occurrence may increase in relation to temperature and/or eutrophication, and that CCA diseases may have far reaching ecological impacts on coral reef ecosystems by weakening CCA skeleton and impairing coral recruitment. Our findings suggest that global action to limit ocean warming and acidification as well as local actions to reduce water pollution are essential to prevent disease outbreaks and their detrimental effects on coral reefs.