High alkalinity reduces growth of the invasive microalga Gonyostomum semen (Raphidophyceae)

Abstract

The unicellular Gonyostomum semen (class Raphidophyceae) is an invasive microalga which can dominate phytoplankton communities during blooms, with economic and ecological consequences. Its abundance in European freshwaters has been mainly explained by water temperature, colour, and acidity. While low pH and brown colour are common features of lakes containing the species, it has also been found in lower water colour and in pH up to 8. There is a need for a more constant predictor to be determined, if mitigation efforts are to be optimised.
In this study, the effect of alkalinity as a direct link between water colour and pH is examined. Laboratory experiments were used to test the effect of calcium and sodium additions in the form of chlorides and carbonates on the growth of G. semen monocultures. This was supported by analysis of biovolume data from a Swedish environmental monitoring database, performed on a dataset including limed and non-limed lakes. Mesocosms containing natural lake water from Lake Bökesjön in southern Sweden were set up to pilot test an up-scaled experimental setups with the species.
There were no clear effects of NaCl or CaCl2 on G. semen cell density, but Na2CO3 and even more prominently CaCO3 both had detrimental effects on growth rates. Limed lakes showed a trend toward having a lower mean August G. semen biovolume than non-limed, but this varied with data summary method. Nonetheless, our experimental results suggested that carbonate to a large extent, and calcium to a certain extent, impact G. semen growth negatively.

Popular Abstract

An Annoying Algae and Alkalinity

Gonyostomum semen (Swedish: gubbslem) consists of only a single cell, and yet is able to dominate lakes. The microscopic alga likes to swim and grow in the warm lake waters of peak summer. Such behaviour is not unlike human behaviour, and this coexistence can cause problems for us. Structures in the cells called trichocysts excrete a slimy, itchy substance when disturbed, for example by a swimmer’s arm. During a bloom, cells divide to great numbers and may create enough of this slime to cover bathers and water filters alike. While G. semen is more annoying than dangerous to humans, its blooms can take over the lake community leaving not enough space, nutrients, or light for other species of phytoplankton to grow. Cascading harm to the rest of the lake’s organisms follows, as trophic webs are disturbed and biodiversity is lowered. How can we predict which lakes are under risk of being invaded next?

G. semen prefers brown, slightly acidic waters, but is by no means limited to them. An abiotic factor linked to the two just mentioned is alkalinity, which refers to pH buffering capacity. In addition, one of the most common alkalinity-raising compounds is calcite (CaCO3), whose calcium ions (Ca2+) may bind to water-browning humic substances, removing these. In other words, alkalinity and calcium levels make excellent candidates for factors that can explain G. semen mass appearance. Therefore, we grew clones of G. semen cultures in laboratory growth media of different calcium chloride (CaCl2), rock salt (NaCl), calcite (CaCO3), or soda ash (Na2CO3) concentrations. Cell density of the cultures was measured before and after to see if the treatments affected growth. In addition, we analysed data on G. semen biovolume in Swedish limed and non-limed lakes. Liming is a restoration method used to increase alkalinity, and thus pH buffer capacity, of acidified waters, applied in Sweden since the 1970s. It is done by adding calcite, and thus also calcium ions.

The experimental results showed that calcium chloride has a limited, but trending toward negative, effect on G. semen growth. Soda ash and calcite both eradicated the starting populations. We deduce that the carbonate (CO32-) ion is the main cause of the negative growth in these experiments, rather than the cation (Ca2+ or Na+) part. However, in both cases the Ca2+-exposed treatments (calcium chloride and calcite) grew worse than their respective Na+ counterpart (rock salt and soda ash respectively), implying that the calcium itself also has some role in growth suppression. Calcium and carbonate both play major roles in liming, so let us take a look at the results of the liming investigation:

The database results indicate that limed lakes have lower blooming biovolume of G. semen than non-limed. While this requires further analysis, especially on pre-liming data, it is an indicator worth considering when making predictions about lakes at risk of G. semen invasion or bloom. Together with the results from the experiment, we conclude that calcium can be a potential predictor for G. semen occurrence because of its tendency to decrease growth, but carbonate and thus alkalinity is an even stronger force. This knowledge can add valuable parameters to ecological niche models of potential distribution, which aid efforts in invasion mitigation. In the long run, hopefully our skin and drinking water can be kept free from itchy slime.


Supervisor: Raphael Gollnisch
Aquatic Ecology, Lund University

• author: Alling, Teodor
• supervisor: Raphael Gollnisch
• organization: Degree Projects in Biology
• course: BIOY01 20181
• year: 2018
• type: M2 - Bachelor Degree
• subject: Biology and Life Sciences
• language: English
• id: 8962465
• date added to LUP: 2018-10-26 11:34:15
• date last changed: 2018-10-26 11:34:15